feat(tyck): contextual typing for destructuring and tuple/array flows

[hongjr03]

Propagates contextual types through destructuring and tuple/array constraints, including field-access backprop. Adds tuple destructuring and include for-loop fixtures/snapshots.

Depends on #2385.

[gemini-code-assist]

Summary of Changes

Hello @hongjr03, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly advances the type checking capabilities by implementing contextual type propagation across various language constructs. It focuses on improving type inference for destructuring, tuple/array flows, and field access, which leads to more precise type suggestions and error detection. The changes also refine how paths are typed in import and include statements and enhance type resolution within for loops, making the type system more intelligent and helpful for developers.

Highlights

• Contextual Type Propagation: Enhanced the type checking system to propagate contextual types more effectively through destructuring assignments, tuple and array constraints, and field access expressions.
• Destructuring Type Inference: Introduced comprehensive support for type inference in destructuring patterns, including handling positional and named bindings, and spread operators.
• Improved Path Type Checking: Refined type inference for import and include expressions, ensuring that source paths are correctly constrained and checked.
• Dynamic Key Handling: Improved the evaluation of dynamic keys in dictionary literals and dict.at() calls, allowing constant string keys to be correctly identified and used for type inference.
• Enhanced For Loop Typing: Added contextual typing for for loops, enabling more accurate type inference for elements iterated over in arrays or tuples.
• New Test Fixtures: Added a suite of new completion and post-type-check fixtures to validate the correctness and robustness of the new contextual typing features.

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[gemini-code-assist]

Code Review

This pull request introduces significant enhancements to the type checking system, enabling contextual type propagation for destructuring assignments and flows involving tuples and arrays. Key changes include back-propagation of types for field access (e.g., through dict.at("key")), improved type inference for for loops, and constant folding for string concatenation. The changes are extensive and well-supported by new test cases.

I have identified a couple of areas for improvement. One is a potential bug in destructuring logic that could lead to loss of type information when spreads are used. The other is a suggestion to generalize the new for loop type inference to support more complex patterns beyond simple identifiers.

Overall, this is a great feature addition that will improve the accuracy of type analysis.

[gemini-code-assist]

This block of code incorrectly clears all inferred positional pattern types if a spread is followed by another positional pattern (e.g., in (a, ..rest, b)). This results in losing the type information for prefix patterns like a. According to the comment on line 443, the intention is to keep the prefix mapping, so this block should be removed to prevent discarding valid type information.

[gemini-code-assist]

The type constraint logic for for loops is currently limited to Pattern::Simple. This prevents type propagation for more complex patterns like destructuring (k, v), which is a common use case. The logic for constraining the iterable and its elements seems general enough to be applied to all pattern types, which would make this feature more powerful and consistent.

Consider removing the if matches!(...) condition to apply the type constraints for all for loop patterns.

        self.constrain(&iter, &Ty::Array(pattern.clone().into()));

        match &iter {
            Ty::Array(elem) => self.constrain(elem, &pattern),
            Ty::Tuple(elems) => {
                for elem in elems.iter() {
                    self.constrain(elem, &pattern);
                }
            }
            _ => {}
        }