[ci] Roll pinned stable toolchain

[google-pr-creation-bot]

Automated changes by create-pull-request GitHub action

[gemini-code-assist]

Summary of Changes

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This pull request primarily focuses on maintaining compatibility with the latest stable Rust ecosystem by rolling forward the pinned stable toolchain version. Concurrently, it updates the expected output of various UI tests to align with the diagnostic changes introduced in the newer Rust compiler, ensuring that tests accurately reflect current compiler behavior and error reporting.

Highlights

• Toolchain Update: The pinned stable Rust toolchain has been updated from version 1.90.0 to 1.91.0 in Cargo.toml.
• UI Test Diagnostic Updates: Numerous UI test .stderr files have been updated to reflect changes in Rust compiler diagnostics. This includes refining trait bound error messages, adding helpful suggestions pointing to type definitions, and incorporating new error codes for transmute size mismatches.

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[gemini-code-assist]

Code Review

This pull request is an automated update to roll the pinned stable Rust toolchain to version 1.91.0. The changes consist of updating the pinned-stable version in Cargo.toml and regenerating the .stderr files for UI tests to match the diagnostic output of the new compiler. The changes to the test outputs are consistent with improvements in Rust compiler diagnostics and appear correct. This is a routine maintenance update that looks good to merge.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.89%. Comparing base (e8eb595) to head (620c535).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2750   +/-   ##
=======================================
  Coverage   88.89%   88.89%           
=======================================
  Files          20       20           
  Lines        5423     5423           
=======================================
  Hits         4821     4821           
  Misses        602      602           

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