bugfix(return-opt): Fixed issue with identical variants on return building. - #10056

Summary

Replaces the (TypeId, Vec<VariableId>) tuple key used in EarlyReturnContext::constructed with a dedicated Construction enum that has distinct Struct and Enum variants. The Enum variant stores a semantic::ConcreteVariant and a single VariableId, rather than a TypeId and a Vec<VariableId>, ensuring that different enum variants with the same input variable cannot collide in the memoization map.

A regression test is added covering the case where two different variants of the same enum (MyEnum::A(x) and MyEnum::B(x)) are constructed from the same input variable, verifying they produce distinct output variables.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

The previous key (TypeId, Vec<VariableId>) for enum constructions used the enum's type ID and the input variable wrapped in a Vec. Because two different variants of the same enum share the same TypeId, constructing MyEnum::A(x) and MyEnum::B(x) would produce identical keys, causing the second construction to incorrectly reuse the variable allocated for the first. This led to wrong lowered IR when both variants appeared in the same early-return path.

What was the behavior or documentation before?

Constructing two different variants of the same enum from the same input variable during return optimization would collide in the constructed map, causing the second enum construct statement to reuse the output variable of the first variant instead of allocating a new one.

What is the behavior or documentation after?

Each enum construction is keyed by its ConcreteVariant (which encodes the specific variant, not just the enum type) and its input VariableId. Struct constructions continue to be keyed by TypeId and the full list of input VariableIds. The two cases are now structurally separate and cannot collide.

Related issue or discussion (if any)

None.

Additional context

The new Construction enum derives Debug, Clone, PartialEq, Eq, and Hash, making it a well-typed, self-documenting replacement for the ad-hoc tuple key.