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Welcome to the rust-clippy wiki!

Here we aim to collect further explanations on the lints clippy provides. So without further ado:

Those lints are Deny by default:

cmp_nan invalid_regex out_of_bounds_indexing unused_io_amount zero_width_space

Those lints are Warn by default:

absurd_extreme_comparisons almost_swapped approx_constant assign_op_pattern bad_bit_mask blacklisted_name block_in_if_condition_expr block_in_if_condition_stmt bool_comparison box_vec boxed_local builtin_type_shadow char_lit_as_u8 chars_next_cmp clone_double_ref clone_on_copy cmp_null cmp_owned collapsible_if crosspointer_transmute cyclomatic_complexity deprecated_semver deref_addrof derive_hash_xor_eq diverging_sub_expression doc_markdown double_neg double_parens drop_ref duplicate_underscore_argument empty_loop enum_clike_unportable_variant enum_variant_names eq_op eval_order_dependence expl_impl_clone_on_copy explicit_counter_loop explicit_into_iter_loop explicit_iter_loop filter_next float_cmp for_kv_map for_loop_over_option for_loop_over_result forget_ref get_unwrap identity_op if_let_redundant_pattern_matching if_let_some_result if_same_then_else ifs_same_cond ineffective_bit_mask inline_always iter_next_loop iter_nth iter_skip_next large_enum_variant len_without_is_empty len_zero let_and_return let_unit_value linkedlist logic_bug manual_swap many_single_char_names map_clone map_entry match_bool match_overlapping_arm match_ref_pats match_same_arms min_max misrefactored_assign_op mixed_case_hex_literals module_inception modulo_one mutex_atomic needless_bool needless_borrow needless_lifetimes needless_range_loop needless_return needless_update neg_multiply new_ret_no_self new_without_default new_without_default_derive no_effect nonsensical_open_options not_unsafe_ptr_arg_deref ok_expect or_fun_call overflow_check_conditional panic_params partialeq_ne_impl precedence print_with_newline ptr_arg range_step_by_zero range_zip_with_len redundant_closure redundant_closure_call redundant_pattern regex_macro reverse_range_loop search_is_some serde_api_misuse short_circuit_statement should_implement_trait single_char_pattern single_match string_extend_chars string_lit_as_bytes suspicious_assignment_formatting suspicious_else_formatting temporary_assignment temporary_cstring_as_ptr too_many_arguments toplevel_ref_arg transmute_ptr_to_ref trivial_regex type_complexity unit_cmp unnecessary_mut_passed unnecessary_operation unneeded_field_pattern unsafe_removed_from_name unused_collect unused_label unused_lifetimes useless_attribute useless_format useless_let_if_seq useless_transmute useless_vec while_let_loop while_let_on_iterator wrong_self_convention wrong_transmute zero_divided_by_zero zero_prefixed_literal

Those lints are Allow by default:

assign_ops cast_possible_truncation cast_possible_wrap cast_precision_loss cast_sign_loss enum_glob_use filter_map float_arithmetic if_not_else indexing_slicing integer_arithmetic invalid_upcast_comparisons items_after_statements mem_forget missing_docs_in_private_items mut_mut mutex_integer non_ascii_literal nonminimal_bool option_map_unwrap_or option_map_unwrap_or_else option_unwrap_used print_stdout pub_enum_variant_names result_unwrap_used shadow_reuse shadow_same shadow_unrelated similar_names single_match_else string_add string_add_assign stutter unicode_not_nfc unseparated_literal_suffix use_debug used_underscore_binding wrong_pub_self_convention

Those lints are deprecated:

extend_from_slice str_to_string string_to_string unstable_as_mut_slice unstable_as_slice

A word of warning

Clippy works as a plugin to the compiler, which means using an unstable internal API. We have gotten quite good at keeping pace with the API evolution, but the consequence is that clippy absolutely needs to be compiled with the version of rustc it will run on, otherwise you will get strange errors of missing symbols.

absurd_extreme_comparisons

Default level: Warn

What it does: Checks for comparisons where one side of the relation is either the minimum or maximum value for its type and warns if it involves a case that is always true or always false. Only integer and boolean types are checked.

Why is this bad? An expression like min <= x may misleadingly imply that is is possible for x to be less than the minimum. Expressions like max < x are probably mistakes.

Known problems: None.

Example:

vec.len() <= 0
100 > std::i32::MAX

almost_swapped

Default level: Warn

What it does: Checks for foo = bar; bar = foo sequences.

Why is this bad? This looks like a failed attempt to swap.

Known problems: None.

Example:

a = b;
b = a;

approx_constant

Default level: Warn

What it does: Checks for floating point literals that approximate constants which are defined in std::f32::consts or std::f64::consts, respectively, suggesting to use the predefined constant.

Why is this bad? Usually, the definition in the standard library is more precise than what people come up with. If you find that your definition is actually more precise, please file a Rust issue.

Known problems: If you happen to have a value that is within 1/8192 of a known constant, but is not and should not be the same, this lint will report your value anyway. We have not yet noticed any false positives in code we tested clippy with (this includes servo), but YMMV.

Example:

let x = 3.14;

assign_op_pattern

Default level: Warn

What it does: Checks for a = a op b or a = b commutative_op a patterns.

Why is this bad? These can be written as the shorter a op= b.

Known problems: While forbidden by the spec, OpAssign traits may have implementations that differ from the regular Op impl.

Example:

let mut a = 5;
...
a = a + b;

assign_ops

Default level: Allow

What it does: Checks for compound assignment operations (+= and similar).

Why is this bad? Projects with many developers from languages without those operations may find them unreadable and not worth their weight.

Known problems: Types implementing OpAssign don't necessarily implement Op.

Example:

a += 1;

bad_bit_mask

Default level: Warn

What it does: Checks for incompatible bit masks in comparisons.

The formula for detecting if an expression of the type _ <bit_op> m <cmp_op> c (where <bit_op> is one of {&, |} and <cmp_op> is one of {!=, >=, >, !=, >=, >}) can be determined from the following table:

Comparison	Bit Op	Example	is always	Formula
== or !=	&	x & 2 == 3	false	c & m != c
< or >=	&	x & 2 < 3	true	m < c
> or <=	&	x & 1 > 1	false	m <= c
== or !=	`	`	`x	1 == 0`
< or >=	`	`	`x	1 < 1`
<= or >	`	`	`x	1 > 0`

Why is this bad? If the bits that the comparison cares about are always set to zero or one by the bit mask, the comparison is constant true or false (depending on mask, compared value, and operators).

So the code is actively misleading, and the only reason someone would write this intentionally is to win an underhanded Rust contest or create a test-case for this lint.

Known problems: None.

Example:

if (x & 1 == 2) { … }

blacklisted_name

Default level: Warn

What it does: Checks for usage of blacklisted names for variables, such as foo.

Why is this bad? These names are usually placeholder names and should be avoided.

Known problems: None.

Example:

let foo = 3.14;

Configuration: This lint has the following configuration variables:

• blacklisted-names: Vec<String>: The list of blacklisted names to lint about (defaults to ["foo", "bar", "baz", "quux"]).

block_in_if_condition_expr

Default level: Warn

What it does: Checks for if conditions that use blocks to contain an expression.

Why is this bad? It isn't really Rust style, same as using parentheses to contain expressions.

Known problems: None.

Example:

if { true } ..

block_in_if_condition_stmt

Default level: Warn

What it does: Checks for if conditions that use blocks containing statements, or conditions that use closures with blocks.

Why is this bad? Using blocks in the condition makes it hard to read.

Known problems: None.

Example:

if { let x = somefunc(); x } ..
// or
if somefunc(|x| { x == 47 }) ..

bool_comparison

Default level: Warn

What it does: Checks for expressions of the form x == true (or vice versa) and suggest using the variable directly.

Why is this bad? Unnecessary code.

Known problems: None.

Example:

if x == true { }  // could be `if x { }`

box_vec

Default level: Warn

What it does: Checks for use of Box<Vec<_>> anywhere in the code.

Why is this bad? Vec already keeps its contents in a separate area on the heap. So if you Box it, you just add another level of indirection without any benefit whatsoever.

Known problems: None.

Example:

struct X {
    values: Box<Vec<Foo>>,
}

boxed_local

Default level: Warn

What it does: Checks for usage of Box<T> where an unboxed T would work fine.

Why is this bad? This is an unnecessary allocation, and bad for performance. It is only necessary to allocate if you wish to move the box into something.

Known problems: None.

Example:

fn main() {
    let x = Box::new(1);
    foo(*x);
    println!("{}", *x);
}

Configuration: This lint has the following configuration variables:

• too-large-for-stack: u64: The maximum size of objects (in bytes) that will be linted. Larger objects are ok on the heap (defaults to 200).

builtin_type_shadow

Default level: Warn

What it does: Warns if a generic shadows a built-in type.

Why is this bad? This gives surprising type errors.

Known problems: None.

Example:

impl<u32> Foo<u32> {
    fn impl_func(&self) -> u32 {
        42
    }
}

cast_possible_truncation

Default level: Allow

What it does: Checks for on casts between numerical types that may truncate large values. This is expected behavior, so the cast is Allow by default.

Why is this bad? In some problem domains, it is good practice to avoid truncation. This lint can be activated to help assess where additional checks could be beneficial.

Known problems: None.

Example:

fn as_u8(x: u64) -> u8 { x as u8 }

cast_possible_wrap

Default level: Allow

What it does: Checks for casts from an unsigned type to a signed type of the same size. Performing such a cast is a 'no-op' for the compiler, i.e. nothing is changed at the bit level, and the binary representation of the value is reinterpreted. This can cause wrapping if the value is too big for the target signed type. However, the cast works as defined, so this lint is Allow by default.

Why is this bad? While such a cast is not bad in itself, the results can be surprising when this is not the intended behavior, as demonstrated by the example below.

Known problems: None.

Example:

u32::MAX as i32  // will yield a value of `-1`

cast_precision_loss

Default level: Allow

What it does: Checks for casts from any numerical to a float type where the receiving type cannot store all values from the original type without rounding errors. This possible rounding is to be expected, so this lint is Allow by default.

Basically, this warns on casting any integer with 32 or more bits to f32 or any 64-bit integer to f64.

Why is this bad? It's not bad at all. But in some applications it can be helpful to know where precision loss can take place. This lint can help find those places in the code.

Known problems: None.

Example:

let x = u64::MAX; x as f64

cast_sign_loss

Default level: Allow

What it does: Checks for casts from a signed to an unsigned numerical type. In this case, negative values wrap around to large positive values, which can be quite surprising in practice. However, as the cast works as defined, this lint is Allow by default.

Why is this bad? Possibly surprising results. You can activate this lint as a one-time check to see where numerical wrapping can arise.

Known problems: None.

Example:

let y: i8 = -1;
y as u128  // will return 18446744073709551615

char_lit_as_u8

Default level: Warn

What it does: Checks for expressions where a character literal is cast to u8 and suggests using a byte literal instead.

Why is this bad? In general, casting values to smaller types is error-prone and should be avoided where possible. In the particular case of converting a character literal to u8, it is easy to avoid by just using a byte literal instead. As an added bonus, b'a' is even slightly shorter than 'a' as u8.

Known problems: None.

Example:

'x' as u8

chars_next_cmp

Default level: Warn

What it does: Checks for usage of .chars().next() on a str to check if it starts with a given char.

Why is this bad? Readability, this can be written more concisely as _.starts_with(_).

Known problems: None.

Example:

name.chars().next() == Some('_')

clone_double_ref

Default level: Warn

What it does: Checks for usage of .clone() on an &&T.

Why is this bad? Cloning an &&T copies the inner &T, instead of cloning the underlying T.

Known problems: None.

Example:

fn main() {
   let x = vec![1];
   let y = &&x;
   let z = y.clone();
   println!("{:p} {:p}",*y, z); // prints out the same pointer
}

clone_on_copy

Default level: Warn

What it does: Checks for usage of .clone() on a Copy type.

Why is this bad? The only reason Copy types implement Clone is for generics, not for using the clone method on a concrete type.

Known problems: None.

Example:

42u64.clone()

cmp_nan

Default level: Deny

What it does: Checks for comparisons to NaN.

Why is this bad? NaN does not compare meaningfully to anything – not even itself – so those comparisons are simply wrong.

Known problems: None.

Example:

x == NAN

cmp_null

Default level: Warn

What it does: This lint checks for equality comparisons with ptr::null

Why is this bad? It's easier and more readable to use the inherent .is_null() method instead

Known problems: None.

Example:

if x == ptr::null { .. }

cmp_owned

Default level: Warn

What it does: Checks for conversions to owned values just for the sake of a comparison.

Why is this bad? The comparison can operate on a reference, so creating an owned value effectively throws it away directly afterwards, which is needlessly consuming code and heap space.

Known problems: None.

Example:

x.to_owned() == y

collapsible_if

Default level: Warn

What it does: Checks for nested if statements which can be collapsed by &&-combining their conditions and for else { if ... } expressions that can be collapsed to else if ....

Why is this bad? Each if-statement adds one level of nesting, which makes code look more complex than it really is.

Known problems: None.

Example:

if x {
    if y {
        …
    }
}

// or

if x {
    …
} else {
    if y {
        …
    }
}

Should be written:

if x && y {
    …
}

// or

if x {
    …
} else if y {
    …
}

crosspointer_transmute

Default level: Warn

What it does: Checks for transmutes between a type T and *T.

Why is this bad? It's easy to mistakenly transmute between a type and a pointer to that type.

Known problems: None.

Example:

core::intrinsics::transmute(t)` // where the result type is the same as `*t` or `&t`'s

cyclomatic_complexity

Default level: Warn

What it does: Checks for methods with high cyclomatic complexity.

Why is this bad? Methods of high cyclomatic complexity tend to be badly readable. Also LLVM will usually optimize small methods better.

Known problems: Sometimes it's hard to find a way to reduce the complexity.

Example: No. You'll see it when you get the warning.

Configuration: This lint has the following configuration variables:

• cyclomatic-complexity-threshold: u64: The maximum cyclomatic complexity a function can have (defaults to 25).

deprecated_semver

Default level: Warn

What it does: Checks for #[deprecated] annotations with a since field that is not a valid semantic version.

Why is this bad? For checking the version of the deprecation, it must be a valid semver. Failing that, the contained information is useless.

Known problems: None.

Example:

#[deprecated(since = "forever")]
fn something_else(..) { ... }

deref_addrof

Default level: Warn

What it does: Checks for usage of *& and *&mut in expressions.

Why is this bad? Immediately dereferencing a reference is no-op and makes the code less clear.

Known problems: Multiple dereference/addrof pairs are not handled so the suggested fix for x = **&&y is x = *&y, which is still incorrect.

Example:

let a = f(*&mut b);
let c = *&d;

derive_hash_xor_eq

Default level: Warn

What it does: Checks for deriving Hash but implementing PartialEq explicitly.

Why is this bad? The implementation of these traits must agree (for example for use with HashMap) so it’s probably a bad idea to use a default-generated Hash implementation with an explicitly defined PartialEq. In particular, the following must hold for any type:

k1 == k2 ⇒ hash(k1) == hash(k2)

Known problems: None.

Example:

#[derive(Hash)]
struct Foo;

impl PartialEq for Foo {
    ...
}

diverging_sub_expression

Default level: Warn

What it does: Checks for diverging calls that are not match arms or statements.

Why is this bad? It is often confusing to read. In addition, the sub-expression evaluation order for Rust is not well documented.

Known problems: Someone might want to use some_bool || panic!() as a shorthand.

Example:

let a = b() || panic!() || c();
// `c()` is dead, `panic!()` is only called if `b()` returns `false`
let x = (a, b, c, panic!());
// can simply be replaced by `panic!()`

doc_markdown

Default level: Warn

What it does: Checks for the presence of _, :: or camel-case words outside ticks in documentation.

Why is this bad? Rustdoc supports markdown formatting, _, :: and camel-case probably indicates some code which should be included between ticks. _ can also be used for empasis in markdown, this lint tries to consider that.

Known problems: Lots of bad docs won’t be fixed, what the lint checks for is limited, and there are still false positives.

Examples:

/// Do something with the foo_bar parameter. See also that::other::module::foo.
// ^ `foo_bar` and `that::other::module::foo` should be ticked.
fn doit(foo_bar) { .. }

Configuration: This lint has the following configuration variables:

• doc-valid-idents: Vec<String>: The list of words this lint should not consider as identifiers needing ticks (defaults to [ "MiB", "GiB", "TiB", "PiB", "EiB", "DirectX", "GPLv2", "GPLv3", "GitHub", "IPv4", "IPv6", "JavaScript", "NaN", "OAuth", "OpenGL", "TrueType", "iOS", "macOS", ]).

double_neg

Default level: Warn

What it does: Detects expressions of the form --x.

Why is this bad? It can mislead C/C++ programmers to think x was decremented.

Known problems: None.

Example:

--x;

double_parens

Default level: Warn

What it does: Checks for unnecessary double parentheses.

Why is this bad? This makes code harder to read and might indicate a mistake.

Known problems: None.

Example:

((0))
foo((0))
((1, 2))

drop_ref

Default level: Warn

What it does: Checks for calls to std::mem::drop with a reference instead of an owned value.

Why is this bad? Calling drop on a reference will only drop the reference itself, which is a no-op. It will not call the drop method (from the Drop trait implementation) on the underlying referenced value, which is likely what was intended.

Known problems: None.

Example:

let mut lock_guard = mutex.lock();
std::mem::drop(&lock_guard) // Should have been drop(lock_guard), mutex still locked
operation_that_requires_mutex_to_be_unlocked();

duplicate_underscore_argument

Default level: Warn

What it does: Checks for function arguments having the similar names differing by an underscore.

Why is this bad? It affects code readability.

Known problems: None.

Example:

fn foo(a: i32, _a: i32) {}

empty_loop

Default level: Warn

What it does: Checks for empty loop expressions.

Why is this bad? Those busy loops burn CPU cycles without doing anything. Think of the environment and either block on something or at least make the thread sleep for some microseconds.

Known problems: None.

Example:

loop {}

enum_clike_unportable_variant

Default level: Warn

What it does: Checks for C-like enumerations that are repr(isize/usize) and have values that don't fit into an i32.

Why is this bad? This will truncate the variant value on 32 bit architectures, but works fine on 64 bit.

Known problems: None.

Example:

#[repr(usize)]
enum NonPortable {
    X = 0x1_0000_0000,
    Y = 0
}

enum_glob_use

Default level: Allow

What it does: Checks for use Enum::*.

Why is this bad? It is usually better style to use the prefixed name of an enumeration variant, rather than importing variants.

Known problems: Old-style enumerations that prefix the variants are still around.

Example:

use std::cmp::Ordering::*;

enum_variant_names

Default level: Warn

What it does: Detects enumeration variants that are prefixed or suffixed by the same characters.

Why is this bad? Enumeration variant names should specify their variant, not repeat the enumeration name.

Known problems: None.

Example:

enum Cake {
    BlackForestCake,
    HummingbirdCake,
}

Configuration: This lint has the following configuration variables:

• enum-variant-name-threshold: u64: The minimum number of enum variants for the lints about variant names to trigger (defaults to 3).

eq_op

Default level: Warn

What it does: Checks for equal operands to comparison, logical and bitwise, difference and division binary operators (==, >, etc., &&, ||, &, |, ^, - and /).

Why is this bad? This is usually just a typo or a copy and paste error.

Known problems: False negatives: We had some false positives regarding calls (notably racer had one instance of x.pop() && x.pop()), so we removed matching any function or method calls. We may introduce a whitelist of known pure functions in the future.

Example:

x + 1 == x + 1

eval_order_dependence

Default level: Warn

What it does: Checks for a read and a write to the same variable where whether the read occurs before or after the write depends on the evaluation order of sub-expressions.

Why is this bad? It is often confusing to read. In addition, the sub-expression evaluation order for Rust is not well documented.

Known problems: Code which intentionally depends on the evaluation order, or which is correct for any evaluation order.

Example:

let mut x = 0;
let a = {x = 1; 1} + x;
// Unclear whether a is 1 or 2.

expl_impl_clone_on_copy

Default level: Warn

What it does: Checks for explicit Clone implementations for Copy types.

Why is this bad? To avoid surprising behaviour, these traits should agree and the behaviour of Copy cannot be overridden. In almost all situations a Copy type should have a Clone implementation that does nothing more than copy the object, which is what #[derive(Copy, Clone)] gets you.

Known problems: None.

Example:

#[derive(Copy)]
struct Foo;

impl Clone for Foo {
    ..
}

explicit_counter_loop

Default level: Warn

What it does: Checks for loops over slices with an explicit counter and suggests the use of .enumerate().

Why is it bad? Not only is the version using .enumerate() more readable, the compiler is able to remove bounds checks which can lead to faster code in some instances.

Known problems: None.

Example:

for i in 0..v.len() { foo(v[i]);
for i in 0..v.len() { bar(i, v[i]); }

explicit_into_iter_loop

Default level: Warn

What it does: Checks for loops on y.into_iter() where y will do, and suggests the latter.

Why is this bad? Readability.

Known problems: None

Example:

// with `y` a `Vec` or slice:
for x in y.into_iter() { .. }

explicit_iter_loop

Default level: Warn

What it does: Checks for loops on x.iter() where &x will do, and suggests the latter.

Why is this bad? Readability.

Known problems: False negatives. We currently only warn on some known types.

Example:

// with `y` a `Vec` or slice:
for x in y.iter() { .. }

extend_from_slice

Default level: Deprecated

What it does: Nothing. This lint has been deprecated.

Deprecation reason: This used to check for Vec::extend, which was slower than Vec::extend_from_slice. Thanks to specialization, this is no longer true.

filter_map

Default level: Allow

What it does: Checks for usage of _.filter(_).map(_), _.filter(_).flat_map(_), _.filter_map(_).flat_map(_) and similar.

Why is this bad? Readability, this can be written more concisely as a single method call.

Known problems: Often requires a condition + Option/Iterator creation inside the closure.

Example:

iter.filter(|x| x == 0).map(|x| x * 2)

filter_next

Default level: Warn

What it does: Checks for usage of _.filter(_).next().

Why is this bad? Readability, this can be written more concisely as _.find(_).

Known problems: None.

Example:

iter.filter(|x| x == 0).next()

float_arithmetic

Default level: Allow

What it does: Checks for float arithmetic.

Why is this bad? For some embedded systems or kernel development, it can be useful to rule out floating-point numbers.

Known problems: None.

Example:

a + 1.0

float_cmp

Default level: Warn

What it does: Checks for (in-)equality comparisons on floating-point values (apart from zero), except in functions called *eq* (which probably implement equality for a type involving floats).

Why is this bad? Floating point calculations are usually imprecise, so asking if two values are exactly equal is asking for trouble. For a good guide on what to do, see the floating point guide.

Known problems: None.

Example:

y == 1.23f64
y != x  // where both are floats

for_kv_map

Default level: Warn

What it does: Checks for iterating a map (HashMap or BTreeMap) and ignoring either the keys or values.

Why is this bad? Readability. There are keys and values methods that can be used to express that don't need the values or keys.

Known problems: None.

Example:

for (k, _) in &map { .. }

could be replaced by

for k in map.keys() { .. }

for_loop_over_option

Default level: Warn

What it does: Checks for for loops over Option values.

Why is this bad? Readability. This is more clearly expressed as an if let.

Known problems: None.

Example:

for x in option { .. }

This should be

if let Some(x) = option { .. }

for_loop_over_result

Default level: Warn

What it does: Checks for for loops over Result values.

Why is this bad? Readability. This is more clearly expressed as an if let.

Known problems: None.

Example:

for x in result { .. }

This should be

if let Ok(x) = result { .. }

forget_ref

Default level: Warn

What it does: Checks for calls to std::mem::forget with a reference instead of an owned value.

Why is this bad? Calling forget on a reference will only forget the reference itself, which is a no-op. It will not forget the underlying referenced value, which is likely what was intended.

Known problems: None.

Example:

let x = Box::new(1);
std::mem::forget(&x) // Should have been forget(x), x will still be dropped

get_unwrap

Default level: Warn

What it does: Checks for use of .get().unwrap() (or .get_mut().unwrap) on a standard library type which implements Index

Why is this bad? Using the Index trait ([]) is more clear and more concise.

Known problems: None.

Example:

let some_vec = vec![0, 1, 2, 3];
let last = some_vec.get(3).unwrap();
*some_vec.get_mut(0).unwrap() = 1;

The correct use would be:

let some_vec = vec![0, 1, 2, 3];
let last = some_vec[3];
some_vec[0] = 1;

identity_op

Default level: Warn

What it does: Checks for identity operations, e.g. x + 0.

Why is this bad? This code can be removed without changing the meaning. So it just obscures what's going on. Delete it mercilessly.

Known problems: None.

Example:

x / 1 + 0 * 1 - 0 | 0

if_let_redundant_pattern_matching

Default level: Warn

What it does:* Lint for redundant pattern matching over Result or Option

Why is this bad? It's more concise and clear to just use the proper utility function

Known problems: None.

Example:

if let Ok(_) = Ok::<i32, i32>(42) {}
if let Err(_) = Err::<i32, i32>(42) {}
if let None = None::<()> {}
if let Some(_) = Some(42) {}

The more idiomatic use would be:

if Ok::<i32, i32>(42).is_ok() {}
if Err::<i32, i32>(42).is_err() {}
if None::<()>.is_none() {}
if Some(42).is_some() {}

if_let_some_result

Default level: Warn

What it does:* Checks for unnecessary ok() in if let.

Why is this bad? Calling ok() in if let is unnecessary, instead match on Ok(pat)

Known problems: None.

Example:

for result in iter {
    if let Some(bench) = try!(result).parse().ok() {
        vec.push(bench)
    }
}

Could be written:

for result in iter {
    if let Ok(bench) = try!(result).parse() {
        vec.push(bench)
    }
}

if_not_else

Default level: Allow

What it does: Checks for usage of ! or != in an if condition with an else branch.

Why is this bad? Negations reduce the readability of statements.

Known problems: None.

Example:

if !v.is_empty() {
    a()
} else {
    b()
}

Could be written:

if v.is_empty() {
    b()
} else {
    a()
}

if_same_then_else

Default level: Warn

What it does: Checks for if/else with the same body as the then part and the else part.

Why is this bad? This is probably a copy & paste error.

Known problems: Hopefully none.

Example:

let foo = if … {
    42
} else {
    42
};

ifs_same_cond

Default level: Warn

What it does: Checks for consecutive ifs with the same condition.

Why is this bad? This is probably a copy & paste error.

Known problems: Hopefully none.

Example:

if a == b {
    …
} else if a == b {
    …
}

Note that this lint ignores all conditions with a function call as it could have side effects:

if foo() {
    …
} else if foo() { // not linted
    …
}

indexing_slicing

Default level: Allow

What it does: Checks for usage of indexing or slicing.

Why is this bad? Usually, this can be safely allowed. However, in some domains such as kernel development, a panic can cause the whole operating system to crash.

Known problems: Hopefully none.

Example:

...
x[2];
&x[0..2];

ineffective_bit_mask

Default level: Warn

What it does: Checks for bit masks in comparisons which can be removed without changing the outcome. The basic structure can be seen in the following table:

Comparison	Bit Op	Example	equals
> / <=	`	/^`	`x
< / >=	`	/^`	x ^ 1 < 4

Why is this bad? Not equally evil as bad_bit_mask, but still a bit misleading, because the bit mask is ineffective.

Known problems: False negatives: This lint will only match instances where we have figured out the math (which is for a power-of-two compared value). This means things like x | 1 >= 7 (which would be better written as x >= 6) will not be reported (but bit masks like this are fairly uncommon).

Example:

if (x | 1 > 3) { … }

inline_always

Default level: Warn

What it does: Checks for items annotated with #[inline(always)], unless the annotated function is empty or simply panics.

Why is this bad? While there are valid uses of this annotation (and once you know when to use it, by all means allow this lint), it's a common newbie-mistake to pepper one's code with it.

As a rule of thumb, before slapping #[inline(always)] on a function, measure if that additional function call really affects your runtime profile sufficiently to make up for the increase in compile time.

Known problems: False positives, big time. This lint is meant to be deactivated by everyone doing serious performance work. This means having done the measurement.

Example:

#[inline(always)]
fn not_quite_hot_code(..) { ... }

integer_arithmetic

Default level: Allow

What it does: Checks for plain integer arithmetic.

Why is this bad? This is only checked against overflow in debug builds. In some applications one wants explicitly checked, wrapping or saturating arithmetic.

Known problems: None.

Example:

a + 1

invalid_regex

Default level: Deny

What it does: Checks regex creation (with Regex::new, RegexBuilder::new or RegexSet::new) for correct regex syntax.

Why is this bad? This will lead to a runtime panic.

Known problems: None.

Example:

Regex::new("|")

invalid_upcast_comparisons

Default level: Allow

What it does: Checks for comparisons where the relation is always either true or false, but where one side has been upcast so that the comparison is necessary. Only integer types are checked.

Why is this bad? An expression like let x : u8 = ...; (x as u32) > 300 will mistakenly imply that it is possible for x to be outside the range of u8.

Known problems: https://github.com/Manishearth/rust-clippy/issues/886

Example:

let x : u8 = ...; (x as u32) > 300

items_after_statements

Default level: Allow

What it does: Checks for items declared after some statement in a block.

Why is this bad? Items live for the entire scope they are declared in. But statements are processed in order. This might cause confusion as it's hard to figure out which item is meant in a statement.

Known problems: None.

Example:

fn foo() {
    println!("cake");
}

fn main() {
    foo(); // prints "foo"
    fn foo() {
        println!("foo");
    }
    foo(); // prints "foo"
}

iter_next_loop

Default level: Warn

What it does: Checks for loops on x.next().

Why is this bad? next() returns either Some(value) if there was a value, or None otherwise. The insidious thing is that Option<_> implements IntoIterator, so that possibly one value will be iterated, leading to some hard to find bugs. No one will want to write such code except to win an Underhanded Rust Contest.

Known problems: None.

Example:

for x in y.next() { .. }

iter_nth

Default level: Warn

What it does: Checks for use of .iter().nth() (and the related .iter_mut().nth()) on standard library types with O(1) element access.

Why is this bad? .get() and .get_mut() are more efficient and more readable.

Known problems: None.

Example:

let some_vec = vec![0, 1, 2, 3];
let bad_vec = some_vec.iter().nth(3);
let bad_slice = &some_vec[..].iter().nth(3);

The correct use would be:

let some_vec = vec![0, 1, 2, 3];
let bad_vec = some_vec.get(3);
let bad_slice = &some_vec[..].get(3);

iter_skip_next

Default level: Warn

What it does: Checks for use of .skip(x).next() on iterators.

Why is this bad? .nth(x) is cleaner

Known problems: None.

Example:

let some_vec = vec![0, 1, 2, 3];
let bad_vec = some_vec.iter().skip(3).next();
let bad_slice = &some_vec[..].iter().skip(3).next();

The correct use would be:

let some_vec = vec![0, 1, 2, 3];
let bad_vec = some_vec.iter().nth(3);
let bad_slice = &some_vec[..].iter().nth(3);

large_enum_variant

Default level: Warn

What it does: Checks for large variants on enums.

Why is this bad? Enum size is bounded by the largest variant. Having a large variant can penalize the memory layout of that enum.

Known problems: None.

Example:

enum Test {
   A(i32),
   B([i32; 8000]),
}

Configuration: This lint has the following configuration variables:

• enum-variant-size-threshold: u64: The maximum size of a emum's variant to avoid box suggestion (defaults to 200).

len_without_is_empty

Default level: Warn

What it does: Checks for items that implement .len() but not .is_empty().

Why is this bad? It is good custom to have both methods, because for some data structures, asking about the length will be a costly operation, whereas .is_empty() can usually answer in constant time. Also it used to lead to false positives on the len_zero lint – currently that lint will ignore such entities.

Known problems: None.

Example:

impl X {
    pub fn len(&self) -> usize { .. }
}

len_zero

Default level: Warn

What it does: Checks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable.

Why is this bad? Some structures can answer .is_empty() much faster than calculating their length. So it is good to get into the habit of using .is_empty(), and having it is cheap. Besides, it makes the intent clearer than a comparison.

Known problems: None.

Example:

if x.len() == 0 { .. }

let_and_return

Default level: Warn

What it does: Checks for let-bindings, which are subsequently returned.

Why is this bad? It is just extraneous code. Remove it to make your code more rusty.

Known problems: None.

Example:

{ let x = ..; x }

let_unit_value

Default level: Warn

What it does: Checks for binding a unit value.

Why is this bad? A unit value cannot usefully be used anywhere. So binding one is kind of pointless.

Known problems: None.

Example:

let x = { 1; };

linkedlist

Default level: Warn

What it does: Checks for usage of any LinkedList, suggesting to use a Vec or a VecDeque (formerly called RingBuf).

Why is this bad? Gankro says:

The TL;DR of LinkedList is that it's built on a massive amount of pointers and indirection. It wastes memory, it has terrible cache locality, and is all-around slow. RingBuf, while "only" amortized for push/pop, should be faster in the general case for almost every possible workload, and isn't even amortized at all if you can predict the capacity you need.

LinkedLists are only really good if you're doing a lot of merging or splitting of lists. This is because they can just mangle some pointers instead of actually copying the data. Even if you're doing a lot of insertion in the middle of the list, RingBuf can still be better because of how expensive it is to seek to the middle of a LinkedList.

Known problems: False positives – the instances where using a LinkedList makes sense are few and far between, but they can still happen.

Example:

let x = LinkedList::new();

logic_bug

Default level: Warn

What it does: Checks for boolean expressions that contain terminals that can be eliminated.

Why is this bad? This is most likely a logic bug.

Known problems: Ignores short circuiting behavior.

Example:

if a && b || a { ... }

The b is unnecessary, the expression is equivalent to if a.

manual_swap

Default level: Warn

What it does: Checks for manual swapping.

Why is this bad? The std::mem::swap function exposes the intent better without deinitializing or copying either variable.

Known problems: None.

Example:

let t = b;
b = a;
a = t;

many_single_char_names

Default level: Warn

What it does: Checks for too many variables whose name consists of a single character.

Why is this bad? It's hard to memorize what a variable means without a descriptive name.

Known problems: None?

Example:

let (a, b, c, d, e, f, g) = (...);

Configuration: This lint has the following configuration variables:

• single-char-binding-names-threshold: u64: The maximum number of single char bindings a scope may have (defaults to 5).

map_clone

Default level: Warn

What it does: Checks for mapping clone() over an iterator.

Why is this bad? It makes the code less readable than using the .cloned() adapter.

Known problems: None.

Example:

x.map(|e| e.clone());

map_entry

Default level: Warn

What it does: Checks for uses of contains_key + insert on HashMap or BTreeMap.

Why is this bad? Using entry is more efficient.

Known problems: Some false negatives, eg.:

let k = &key;
if !m.contains_key(k) { m.insert(k.clone(), v); }

Example:

if !m.contains_key(&k) { m.insert(k, v) }

can be rewritten as:

m.entry(k).or_insert(v);

match_bool

Default level: Warn

What it does: Checks for matches where match expression is a bool. It suggests to replace the expression with an if...else block.

Why is this bad? It makes the code less readable.

Known problems: None.

Example:

let condition: bool = true;
match condition {
    true => foo(),
    false => bar(),
}

match_overlapping_arm

Default level: Warn

What it does: Checks for overlapping match arms.

Why is this bad? It is likely to be an error and if not, makes the code less obvious.

Known problems: None.

Example:

let x = 5;
match x {
    1 ... 10 => println!("1 ... 10"),
    5 ... 15 => println!("5 ... 15"),
    _ => (),
}

match_ref_pats

Default level: Warn

What it does: Checks for matches where all arms match a reference, suggesting to remove the reference and deref the matched expression instead. It also checks for if let &foo = bar blocks.

Why is this bad? It just makes the code less readable. That reference destructuring adds nothing to the code.

Known problems: None.

Example:

match x {
    &A(ref y) => foo(y),
    &B => bar(),
    _ => frob(&x),
}

match_same_arms

Default level: Warn

What it does: Checks for match with identical arm bodies.

Why is this bad? This is probably a copy & paste error. If arm bodies are the same on purpose, you can factor them using |.

Known problems: False positive possible with order dependent match (see issue #860).

Example:

match foo {
    Bar => bar(),
    Quz => quz(),
    Baz => bar(), // <= oops
}

This should probably be

match foo {
    Bar => bar(),
    Quz => quz(),
    Baz => baz(), // <= fixed
}

or if the original code was not a typo:

match foo {
    Bar | Baz => bar(), // <= shows the intent better
    Quz => quz(),
}

mem_forget

Default level: Allow

What it does: Checks for usage of std::mem::forget(t) where t is Drop.

Why is this bad? std::mem::forget(t) prevents t from running its destructor, possibly causing leaks.

Known problems: None.

Example:

mem::forget(Rc::new(55)))

min_max

Default level: Warn

What it does: Checks for expressions where std::cmp::min and max are used to clamp values, but switched so that the result is constant.

Why is this bad? This is in all probability not the intended outcome. At the least it hurts readability of the code.

Known problems: None

Example:

min(0, max(100, x))

It will always be equal to 0. Probably the author meant to clamp the value between 0 and 100, but has erroneously swapped min and max.

misrefactored_assign_op

Default level: Warn

What it does: Checks for a op= a op b or a op= b op a patterns.

Why is this bad? Most likely these are bugs where one meant to write a op= b.

Known problems: Someone might actually mean a op= a op b, but that should rather be written as a = (2 * a) op b where applicable.

Example:

let mut a = 5;
...
a += a + b;

missing_docs_in_private_items

Default level: Allow

What it does: Warns if there is missing doc for any documentable item (public or private).

Why is this bad? Doc is good. rustc has a MISSING_DOCS allowed-by-default lint for public members, but has no way to enforce documentation of private items. This lint fixes that.

Known problems: None.

mixed_case_hex_literals

Default level: Warn

What it does: Warns on hexadecimal literals with mixed-case letter digits.

Why is this bad? It looks confusing.

Known problems: None.

Example:

let y = 0x1a9BAcD;

module_inception

Default level: Warn

What it does: Checks for modules that have the same name as their parent module

Why is this bad? A typical beginner mistake is to have mod foo; and again mod foo { .. } in foo.rs. The expectation is that items inside the inner mod foo { .. } are then available through foo::x, but they are only available through foo::foo::x. If this is done on purpose, it would be better to choose a more representative module name.

Known problems: None.

Example:

// lib.rs
mod foo;
// foo.rs
mod foo {
    ...
}

modulo_one

Default level: Warn

What it does: Checks for getting the remainder of a division by one.

Why is this bad? The result can only ever be zero. No one will write such code deliberately, unless trying to win an Underhanded Rust Contest. Even for that contest, it's probably a bad idea. Use something more underhanded.

Known problems: None.

Example:

x % 1

mut_mut

Default level: Allow

What it does: Checks for instances of mut mut references.

Why is this bad? Multiple muts don't add anything meaningful to the source. This is either a copy'n'paste error, or it shows a fundamental misunderstanding of references.

Known problems: None.

Example:

let x = &mut &mut y;

mutex_atomic

Default level: Warn

What it does: Checks for usages of Mutex<X> where an atomic will do.

Why is this bad? Using a mutex just to make access to a plain bool or reference sequential is shooting flies with cannons. std::atomic::AtomicBool and std::atomic::AtomicPtr are leaner and faster.

Known problems: This lint cannot detect if the mutex is actually used for waiting before a critical section.

Example:

let x = Mutex::new(&y);

mutex_integer

Default level: Allow

What it does: Checks for usages of Mutex<X> where X is an integral type.

Why is this bad? Using a mutex just to make access to a plain integer sequential is shooting flies with cannons. std::atomic::usize is leaner and faster.

Known problems: This lint cannot detect if the mutex is actually used for waiting before a critical section.

Example:

let x = Mutex::new(0usize);

needless_bool

Default level: Warn

What it does: Checks for expressions of the form if c { true } else { false } (or vice versa) and suggest using the condition directly.

Why is this bad? Redundant code.

Known problems: Maybe false positives: Sometimes, the two branches are painstakingly documented (which we of course do not detect), so they may have some value. Even then, the documentation can be rewritten to match the shorter code.

Example:

if x { false } else { true }

needless_borrow

Default level: Warn

What it does: Checks for address of operations (&) that are going to be dereferenced immediately by the compiler.

Why is this bad? Suggests that the receiver of the expression borrows the expression.

Known problems: None.

Example:

let x: &i32 = &&&&&&5;

needless_lifetimes

Default level: Warn

What it does: Checks for lifetime annotations which can be removed by relying on lifetime elision.

Why is this bad? The additional lifetimes make the code look more complicated, while there is nothing out of the ordinary going on. Removing them leads to more readable code.

Known problems: Potential false negatives: we bail out if the function has a where clause where lifetimes are mentioned.

Example:

fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 { x }

needless_range_loop

Default level: Warn

What it does: Checks for looping over the range of 0..len of some collection just to get the values by index.

Why is this bad? Just iterating the collection itself makes the intent more clear and is probably faster.

Known problems: None.

Example:

for i in 0..vec.len() {
    println!("{}", vec[i]);
}

needless_return

Default level: Warn

What it does: Checks for return statements at the end of a block.

Why is this bad? Removing the return and semicolon will make the code more rusty.

Known problems: None.

Example:

fn foo(x: usize) { return x; }

needless_update

Default level: Warn

What it does: Checks for needlessly including a base struct on update when all fields are changed anyway.

Why is this bad? This will cost resources (because the base has to be somewhere), and make the code less readable.

Known problems: None.

Example:

Point { x: 1, y: 0, ..zero_point }

neg_multiply

Default level: Warn

What it does: Checks for multiplication by -1 as a form of negation.

Why is this bad? It's more readable to just negate.

Known problems: This only catches integers (for now).

Example:

x * -1

new_ret_no_self

Default level: Warn

What it does: Checks for new not returning Self.

Why is this bad? As a convention, new methods are used to make a new instance of a type.

Known problems: None.

Example:

impl Foo {
    fn new(..) -> NotAFoo {
    }
}

new_without_default

Default level: Warn

What it does: Checks for types with a fn new() -> Self method and no implementation of Default.

Why is this bad? The user might expect to be able to use Default as the type can be constructed without arguments.

Known problems: Hopefully none.

Example:

struct Foo(Bar);

impl Foo {
    fn new() -> Self {
        Foo(Bar::new())
    }
}

Instead, use:

struct Foo(Bar);

impl Default for Foo {
    fn default() -> Self {
        Foo(Bar::new())
    }
}

You can also have new() call Default::default().

new_without_default_derive

Default level: Warn

What it does: Checks for types with a fn new() -> Self method and no implementation of Default, where the Default can be derived by #[derive(Default)].

Why is this bad? The user might expect to be able to use Default as the type can be constructed without arguments.

Known problems: Hopefully none.

Example:

struct Foo;

impl Foo {
    fn new() -> Self {
        Foo
    }
}

Just prepend #[derive(Default)] before the struct definition.

no_effect

Default level: Warn

What it does: Checks for statements which have no effect.

Why is this bad? Similar to dead code, these statements are actually executed. However, as they have no effect, all they do is make the code less readable.

Known problems: None.

Example:

0;

non_ascii_literal

Default level: Allow

What it does: Checks for non-ASCII characters in string literals.

Why is this bad? Yeah, we know, the 90's called and wanted their charset back. Even so, there still are editors and other programs out there that don't work well with Unicode. So if the code is meant to be used internationally, on multiple operating systems, or has other portability requirements, activating this lint could be useful.

Known problems: None.

Example:

let x = "Hä?"

nonminimal_bool

Default level: Allow

What it does: Checks for boolean expressions that can be written more concisely.

Why is this bad? Readability of boolean expressions suffers from unnecessary duplication.

Known problems: Ignores short circuiting behavior of || and &&. Ignores |, & and ^.

Example:

if a && true  // should be: if a
if !(a == b)  // should be: if a != b

nonsensical_open_options

Default level: Warn

What it does: Checks for duplicate open options as well as combinations that make no sense.

Why is this bad? In the best case, the code will be harder to read than necessary. I don't know the worst case.

Known problems: None.

Example:

OpenOptions::new().read(true).truncate(true)

not_unsafe_ptr_arg_deref

Default level: Warn

What it does: Checks for public functions that dereferences raw pointer arguments but are not marked unsafe.

Why is this bad? The function should probably be marked unsafe, since for an arbitrary raw pointer, there is no way of telling for sure if it is valid.

Known problems:

• It does not check functions recursively so if the pointer is passed to a private non-unsafe function which does the dereferencing, the lint won't trigger.
• It only checks for arguments whose type are raw pointers, not raw pointers got from an argument in some other way (fn foo(bar: &[*const u8]) or some_argument.get_raw_ptr()).

Example:

pub fn foo(x: *const u8) { println!("{}", unsafe { *x }); }

ok_expect

Default level: Warn

What it does: Checks for usage of ok().expect(..).

Why is this bad? Because you usually call expect() on the Result directly to get a better error message.

Known problems: None.

Example:

x.ok().expect("why did I do this again?")

option_map_unwrap_or

Default level: Allow

What it does: Checks for usage of _.map(_).unwrap_or(_).

Why is this bad? Readability, this can be written more concisely as _.map_or(_, _).

Known problems: None.

Example:

x.map(|a| a + 1).unwrap_or(0)

option_map_unwrap_or_else

Default level: Allow

What it does: Checks for usage of _.map(_).unwrap_or_else(_).

Why is this bad? Readability, this can be written more concisely as _.map_or_else(_, _).

Known problems: None.

Example:

x.map(|a| a + 1).unwrap_or_else(some_function)

option_unwrap_used

Default level: Allow

What it does: Checks for .unwrap() calls on Options.

Why is this bad? Usually it is better to handle the None case, or to at least call .expect(_) with a more helpful message. Still, for a lot of quick-and-dirty code, unwrap is a good choice, which is why this lint is Allow by default.

Known problems: None.

Example:

x.unwrap()

or_fun_call

Default level: Warn

What it does: Checks for calls to .or(foo(..)), .unwrap_or(foo(..)), etc., and suggests to use or_else, unwrap_or_else, etc., or unwrap_or_default instead.

Why is this bad? The function will always be called and potentially allocate an object acting as the default.

Known problems: If the function has side-effects, not calling it will change the semantic of the program, but you shouldn't rely on that anyway.

Example:

foo.unwrap_or(String::new())

this can instead be written:

foo.unwrap_or_else(String::new)

or

foo.unwrap_or_default()

out_of_bounds_indexing

Default level: Deny

What it does: Checks for out of bounds array indexing with a constant index.

Why is this bad? This will always panic at runtime.

Known problems: Hopefully none.

Example:

let x = [1,2,3,4];
...
x[9];
&x[2..9];

overflow_check_conditional

Default level: Warn

panic_params

Default level: Warn

What it does: Checks for missing parameters in panic!.

Why is this bad? Contrary to the format! family of macros, there are two forms of panic!: if there are no parameters given, the first argument is not a format string and used literally. So while format!("{}") will fail to compile, panic!("{}") will not.

Known problems: Should you want to use curly brackets in panic! without any parameter, this lint will warn.

Example:

panic!("This `panic!` is probably missing a parameter there: {}");

partialeq_ne_impl

Default level: Warn

What it does: Checks for manual re-implementations of PartialEq::ne.

Why is this bad? PartialEq::ne is required to always return the negated result of PartialEq::eq, which is exactly what the default implementation does. Therefore, there should never be any need to re-implement it.

Known problems: None.

Example:

struct Foo;

impl PartialEq for Foo {
   fn eq(&self, other: &Foo) -> bool { ... }
   fn ne(&self, other: &Foo) -> bool { !(self == other) }
}

precedence

Default level: Warn

What it does: Checks for operations where precedence may be unclear and suggests to add parentheses. Currently it catches the following:

• mixed usage of arithmetic and bit shifting/combining operators without parentheses
• a "negative" numeric literal (which is really a unary - followed by a numeric literal) followed by a method call

Why is this bad? Not everyone knows the precedence of those operators by heart, so expressions like these may trip others trying to reason about the code.

Known problems: None.

Example:

• 1 << 2 + 3 equals 32, while (1 << 2) + 3 equals 7
• -1i32.abs() equals -1, while (-1i32).abs() equals 1

print_stdout

Default level: Allow

What it does: Checks for printing on stdout. The purpose of this lint is to catch debugging remnants.

Why is this bad? People often print on stdout while debugging an application and might forget to remove those prints afterward.

Known problems: Only catches print! and println! calls.

Example:

println!("Hello world!");

print_with_newline

Default level: Warn

What it does: This lint warns when you using print!() with a format string that ends in a newline.

Why is this bad? You should use println!() instead, which appends the newline.

Known problems: None.

Example:

print!("Hello {}!\n", name);

ptr_arg

Default level: Warn

What it does: This lint checks for function arguments of type &String or &Vec unless the references are mutable.

Why is this bad? Requiring the argument to be of the specific size makes the function less useful for no benefit; slices in the form of &[T] or &str usually suffice and can be obtained from other types, too.

Known problems: None.

Example:

fn foo(&Vec<u32>) { .. }

pub_enum_variant_names

Default level: Allow

What it does: Detects enumeration variants that are prefixed or suffixed by the same characters.

Why is this bad? Enumeration variant names should specify their variant, not repeat the enumeration name.

Known problems: None.

Example:

enum Cake {
    BlackForestCake,
    HummingbirdCake,
}

range_step_by_zero

Default level: Warn

What it does: Checks for iterating over ranges with a .step_by(0), which never terminates.

Why is this bad? This very much looks like an oversight, since with loop { .. } there is an obvious better way to endlessly loop.

Known problems: None.

Example:

for x in (5..5).step_by(0) { .. }

range_zip_with_len

Default level: Warn

What it does: Checks for zipping a collection with the range of 0.._.len().

Why is this bad? The code is better expressed with .enumerate().

Known problems: None.

Example:

x.iter().zip(0..x.len())

redundant_closure

Default level: Warn

What it does: Checks for closures which just call another function where the function can be called directly. unsafe functions or calls where types get adjusted are ignored.

Why is this bad? Needlessly creating a closure adds code for no benefit and gives the optimizer more work.

Known problems: None.

Example:

xs.map(|x| foo(x))

where foo(_) is a plain function that takes the exact argument type of x.

redundant_closure_call

Default level: Warn

What it does: Detects closures called in the same expression where they are defined.

Why is this bad? It is unnecessarily adding to the expression's complexity.

Known problems: None.

Example:

(|| 42)()

redundant_pattern

Default level: Warn

What it does: Checks for patterns in the form name @ _.

Why is this bad? It's almost always more readable to just use direct bindings.

Known problems: None.

Example:

match v {
    Some(x) => (),
    y @ _   => (), // easier written as `y`,
}

regex_macro

Default level: Warn

What it does: Checks for usage of regex!(_) which (as of now) is usually slower than Regex::new(_) unless called in a loop (which is a bad idea anyway).

Why is this bad? Performance, at least for now. The macro version is likely to catch up long-term, but for now the dynamic version is faster.

Known problems: None.

Example:

regex!("foo|bar")

result_unwrap_used

Default level: Allow

What it does: Checks for .unwrap() calls on Results.

Why is this bad? result.unwrap() will let the thread panic on Err values. Normally, you want to implement more sophisticated error handling, and propagate errors upwards with try!.

Even if you want to panic on errors, not all Errors implement good messages on display. Therefore it may be beneficial to look at the places where they may get displayed. Activate this lint to do just that.

Known problems: None.

Example:

x.unwrap()

reverse_range_loop

Default level: Warn

What it does: Checks for loops over ranges x..y where both x and y are constant and x is greater or equal to y, unless the range is reversed or has a negative .step_by(_).

Why is it bad? Such loops will either be skipped or loop until wrap-around (in debug code, this may panic!()). Both options are probably not intended.

Known problems: The lint cannot catch loops over dynamically defined ranges. Doing this would require simulating all possible inputs and code paths through the program, which would be complex and error-prone.

Example:

for x in 5..10-5 { .. } // oops, stray `-`

search_is_some

Default level: Warn

What it does: Checks for an iterator search (such as find(), position(), or rposition()) followed by a call to is_some().

Why is this bad? Readability, this can be written more concisely as _.any(_).

Known problems: None.

Example:

iter.find(|x| x == 0).is_some()

serde_api_misuse

Default level: Warn

What it does: Checks for mis-uses of the serde API.

Why is this bad? Serde is very finnicky about how its API should be used, but the type system can't be used to enforce it (yet).

Known problems: None.

Example: Implementing Visitor::visit_string but not Visitor::visit_str.

shadow_reuse

Default level: Allow

What it does: Checks for bindings that shadow other bindings already in scope, while reusing the original value.

Why is this bad? Not too much, in fact it's a common pattern in Rust code. Still, some argue that name shadowing like this hurts readability, because a value may be bound to different things depending on position in the code.

Known problems: This lint, as the other shadowing related lints, currently only catches very simple patterns.

Example:

let x = x + 1;

shadow_same

Default level: Allow

What it does: Checks for bindings that shadow other bindings already in scope, while just changing reference level or mutability.

Why is this bad? Not much, in fact it's a very common pattern in Rust code. Still, some may opt to avoid it in their code base, they can set this lint to Warn.

Known problems: This lint, as the other shadowing related lints, currently only catches very simple patterns.

Example:

let x = &x;

shadow_unrelated

Default level: Allow

What it does: Checks for bindings that shadow other bindings already in scope, either without a initialization or with one that does not even use the original value.

Why is this bad? Name shadowing can hurt readability, especially in large code bases, because it is easy to lose track of the active binding at any place in the code. This can be alleviated by either giving more specific names to bindings ore introducing more scopes to contain the bindings.

Known problems: This lint, as the other shadowing related lints, currently only catches very simple patterns.

Example:

let x = y; let x = z; // shadows the earlier binding

short_circuit_statement

Default level: Warn

What it does: Checks for the use of short circuit boolean conditions as a statement.

Why is this bad? Using a short circuit boolean condition as a statement may hide the fact that the second part is executed or not depending on the outcome of the first part.

Known problems: None.

Example:

f() && g();  // We should write `if f() { g(); }`.

should_implement_trait

Default level: Warn

What it does: Checks for methods that should live in a trait implementation of a std trait (see llogiq's blog post for further information) instead of an inherent implementation.

Why is this bad? Implementing the traits improve ergonomics for users of the code, often with very little cost. Also people seeing a mul(...) method may expect * to work equally, so you should have good reason to disappoint them.

Known problems: None.

Example:

struct X;
impl X {
   fn add(&self, other: &X) -> X { .. }
}

similar_names

Default level: Allow

What it does: Checks for names that are very similar and thus confusing.

Why is this bad? It's hard to distinguish between names that differ only by a single character.

Known problems: None?

Example:

let checked_exp = something;
let checked_expr = something_else;

single_char_pattern

Default level: Warn

What it does: Checks for string methods that receive a single-character str as an argument, e.g. _.split("x").

Why is this bad? Performing these methods using a char is faster than using a str.

Known problems: Does not catch multi-byte unicode characters.

Example: _.split("x") could be _.split('x')

single_match

Default level: Warn

What it does: Checks for matches with a single arm where an if let will usually suffice.

Why is this bad? Just readability – if let nests less than a match.

Known problems: None.

Example:

match x {
    Some(ref foo) => bar(foo),
    _ => ()
}

single_match_else

Default level: Allow

What it does: Checks for matches with a two arms where an if let will usually suffice.

Why is this bad? Just readability – if let nests less than a match.

Known problems: Personal style preferences may differ.

Example:

match x {
    Some(ref foo) => bar(foo),
    _ => bar(other_ref),
}

str_to_string

Default level: Deprecated

What it does: Nothing. This lint has been deprecated.

Deprecation reason: This used to check for .to_string() method calls on values of type &str. This is not unidiomatic and with specialization coming, to_string could be specialized to be as efficient as to_owned.

string_add

Default level: Allow

What it does: Checks for all instances of x + _ where x is of type String, but only if string_add_assign does not match.

Why is this bad? It's not bad in and of itself. However, this particular Add implementation is asymmetric (the other operand need not be String, but x does), while addition as mathematically defined is symmetric, also the String::push_str(_) function is a perfectly good replacement. Therefore some dislike it and wish not to have it in their code.

That said, other people think that string addition, having a long tradition in other languages is actually fine, which is why we decided to make this particular lint allow by default.

Known problems: None.

Example:

let x = "Hello".to_owned();
x + ", World"

string_add_assign

Default level: Allow

What it does: Checks for string appends of the form x = x + y (without let!).

Why is this bad? It's not really bad, but some people think that the .push_str(_) method is more readable.

Known problems: None.

Example:

let mut x = "Hello".to_owned();
x = x + ", World";

string_extend_chars

Default level: Warn

string_lit_as_bytes

Default level: Warn

What it does: Checks for the as_bytes method called on string literals that contain only ASCII characters.

Why is this bad? Byte string literals (e.g. b"foo") can be used instead. They are shorter but less discoverable than as_bytes().

Known Problems: None.

Example:

let bs = "a byte string".as_bytes();

string_to_string

Default level: Deprecated

What it does: Nothing. This lint has been deprecated.

Deprecation reason: This used to check for .to_string() method calls on values of type String. This is not unidiomatic and with specialization coming, to_string could be specialized to be as efficient as clone.

stutter

Default level: Allow

What it does: Detects type names that are prefixed or suffixed by the containing module's name.

Why is this bad? It requires the user to type the module name twice.

Known problems: None.

Example:

mod cake {
    struct BlackForestCake;
}

suspicious_assignment_formatting

Default level: Warn

What it does: Checks for use of the non-existent =*, =! and =- operators.

Why is this bad? This is either a typo of *=, != or -= or confusing.

Known problems: None.

Example:

a =- 42; // confusing, should it be `a -= 42` or `a = -42`?

suspicious_else_formatting

Default level: Warn

What it does: Checks for formatting of else if. It lints if the else and if are not on the same line or the else seems to be missing.

Why is this bad? This is probably some refactoring remnant, even if the code is correct, it might look confusing.

Known problems: None.

Example:

if foo {
} if bar { // looks like an `else` is missing here
}

if foo {
} else

if bar { // this is the `else` block of the previous `if`, but should it be?
}

temporary_assignment

Default level: Warn

What it does: Checks for construction of a structure or tuple just to assign a value in it.

Why is this bad? Readability. If the structure is only created to be updated, why not write the structure you want in the first place?

Known problems: None.

Example:

(0, 0).0 = 1

temporary_cstring_as_ptr

Default level: Warn

What it does: Checks for getting the inner pointer of a temporary CString.

Why is this bad? The inner pointer of a CString is only valid as long as the CString is alive.

Known problems: None.

Example:

let c_str = CString::new("foo").unwrap().as_ptr();
unsafe {
call_some_ffi_func(c_str);
}

Here c_str point to a freed address. The correct use would be:

let c_str = CString::new("foo").unwrap();
unsafe {
    call_some_ffi_func(c_str.as_ptr());
}

too_many_arguments

Default level: Warn

What it does: Checks for functions with too many parameters.

Why is this bad? Functions with lots of parameters are considered bad style and reduce readability (“what does the 5th parameter mean?”). Consider grouping some parameters into a new type.

Known problems: None.

Example:

fn foo(x: u32, y: u32, name: &str, c: Color, w: f32, h: f32, a: f32, b: f32) { .. }

Configuration: This lint has the following configuration variables:

• too-many-arguments-threshold: u64: The maximum number of argument a function or method can have (defaults to 7).

toplevel_ref_arg

Default level: Warn

What it does: Checks for function arguments and let bindings denoted as ref.

Why is this bad? The ref declaration makes the function take an owned value, but turns the argument into a reference (which means that the value is destroyed when exiting the function). This adds not much value: either take a reference type, or take an owned value and create references in the body.

For let bindings, let x = &foo; is preferred over let ref x = foo. The type of x is more obvious with the former.

Known problems: If the argument is dereferenced within the function, removing the ref will lead to errors. This can be fixed by removing the dereferences, e.g. changing *x to x within the function.

Example:

fn foo(ref x: u8) -> bool { .. }

transmute_ptr_to_ref

Default level: Warn

What it does: Checks for transmutes from a pointer to a reference.

Why is this bad? This can always be rewritten with & and *.

Known problems: None.

Example:

let _: &T = std::mem::transmute(p); // where p: *const T
// can be written:
let _: &T = &*p;

trivial_regex

Default level: Warn

What it does: Checks for trivial regex creation (with Regex::new, RegexBuilder::new or RegexSet::new).

Why is this bad? Matching the regex can likely be replaced by == or str::starts_with, str::ends_with or std::contains or other str methods.

Known problems: None.

Example:

Regex::new("^foobar")

type_complexity

Default level: Warn

What it does: Checks for types used in structs, parameters and let declarations above a certain complexity threshold.

Why is this bad? Too complex types make the code less readable. Consider using a type definition to simplify them.

Known problems: None.

Example:

struct Foo { inner: Rc<Vec<Vec<Box<(u32, u32, u32, u32)>>>> }

Configuration: This lint has the following configuration variables:

• type-complexity-threshold: u64: The maximum complexity a type can have (defaults to 250).

unicode_not_nfc

Default level: Allow

What it does: Checks for string literals that contain Unicode in a form that is not equal to its NFC-recomposition.

Why is this bad? If such a string is compared to another, the results may be surprising.

Known problems None.

Example: You may not see it, but “à” and “à” aren't the same string. The former when escaped is actually "a\u{300}" while the latter is "\u{e0}".

unit_cmp

Default level: Warn

What it does: Checks for comparisons to unit.

Why is this bad? Unit is always equal to itself, and thus is just a clumsily written constant. Mostly this happens when someone accidentally adds semicolons at the end of the operands.

Known problems: None.

Example:

if { foo(); } == { bar(); } { baz(); }

is equal to

{ foo(); bar(); baz(); }

unnecessary_mut_passed

Default level: Warn

What it does: Detects giving a mutable reference to a function that only requires an immutable reference.

Why is this bad? The immutable reference rules out all other references to the value. Also the code misleads about the intent of the call site.

Known problems: None.

Example:

my_vec.push(&mut value)

unnecessary_operation

Default level: Warn

What it does: Checks for expression statements that can be reduced to a sub-expression.

Why is this bad? Expressions by themselves often have no side-effects. Having such expressions reduces readability.

Known problems: None.

Example:

compute_array()[0];

unneeded_field_pattern

Default level: Warn

What it does: Checks for structure field patterns bound to wildcards.

Why is this bad? Using .. instead is shorter and leaves the focus on the fields that are actually bound.

Known problems: None.

Example:

let { a: _, b: ref b, c: _ } = ..

unsafe_removed_from_name

Default level: Warn

What it does: Checks for imports that remove "unsafe" from an item's name.

Why is this bad? Renaming makes it less clear which traits and structures are unsafe.

Known problems: None.

Example:

use std::cell::{UnsafeCell as TotallySafeCell};

extern crate crossbeam;
use crossbeam::{spawn_unsafe as spawn};

unseparated_literal_suffix

Default level: Allow

What it does: Warns if literal suffixes are not separated by an underscore.

Why is this bad? It is much less readable.

Known problems: None.

Example:

let y = 123832i32;

unstable_as_mut_slice

Default level: Deprecated

What it does: Nothing. This lint has been deprecated.

Deprecation reason: This used to check for Vec::as_mut_slice, which was unstable with good stable alternatives. Vec::as_mut_slice has now been stabilized.

unstable_as_slice

Default level: Deprecated

What it does: Nothing. This lint has been deprecated.

Deprecation reason: This used to check for Vec::as_slice, which was unstable with good stable alternatives. Vec::as_slice has now been stabilized.

unused_collect

Default level: Warn

What it does: Checks for using collect() on an iterator without using the result.

Why is this bad? It is more idiomatic to use a for loop over the iterator instead.

Known problems: None.

Example:

vec.iter().map(|x| /* some operation returning () */).collect::<Vec<_>>();

unused_io_amount

Default level: Deny

What it does: Checks for unused written/read amount.

Why is this bad? io::Write::write and io::Read::read are not guaranteed to process the entire buffer. They return how many bytes were processed, which might be smaller than a given buffer's length. If you don't need to deal with partial-write/read, use write_all/read_exact instead.

Known problems: Detects only common patterns.

Example:

use std::io;
fn foo<W: io::Write>(w: &mut W) -> io::Result<()> {
    // must be `w.write_all(b"foo")?;`
    w.write(b"foo")?;
    Ok(())
}

unused_label

Default level: Warn

What it does: Checks for unused labels.

Why is this bad? Maybe the label should be used in which case there is an error in the code or it should be removed.

Known problems: Hopefully none.

Example:

fn unused_label() {
    'label: for i in 1..2 {
        if i > 4 { continue }
    }

unused_lifetimes

Default level: Warn

What it does: Checks for lifetimes in generics that are never used anywhere else.

Why is this bad? The additional lifetimes make the code look more complicated, while there is nothing out of the ordinary going on. Removing them leads to more readable code.

Known problems: None.

Example:

fn unused_lifetime<'a>(x: u8) { .. }

use_debug

Default level: Allow

What it does: Checks for use of Debug formatting. The purpose of this lint is to catch debugging remnants.

Why is this bad? The purpose of the Debug trait is to facilitate debugging Rust code. It should not be used in in user-facing output.

Example:

println!("{:?}", foo);

used_underscore_binding

Default level: Allow

What it does: Checks for the use of bindings with a single leading underscore.

Why is this bad? A single leading underscore is usually used to indicate that a binding will not be used. Using such a binding breaks this expectation.

Known problems: The lint does not work properly with desugaring and macro, it has been allowed in the mean time.

Example:

let _x = 0;
let y = _x + 1; // Here we are using `_x`, even though it has a leading underscore.
                // We should rename `_x` to `x`

useless_attribute

Default level: Warn

What it does: Checks for extern crate and use items annotated with lint attributes

Why is this bad? Lint attributes have no effect on crate imports. Most likely a ! was forgotten

Known problems: Technically one might allow unused_import on a use item, but it's easier to remove the unused item.

Example:

#[deny(dead_code)]
extern crate foo;
#[allow(unused_import)]
use foo::bar;

useless_format

Default level: Warn

What it does: Checks for the use of format!("string literal with no argument") and format!("{}", foo) where foo is a string.

Why is this bad? There is no point of doing that. format!("too") can be replaced by "foo".to_owned() if you really need a String. The even worse &format!("foo") is often encountered in the wild. format!("{}", foo) can be replaced by foo.clone() if foo: String or foo.to_owned() if foo: &str.

Known problems: None.

Examples:

format!("foo")
format!("{}", foo)

useless_let_if_seq

Default level: Warn

What it does: Checks for variable declarations immediately followed by a conditional affectation.

Why is this bad? This is not idiomatic Rust.

Known problems: None.

Example:

let foo;

if bar() {
    foo = 42;
} else {
    foo = 0;
}

let mut baz = None;

if bar() {
    baz = Some(42);
}

should be written

let foo = if bar() {
    42
} else {
    0
};

let baz = if bar() {
    Some(42)
} else {
    None
};

useless_transmute

Default level: Warn

What it does: Checks for transmutes to the original type of the object and transmutes that could be a cast.

Why is this bad? Readability. The code tricks people into thinking that something complex is going on.

Known problems: None.

Example:

core::intrinsics::transmute(t) // where the result type is the same as `t`'s

useless_vec

Default level: Warn

What it does: Checks for usage of &vec![..] when using &[..] would be possible.

Why is this bad? This is less efficient.

Known problems: None.

Example:

foo(&vec![1, 2])

while_let_loop

Default level: Warn

What it does: Detects loop + match combinations that are easier written as a while let loop.

Why is this bad? The while let loop is usually shorter and more readable.

Known problems: Sometimes the wrong binding is displayed (#383).

Example:

loop {
    let x = match y {
        Some(x) => x,
        None => break,
    }
    // .. do something with x
}
// is easier written as
while let Some(x) = y {
    // .. do something with x
}

while_let_on_iterator

Default level: Warn

What it does: Checks for while let expressions on iterators.

Why is this bad? Readability. A simple for loop is shorter and conveys the intent better.

Known problems: None.

Example:

while let Some(val) = iter() { .. }

wrong_pub_self_convention

Default level: Allow

What it does: This is the same as wrong_self_convention, but for public items.

Why is this bad? See wrong_self_convention.

Known problems: Actually renaming the function may break clients if the function is part of the public interface. In that case, be mindful of the stability guarantees you've given your users.

Example:

impl X {
    pub fn as_str(self) -> &str { .. }
}

wrong_self_convention

Default level: Warn

What it does: Checks for methods with certain name prefixes and which doesn't match how self is taken. The actual rules are:

Prefix	self taken
as_	&self or &mut self
from_	none
into_	self
is_	&self or none
to_	&self

Why is this bad? Consistency breeds readability. If you follow the conventions, your users won't be surprised that they, e.g., need to supply a mutable reference to a as_.. function.

Known problems: None.

Example:

impl X {
    fn as_str(self) -> &str { .. }
}

wrong_transmute

Default level: Warn

What it does: Checks for transmutes that can't ever be correct on any architecture.

Why is this bad? It's basically guaranteed to be undefined behaviour.

Known problems: When accessing C, users might want to store pointer sized objects in extradata arguments to save an allocation.

Example:

let ptr: *const T = core::intrinsics::transmute('x')`

zero_divided_by_zero

Default level: Warn

What it does: Checks for 0.0 / 0.0.

Why is this bad? It's less readable than std::f32::NAN or std::f64::NAN.

Known problems: None.

Example:

0.0f32 / 0.0

zero_prefixed_literal

Default level: Warn

What it does: Warns if an integral constant literal starts with 0.

Why is this bad? In some languages (including the infamous C language and most of its familly), this marks an octal constant. In Rust however, this is a decimal constant. This could be confusing for both the writer and a reader of the constant.

Known problems: None.

Example:

In Rust:

fn main() {
    let a = 0123;
    println!("{}", a);
}

prints 123, while in C:

#include <stdio.h>

int main() {
    int a = 0123;
    printf("%d\n", a);
}

prints 83 (as 83 == 0o123 while 123 == 0o173).

zero_width_space

Default level: Deny

What it does: Checks for the Unicode zero-width space in the code.

Why is this bad? Having an invisible character in the code makes for all sorts of April fools, but otherwise is very much frowned upon.

Known problems: None.

Example: You don't see it, but there may be a zero-width space somewhere in this text.