[v3risec]

Summary

Fix loop contract attribute parsing for loop expressions used directly as match arms.

Previously, the token stream passed to #[kani::loop_invariant], #[kani::loop_modifies], or #[kani::loop_decreases] could include the match arm's trailing comma. Parsing it as a syn::Stmt failed, and the macro could panic instead of expanding the loop or reporting a useful diagnostic.

Changes

• Introduce a shared LoopInput parser for loop, while, and for expressions, supporting an optional statement semicolon or match-arm trailing comma.
• Use the parser consistently in all three loop contract attributes.
• Return normal compile errors for malformed input instead of panicking.
• Add regression tests for match-arm loops, different attribute orders, and invalid attribute placement.
• Add a negative termination-check regression to ensure a nondecreasing measure is actually rejected.

Testing

• Targeted loop-contract expected tests pass, including positive and negative decreases checks.
• Loop attribute UI tests and kani_macros unit tests pass.
• The standalone branch builds successfully; Clippy and formatting checks pass.
• Full repository CI has not been run locally.

Related work

This fixes match-arm attribute parsing, independently of the loop transformation and modifies-clause fixes in #4930, #4947, #5068, and #5071.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[Copilot]

🟢 Approval recommended

No active review issues remain for the loop-contract parsing and diagnostic changes.

No open issues 🎉

What changed in this PR

Fixes Kani’s loop contract macros when a loop appears directly in a match arm, without changing loop transformation or modifies-clause behavior.

Changes:

• Adds a shared parser for loop expressions with an optional semicolon or trailing comma, used by all three loop contract attributes.
• Replaces parsing panics with normal compile errors.
• Adds parser, match-arm, attribute-order, diagnostic, and negative termination-check regressions.

File	Description
tests/​ui/​loop-contract-attributes/​main.rs	Tests invalid attribute placement and malformed invariant input.
tests/​ui/​loop-contract-attributes/​expected	Records expected compile diagnostics.
tests/​expected/​loop-contract/​match_decreases_stale.rs	Tests rejection of a nondecreasing termination measure.
tests/​expected/​loop-contract/​match_decreases_stale.expected	Requires the termination check to fail.
tests/​expected/​loop-contract/​loop_match_expression.rs	Covers match-arm loop forms and different attribute orders.
tests/​expected/​loop-contract/​loop_match_expression.expected	Requires all four harnesses to verify successfully.
library/​kani_macros/​src/​sysroot/​loop_contracts/​mod.rs	Introduces the shared parser, integrates diagnostics, and adds parser unit tests.

_{Review effort: Balanced}