fix missing_asserts_for_indexing ignores match for length

[wasd243]

Fixes #17398

missing_asserts_for_indexing previously ignored match expressions, so indexing inside a match on the slice's length was linted even when the match arms already guarantee the indices are in bounds. For example:

match supported.len() { // `match` without assert
    0 => {},
    1 => println!("{}", supported[0]),
    _ => println!("{} or {}", supported[0], supported[1]),
}

Here the 0 and 1 arms rule out all lengths below 2, so the indexing in the wildcard arm cannot fail and the bounds checks are already elided without an assert!.

This PR suppresses the lint for an index inside a match on slice.len() when the non-wildcard arms are integer literals contiguously covering 0..=max (so the wildcard arm implies len > max) and the index is at most max.

The check is deliberately conservative and falls back to linting as before when:

• the arm literals don't contiguously cover 0..=max (e.g. 5 => .., _ => .., where _ can still be 0)
• any arm has a guard
• an arm uses an or-pattern or any non-literal pattern (e.g. 0 | 1 => .. is safe in principle but not analyzed)

One behavioral note: suppression is per-index. If a match guarantees len > 1 but the wildcard arm indexes [0], [1] and [2], only [2] remains unsuppressed, which is equivalent to a single unchecked access and therefore doesn't lint (consistent with the lint never firing on single accesses).

changelog: [missing_asserts_for_indexing]: fix false positive when a match on the slice length guarantees the indices are in bounds

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[github-actions]

Lintcheck changes for cd5f376

Lint	Added	Removed	Changed
clippy::missing_asserts_for_indexing	0	2	1

This comment will be updated if you push new changes

[Gri-ffin]

This assumes that if we have a wildcard anywhere, then the entire match is safe, even if we are accessing indexes we can't safely assume.

fn foo(supported: &[u8]) {
    match supported.len() {
        0 => {},
        1 => {},
        2 => println!("{} {}", supported[0], supported[2]),
        _ => println!("{} {}", supported[0], supported[2]),
    }
}

View changes since the review

[wasd243]

Fixed -- literal arms now require index < n (the arm pins len to exactly n), instead of the shared index <= max check.
Lints now:

match supported.len() {
    0 => {},
    1 => {},
    2 => println!("{} {}", supported[2], supported[3]), // both out of bounds for len == 2 → lints
    _ => {},
}

The example doesn't lint but for a pre-existing reason unrelated to the suppression:

match supported.len() {
    0 => {},
    1 => {},
    2 => println!("{} {}", supported[0], supported[2]), // [0] in bounds → suppressed; [2] NOT suppressed
    _ => println!("{} {}", supported[0], supported[2]), // both in bounds (len >= 3) → suppressed
}