Lint useless or-pattern alternatives under unreachable_patterns

[iMostfa]

rustc warns when an or-pattern alternative is unreachable (_ | 0), but not when it is reachable yet useless (0 | _): removing it wouldn't change the result of the match.

Teach the usefulness algorithm to detect such alternatives and lint them under unreachable_patterns, with a dedicated "useless pattern" message since they are not unreachable. Being semantic rather than syntactic, this also catches what clippy can't: coverage by a union of siblings (0..=1 | 1..=2 | 2..=3) and duplicates in guarded arms (0 | 0 if g). Alternatives that bind variables are not linted, since which alternative matches determines the bound values ((0, x) | (x, _)).

The lint found two real cases in rustc's own sources, fixed here. Existing consumers of rustc_pattern_analysis (rust-analyzer) keep compiling unchanged.

Fixes #160772

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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   Compiling rustc_metadata v0.0.0 (/checkout/compiler/rustc_metadata)
warning: useless pattern
   --> compiler/rustc_const_eval/src/const_eval/valtrees.rs:201:11
    |
201 |         | ty::Infer(ty::FreshIntTy(_))
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ all the values this pattern matches are already matched by the rest of the or-pattern
...
206 |         | ty::Infer(_)
    |           ------------ matches all the values this pattern does
    |
    = note: `#[warn(unreachable_patterns)]` (part of `#[warn(unused)]`) on by default

warning: useless pattern
   --> compiler/rustc_const_eval/src/const_eval/valtrees.rs:202:11
    |
202 |         | ty::Infer(ty::FreshFloatTy(_))
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ all the values this pattern matches are already matched by the rest of the or-pattern
...
206 |         | ty::Infer(_)
    |           ------------ matches all the values this pattern does

warning: useless pattern
   --> compiler/rustc_const_eval/src/const_eval/valtrees.rs:367:11
    |
367 |         | ty::Infer(ty::FreshIntTy(_))
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ all the values this pattern matches are already matched by the rest of the or-pattern
...
373 |         | ty::Infer(_)
    |           ------------ matches all the values this pattern does

warning: useless pattern
   --> compiler/rustc_const_eval/src/const_eval/valtrees.rs:368:11
    |
368 |         | ty::Infer(ty::FreshFloatTy(_))
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ all the values this pattern matches are already matched by the rest of the or-pattern
...
373 |         | ty::Infer(_)
    |           ------------ matches all the values this pattern does

[RUSTC-TIMING] rustc_middle test:false 139.131

[rust-bors]

☔ The latest upstream changes (presumably #161706) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.