Emit a future incompat lint when overwriting the function body

compiler/rustc_lint_defs/src/builtin.rs

@@ -43,6 +43,7 @@ declare_lint_pass! {
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
         ELIDED_LIFETIMES_IN_PATHS,
         EXPORTED_PRIVATE_DEPENDENCIES,
+        FEATURE_DETECTION,
         FFI_UNWIND_CALLS,
         FORBIDDEN_LINT_GROUPS,
         FUNCTION_ITEM_REFERENCES,
@@ -176,6 +177,34 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `feature_detection` lint detects flawed ways to determine
+    /// whether nightly features can be enabled.
+    ///
+    /// ### Example
+    ///
+    /// The `version_check` crate's `supports_feature` function.
+    ///
+    /// ### Recommended fix
+    ///
+    /// Remove dependencies that rely on `version_check`.
+    ///
+    /// ### Explanation
+    ///
+    /// Detecting if the current compiler is a nightly compiler, without checking if
+    /// the feature
+    /// will actually work for the user. This means that updates to the feature
+    /// will arbitrarily break nightly users that did not opt-in to using nightly
+    /// features.
+    pub FEATURE_DETECTION,
+    Warn,
+    "applying forbid to lint-groups",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::Custom("nightly feature detection is disabled"),
+        reference: "",
+    };
+}
+
 declare_lint! {
     /// The `ill_formed_attribute_input` lint detects ill-formed attribute
     /// inputs that were previously accepted and used in practice.

compiler/rustc_mir_build/messages.ftl

@@ -109,6 +109,9 @@ mir_build_extern_static_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
     .note = extern statics are not controlled by the Rust type system: invalid data, aliasing violations or data races will cause undefined behavior
     .label = use of extern static
 
+mir_build_feature_detection = Rust language and library feature detection via `version_check::feature_enabled` does not actually check whether the feature works correctly
+    .note = the fucntion has been disabled and will always return `None` on a nightly compiler.
+
 mir_build_indirect_structural_match =
     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq)]`
 

compiler/rustc_mir_build/src/build/mod.rs

@@ -1,5 +1,6 @@
 use crate::build::expr::as_place::PlaceBuilder;
 use crate::build::scope::DropKind;
+use crate::errors::FeatureDetection;
 use hir::def_id::LOCAL_CRATE;
 use itertools::Itertools;
 use rustc_apfloat::ieee::{Double, Half, Quad, Single};
@@ -21,6 +22,7 @@ use rustc_middle::mir::*;
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::thir::{self, ExprId, LintLevel, LocalVarId, Param, ParamId, PatKind, Thir};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
+use rustc_session::lint;
 use rustc_span::symbol::sym;
 use rustc_span::Symbol;
 use rustc_span::{Span, DUMMY_SP};
@@ -535,6 +537,14 @@ fn construct_fn<'tcx>(
             ),
         );
         builder.cfg.terminate(block, source_info, TerminatorKind::Return);
+
+        tcx.emit_node_span_lint(
+            lint::builtin::FEATURE_DETECTION,
+            fn_id,
+            span,
+            FeatureDetection { span },
+        );
+
         return builder.finish();
     }
 

compiler/rustc_mir_build/src/errors.rs

@@ -950,3 +950,10 @@ pub enum RustcBoxAttrReason {
     #[note(mir_build_missing_box)]
     MissingBox,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(mir_build_feature_detection)]
+pub struct FeatureDetection {
+    #[note]
+    pub span: Span,
+}

tests/ui/feature-gates/version_check.rs

@@ -1,6 +1,8 @@
 //@ run-pass
 
 fn supports_feature(_: &str) -> Option<bool> {
+    //~^ WARN:`version_check::feature_enabled` does not actually check
+    //~| WARN: nightly feature detection is disabled
     Some(true)
 }
 

tests/ui/feature-gates/version_check.stderr

@@ -0,0 +1,17 @@
+warning: Rust language and library feature detection via `version_check::feature_enabled` does not actually check whether the feature works correctly
+  --> $DIR/version_check.rs:3:1
+   |
+LL | fn supports_feature(_: &str) -> Option<bool> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: nightly feature detection is disabled
+   = note: for more information, see TODO
+note: the fucntion has been disabled and will always return `None` on a nightly compiler.
+  --> $DIR/version_check.rs:3:1
+   |
+LL | fn supports_feature(_: &str) -> Option<bool> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `#[warn(feature_detection)]` on by default
+
+warning: 1 warning emitted
+