fix: Do not use PostAnalysis TypingMode for IDE method resolution

[ChayimFriedman2]

As explained in the comments, PostAnalysis is good for most IDE things but not method resolution.

This fixes a bug which should not be impacted by this at all - return position impl trait in trait. It is currently lowered to an opaque, while it should be lowered to an anonymous associated type. But today when it is lowered as an opaque, this opaque of course has no definition and will normalize to an error, preventing method resolution on it from succeeding in some cases.

Fixes #21749.

[Copilot]

Pull request overview

This PR fixes a bug in IDE method resolution related to Return Position impl Trait in Traits (RPITIT). Previously, with_method_resolution always built an InferCtxt using TypingMode::PostAnalysis, which would attempt to reveal opaque types. However, RPITIT opaques currently lack a proper definition (they're lowered as opaque rather than as anonymous associated types), causing them to normalize to errors and preventing valid method suggestions.

Changes:

• In with_method_resolution, replace the hardcoded TypingMode::PostAnalysis with a context-appropriate typing mode: TypingMode::analysis_in_body when inside a function body, and TypingMode::non_body_analysis() otherwise.
• Add a regression test rpitit_with_reference verifying that method completion works correctly on &impl Foo returned from a trait method.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/hir/src/lib.rs	Replace TypingMode::PostAnalysis with a body-aware typing mode selection in with_method_resolution
crates/ide-completion/src/tests/expression.rs	Add regression test for RPITIT method completions via reference

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[ChayimFriedman2]

Sorry for the Copilot review, I was curious to see how much it will understand from the jargon. Looks like it understand nothing and just repeats what I said, as expected 😂