refactor: shrink check_attr by moving attribute checks to parsers

[herbenderbler]

refactor: shrink check_attr by moving attribute checks into rustc_attr_parsing

Part of #153101.

Problem

compiler/rustc_passes/src/check_attr.rs accumulates a large amount of attribute validation logic. Much of that logic is really about what an attribute means on a particular syntactic target, but it historically lived next to diagnostic emission in the pass, which makes the attribute pipeline harder to follow and duplicates concepts that already live in rustc_attr_parsing.

Issue #153101 tracks shrinking check_attr and moving checks as far toward parsing as information allows. Many rules cannot run during raw attribute parsing because they need HIR / target context (e.g. “is this a trait impl?”, “is this expression a loop?”). This PR takes a concrete step in that direction without pulling rustc_middle into the attribute parser crate.

What we implemented

1. late_validation (compiler/rustc_attr_parsing/src/late_validation.rs)

New module with:

• LateValidationContext — Plain data built in check_attr from TyCtxt / HIR: Target, parent_is_trait_impl, optional expression context (loop vs break), trait-impl constness / impl_of_trait, foreign mod ABI, decl-macro flag for macro_export, etc.
• Validators that return Option<…> (or small enums) describing what is wrong, without emitting diagnostics:
  • #[deprecated] where it has no effect (trait impl members)
  • #[loop_match], #[const_continue] on the wrong expression kind
  • #[link] (foreign mod + not extern "Rust")
  • #[macro_export] on decl macros
  • Diagnostic attributes: do_not_recommend, on_unimplemented (placement), on_const, on_move (placement)
  • for_each_unknown_diagnostic_format_param — shared walk over Directive format parameters vs non-lifetime type parameters (used for on_unimplemented and on_move)

The pass check_attr remains responsible for emitting lints/errors via rustc_passes::errors and existing lint machinery; it only calls into late_validation for the predicate.

2. #[custom_mir] (compiler/rustc_attr_parsing/src/attributes/prototype.rs)

Dialect/phase compatibility and “phase without dialect” are validated in CustomMirParser via a dedicated check_custom_mir helper (same shape as the standalone parser work in #154126: failed flag + session_diagnostics). AttributeKind::CustomMir keeps optional dialect/phase with spans.

Corresponding diagnostics were removed from rustc_passes::errors and live in rustc_attr_parsing::session_diagnostics instead.

3. check_attr cleanup (compiler/rustc_passes/src/check_attr.rs)

• Builds LateValidationContext once per attribute walk and dispatches to the validators above.
• generics_has_type_param_named — small local helper for diagnostic format-parameter checking (shared by on_unimplemented / on_move arms).
• CustomMir arm is a no-op beyond a comment (validation is in the parser).

4. Small HIR doc tweak

• Directive::visit_params in compiler/rustc_hir/src/attrs/diagnostic.rs now points callers at late_validation::for_each_unknown_diagnostic_format_param for the post-HIR check.

Why this design

• Separation of concerns: Predicate logic colocated with attribute parsing in rustc_attr_parsing; rustc_passes keeps tcx/HIR wiring and user-facing diagnostics.
• Dependency layering: late_validation avoids rustc_middle / typeck; only rustc_hir + rustc_span (and Directive for format-parameter validation).
• Honest about limits: Module docs note that the long-term direction in Reduce the size of check_attrs.rs and move as much as possible into the attribute parsers. #153101 is to feed target context into parsing where possible; this module is an interim home for rules that still require a built HIR.

This matches reviewer feedback that we should not pretend check_attr is “done” when logic merely moves crates—it documents the staged approach and keeps validation code in the attribute stack.

Tests

No new tests/ files were added in this PR. Behavior is covered by existing UI / compiler tests (e.g. diagnostic namespace, custom_mir, attribute lints).

Local verification (recommended before merge):

./x.py check compiler/rustc_hir compiler/rustc_attr_parsing compiler/rustc_passes
./x.py test tidy
./x.py test tests/ui/diagnostic_namespace

Documentation

• Rustdoc on late_validation (purpose, LateValidationContext, link to Reduce the size of check_attrs.rs and move as much as possible into the attribute parsers. #153101, note on future target-aware parsing).
• Comment on CustomMir handling in check_attr.
• Directive::visit_params rustdoc cross-reference as above.

---

Rebase note

This branch was rebased onto rust-lang/rust main at 86c839ffb36. If main advances (e.g. after #154126 lands), run:

git fetch https://github.com/rust-lang/rust.git main
git rebase FETCH_HEAD   # or: git rebase rust-lang/main

If #154126 merges first, expect at most a small conflict or redundancy check in prototype.rs around check_custom_mir—the intent is a single shared implementation pattern.

[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @TaKO8Ki

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[JonathanBrouwer]

In the future, it would be nice if you could coordinate your ideas in the issue thread or on zulip before making a PR :)

[herbenderbler]

In the future, it would be nice if you could coordinate your ideas in the issue thread or on zulip before making a PR :)

My apologies, this is my first attempt at landing a PR here 😬

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer