mirth-lab lint-check: lint oracles (check 16); findings 33-36

[zmaril]

Check 16 of docs/checks.md (lint oracles), as mirth-lab lint-check.

Property: a lint fires only when its premise holds, and acting on it changes nothing else. For every standalone UI test that compiles, with 16 allow-by-default lints turned on, and for each lint that warns, the check does four things:

• allow: adds #![allow(lint)]; only that lint's warnings may change.
• delete: removes what dead_code, unreachable_patterns, unreachable_code and unused_extern_crates flag. The result must still compile and, for run-pass tests, print the same output.
• premise: rewrites to what the lint says is equivalent:
  • trivial_casts goes through a coercion site;
  • trivial_numeric_casts drops the cast;
  • the wide-pointer comparison lint's operands go through a const assertion that they are two words (#163840);
  • missing_copy_implementations gets an impl Copy.
• widen-fix: applies the machine-applicable fixes of allow-by-default lints, which suggest-diff never sees, alone and then all of a lint's fixes together.

Edits are located with syn (span-locations). Failures that the lint's design or the edit explains are recorded per test as expected, not as findings.

Sweep: 18,624 UI tests. 8,055 compile without errors; 4,244 of those have a lint warning (211 lints in all); 30,100 compilations.

Findings: in docs/hunt.md and docs/hunt/lint-check.md, with reductions in docs/hunt/tests/lint-check/. Facts only.

• 33: let_underscore_drop fires on let _ = place (nothing is dropped there), and its two fixes break builds or move drops: E0716, a lost type annotation, a lost #[coroutine], rewritten macro bodies. 26 UI tests. Low severity: cargo fix skips alternative suggestions (#104910).
• 34: lifetime-lint fixes that change meaning. A single_use_lifetimes fix moves #[may_dangle] to the next parameter and still compiles. A '_ fix gives E0637. An unused_lifetimes fix removes a for<'a> that kept a bound from being checked.
• 35: dead_code reports a trait as "never used" when it appears only in the where-clause or self type of an impl whose methods are called (stable since at least 1.80.0). It also reports the #[define_opaque] function that is an opaque type's only defining use (nightly).
• 36: trivial_numeric_casts warns on 5 as i16, where the cast is what gives the literal its type.

Known issues reproduced: #110332 and #163369.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QXiEXbESemwqMLYKaWLDbT

[zmaril]

Merged into main together with #33–#38; renumbered at merge so finding numbers are unique: findings 33–36 are now 40–43 (docs/hunt.md).