[ty] Add separate type for typevar "identity"

[dcreager]

As part of #20598, we added is_identical_to methods to TypeVarInstance and BoundTypeVarInstance, which compare when two typevar instances refer to "the same" underlying typevar, even if we have forced their lazy bounds/constraints as part of marking typevars as inferable. (Doing so results in a different salsa interned struct ID, since we've changed the contents of the bounds_or_constraints field.)

It turns out that marking typevars as inferable is not the only way that we might force lazy bounds/constraints; it also happens when we materialize a type containing a typevar. This surfaced as ecosystem report failures on #20677.

That means that we need a more long-term fix to this problem. (is_identical_to, and its underlying original field, were meant to be a temporary fix until we removed the MarkTypeVarsInferable type mapping.)

This PR extracts out a separate type (TypeVarIdentity) that only includes the fields that actually inform whether two typevars are "the same". All other properties of the typevar (default, bounds/constraints, etc) still live in TypeVarInstance. Call sites that care about typevar identity can now either store just TypeVarIdentity (if they never need access to those other properties), or continue to store TypeVarInstance but pull out its identity when performing those "are they the same typevar" comparisons. (All of this also applies respectively to BoundTypeVar{Identity,Instance}.) In particular, constraint sets now work on BoundTypeVarIdentity, and generic contexts still store a BoundTypeVarInstance (since we might need access to defaults when specializing), but are keyed on BoundTypeVarIdentity.

[github-actions]

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-10-13 23:49:17.110848042 +0000
+++ new-output.txt	2025-10-13 23:49:20.423865224 +0000
@@ -1,5 +1,5 @@
-fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_type_statement.py`: `PEP695TypeAliasType < 'db >::value_type_(Id(cc17)): execute: too many cycle iterations`
-fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_typealiastype.py`: `infer_definition_types(Id(1603f)): execute: too many cycle iterations`
+fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_type_statement.py`: `PEP695TypeAliasType < 'db >::value_type_(Id(d017)): execute: too many cycle iterations`
+fatal[panic] Panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/29ab321/src/function/execute.rs:217:25 when checking `/home/runner/work/ruff/ruff/typing/conformance/tests/aliases_typealiastype.py`: `infer_definition_types(Id(1643f)): execute: too many cycle iterations`
 _directives_deprecated_library.py:15:31: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `int`
 _directives_deprecated_library.py:30:26: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `str`
 _directives_deprecated_library.py:36:41: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `Self@__add__`

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

sympy (https://github.com/sympy/sympy)
- sympy/polys/polyclasses.py:1289:31: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- Found 11030 diagnostics
+ Found 11029 diagnostics

No memory usage changes detected ✅

[github-actions]

ecosystem-analyzer results

Lint rule	Added	Removed	Changed
unused-ignore-comment	0	1	0
Total	0	1	0

Full report with detailed diff (timing results)

[sharkdp]

This looks great, thank you!

It looks like we can now revert @carljm's change in signatures.rs and potentially reclaim some performance?

diff --git a/crates/ty_python_semantic/src/types/signatures.rs b/crates/ty_python_semantic/src/types/signatures.rs
index 039b89a6eb..108df8cb00 100644
--- a/crates/ty_python_semantic/src/types/signatures.rs
+++ b/crates/ty_python_semantic/src/types/signatures.rs
@@ -1292,19 +1292,8 @@ impl<'db> Parameters<'db> {
                     let class = nearest_enclosing_class(db, index, scope_id).unwrap();
 
                     Some(
-                        // It looks like unnecessary work here that we create the implicit Self
-                        // annotation using non-inferable typevars and then immediately apply
-                        // `MarkTypeVarsInferable` to it. However, this is currently necessary to
-                        // ensure that implicit-Self and explicit Self annotations are both treated
-                        // the same. Marking type vars inferable will cause reification of lazy
-                        // typevar defaults/bounds/constraints; this needs to happen for both
-                        // implicit and explicit Self so they remain the "same" typevar.
-                        typing_self(db, scope_id, typevar_binding_context, class, &Type::NonInferableTypeVar)
-                            .expect("We should always find the surrounding class for an implicit self: Self annotation").apply_type_mapping(
-                                db,
-                                &TypeMapping::MarkTypeVarsInferable(None),
-                                TypeContext::default()
-                            )
+                        typing_self(db, scope_id, typevar_binding_context, class, &Type::TypeVar)
+                            .expect("We should always find the surrounding class for an implicit self: Self annotation")
                     )
                 } else {
                     // For methods of non-generic classes that are not otherwise generic (e.g. return `Self` or

[dcreager]

It looks like we can now revert @carljm's change in signatures.rs and potentially reclaim some performance?

I currently have that being removed in #20677, where we remove the separate enum variants for inferable vs non-inferable. But I can try it here too and see if everything still passes.

[dcreager]

But I can try it here too and see if everything still passes.

Removing this here added back a no-matching-overload ecosystem diagnostic that had been removed. I'm going to keep this removal on #20677, where I think it will be fully redundant.