feat(completion): path inference for concatenation and let init

[hongjr03]

Enables path completion for string concatenation and let-initialized strings, with a small completion filter refinement. Includes related fixtures/snapshots and minor clippy/fmt touchups.

Depends on #2386.

[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 enhances the path completion capabilities within the system. By introducing advanced type inference for string concatenations, let-initialized variables, destructuring patterns, and for loops, it allows the language server to provide more intelligent and context-aware file path suggestions. The changes also include refinements to how completion items are filtered and inserted, leading to a more precise and helpful completion experience for users.

Highlights

• Enhanced Path Completion: Introduced path completion for string concatenations (e.g., #include "dir/" + "file.typ") and for strings initialized via let bindings (e.g., #let p = "file.typ"; #include p). This significantly improves the developer experience when working with file paths.
• Improved Type Inference: The type checker now better infers types for destructuring assignments, elements within for loops, and dictionary keys, especially when they involve constant string values. This leads to more accurate and context-aware completions.
• Completion Filtering Refinement: A new mechanism has been implemented to filter and strip completion suggestions based on the constant prefix and suffix of concatenated strings, ensuring only relevant and correctly formatted paths are suggested.
• Structural Changes for Import Expressions: The ImportExpr struct now includes a source field, allowing the system to track the origin expression of an import, which is crucial for contextual path completion.
• Dynamic Dictionary Key Handling: The type checker can now treat dict.at("key") calls as direct field access (dict.key) when the key is a constant string, improving type resolution for dynamically accessed dictionary members.

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[gemini-code-assist]

Code Review

This pull request is a great enhancement, adding path completion for string concatenations and let-initialized strings. The changes are substantial and well-implemented, touching on various parts of the type checking and completion logic to enable this new feature. The improvements to type inference, especially for propagating path constraints and constant folding, are impressive. The new test fixtures and snapshots provide good coverage for the new functionality. The code quality is high, and I only have one minor suggestion for simplification.

[gemini-code-assist]

These checks for starts_with and ends_with are redundant. The subsequent strip_prefix and strip_suffix calls already handle these checks by returning None if the prefix/suffix doesn't match, which is then handled by the let ... else { continue; } construct. You can safely remove these initial checks to simplify the code.