Add an `manual_slice_match` lint

[CommanderStorm]

What it does

This suggestion builds upon #17400 (comment)

We should detect match or if statements that check the .len() of a slice and subsequently index into it, suggesting a rewrite to use slice patterns.

Advantage

• Destructuring assigns meaning to the elements (e.g., [_, second, _, _]) instead of requiring the reader to parse and track raw integer indices across a block of code.
• removes panic sites by guaranteeing the check, though I suspect that llvm can optimsie this away. Needs a double check if llvm can do this in all cases though.

-> so a stylistic lint I think.

Drawbacks

None that I can think off.

Example

// Bad
fn foo(slice: &[u8]) -> u8 {
    match slice.len() {
        3 => slice[0] + slice[1] + slice[2],
        4 => slice[1],
        _ => 42,
    }
}

Could be written as:

// Good
fn foo(slice: &[u8]) -> u8 {
    match slice.len() {
        [a, b, c] => a + b + c,
        [_, b, _, _] => b,
        _ => 42,
    }
}

Comparison with existing lints

• clippy::len_zero targets len == 0
• clippy::get_first targets len == 1
• not sure why clippy::get_last does not exist
• clippy::get_last_with_len targets x.get(x.len() - 1)
• rustc::unconditional_panic targets bad indexings
• clippy::missing_asserts_for_indexing targets indexing without len checks

Additional Context

At first, this should be done with these additional constraints:

• slices with copy-able T (given that derfs might be seen as ugly)
• operation inside does not require mut
• up to index 5 (since adding something like 25 here might be fairly ugly).
  • Also goes for match-or patterns.
    Allow such a rewrite again, only if sum(idx for idx in match_or) <= 5, due to complexity.
• if the whole match is not actually .first(), .last(), 0 or impossible (no rustc::unconditional_panic)

[CommanderStorm]

ups.
Noticed that clippy::index_refutable_slice exists, that likely should also cover match patters then.

[CommanderStorm]

opening a false-negative issue instead, sorry for the noise

[CommanderStorm]

Actually, with the proper lint in the search, also finds the proper issue:

• indexing_slicing ignores match for length #17399