fix: TypeId in ContextTag not stable across builds (#217)

* fix: use &str instead of TypeId in ContextTag

* chore: add warning to readme

* chore: fix comment

halo2-base/README.md

@@ -69,6 +69,8 @@ During `synthesize()`, the advice values of all `Context`s are concatenated into
 
 For parallel witness generation, multiple `Context`s are created for each parallel operation. After parallel witness generation, these `Context`'s are combined to form a single "virtual column" as above. Note that while the witness generation can be multi-threaded, the ordering of the contents in each `Context`, and the order of the `Context`s themselves, must be deterministic.
 
+**Warning:** If you create your own `Context` in a new virtual region not provided by our libraries, you must ensure that the `type_id: &str` of the context is a globally unique identifier for the virtual region, distinct from the other `type_id` strings used to identify other virtual regions. In the future we will introduce a macro to check this uniqueness at compile time.
+
 ### [**AssignedValue**](./src/lib.rs):
 
 Despite the name, an `AssignedValue` is a **virtual cell**. It contains the actual witness value as well as a pointer to the location of the virtual cell within a virtual region. The pointer is given by type `ContextCell`. We only store the pointer when not in witness generation only mode as an optimization.

halo2-base/src/gates/flex_gate/threads/single_phase.rs

@@ -1,4 +1,4 @@
-use std::{any::TypeId, cell::RefCell};
+use std::cell::RefCell;
 
 use getset::CopyGetters;
 
@@ -13,10 +13,7 @@ use crate::{
     Context, ContextCell,
 };
 use crate::{
-    halo2_proofs::{
-        circuit::{Region, Value},
-        plonk::{FirstPhase, SecondPhase, ThirdPhase},
-    },
+    halo2_proofs::circuit::{Region, Value},
     virtual_region::manager::VirtualRegionManager,
 };
 
@@ -114,11 +111,11 @@ impl<F: ScalarField> SinglePhaseCoreManager<F> {
     }
 
     /// A distinct tag for this particular type of virtual manager, which is different for each phase.
-    pub fn type_of(&self) -> TypeId {
+    pub fn type_of(&self) -> &'static str {
         match self.phase {
-            0 => TypeId::of::<(Self, FirstPhase)>(),
-            1 => TypeId::of::<(Self, SecondPhase)>(),
-            2 => TypeId::of::<(Self, ThirdPhase)>(),
+            0 => "SinglePhaseCoreManager: FirstPhase",
+            1 => "SinglePhaseCoreManager: SecondPhase",
+            2 => "SinglePhaseCoreManager: ThirdPhase",
             _ => panic!("Unsupported phase"),
         }
     }

halo2-base/src/lib.rs

@@ -9,8 +9,6 @@
 #![warn(clippy::default_numeric_fallback)]
 #![warn(missing_docs)]
 
-use std::any::TypeId;
-
 use getset::CopyGetters;
 use itertools::Itertools;
 // Different memory allocator options:
@@ -104,14 +102,16 @@ impl<F: ScalarField> QuantumCell<F> {
     }
 }
 
-/// Unique tag for a context across all virtual regions
-pub type ContextTag = (TypeId, usize);
+/// Unique tag for a context across all virtual regions.
+/// In the form `(type_id, context_id)` where `type_id` should be a unique identifier
+/// for the virtual region this context belongs to, and `context_id` is a counter local to that virtual region.
+pub type ContextTag = (&'static str, usize);
 
 /// Pointer to the position of a cell at `offset` in an advice column within a [Context] of `context_id`.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
 pub struct ContextCell {
-    /// The [TypeId] of the virtual region that this cell belongs to.
-    pub type_id: TypeId,
+    /// The unique string identifier of the virtual region that this cell belongs to.
+    pub type_id: &'static str,
     /// Identifier of the [Context] that this cell belongs to.
     pub context_id: usize,
     /// Relative offset of the cell within this [Context] advice column.
@@ -120,7 +120,7 @@ pub struct ContextCell {
 
 impl ContextCell {
     /// Creates a new [ContextCell] with the given `type_id`, `context_id`, and `offset`.
-    pub fn new(type_id: TypeId, context_id: usize, offset: usize) -> Self {
+    pub fn new(type_id: &'static str, context_id: usize, offset: usize) -> Self {
         Self { type_id, context_id, offset }
     }
 }
@@ -174,9 +174,11 @@ pub struct Context<F: ScalarField> {
     /// The challenge phase that this [Context] will map to.
     #[getset(get_copy = "pub")]
     phase: usize,
-    /// Identifier for what virtual region this context is in
+    /// Identifier for what virtual region this context is in.
+    /// Warning: the circuit writer must ensure that distinct virtual regions have distinct names as strings to prevent possible errors.
+    /// We do not use [std::any::TypeId] because it is not stable across rust builds or dependencies.
     #[getset(get_copy = "pub")]
-    type_id: TypeId,
+    type_id: &'static str,
     /// Identifier to reference cells from this [Context].
     context_id: usize,
 
@@ -204,7 +206,7 @@ impl<F: ScalarField> Context<F> {
     pub fn new(
         witness_gen_only: bool,
         phase: usize,
-        type_id: TypeId,
+        type_id: &'static str,
         context_id: usize,
         copy_manager: SharedCopyConstraintManager<F>,
     ) -> Self {

halo2-base/src/virtual_region/copy_constraints.rs

@@ -1,4 +1,3 @@
-use std::any::TypeId;
 use std::collections::{BTreeMap, HashMap};
 use std::ops::DerefMut;
 use std::sync::{Arc, Mutex, OnceLock};
@@ -87,7 +86,7 @@ impl<F: Field + Ord> CopyConstraintManager<F> {
     }
 
     fn load_external_cell_impl(&mut self, cell: Option<Cell>) -> ContextCell {
-        let context_cell = ContextCell::new(TypeId::of::<Cell>(), 0, self.external_cell_count);
+        let context_cell = ContextCell::new("External Raw Halo2 Cell", 0, self.external_cell_count);
         self.external_cell_count += 1;
         if let Some(cell) = cell {
             self.assigned_advices.insert(context_cell, cell);