Warn on leading pipes in patterns

[LeoniePhiline]

What it does

Warns on a |x| x pattern, which looks like a closure.

Lint Name

pattern_leading_pipe

Category

suspicious

Advantage

• Avoids confusion of "or" pattern having a leading pipe (which is discarded), with a closure, enclosing arguments in pipes.

Drawbacks

None

Example

    let (|x| x) = match x(..) {
        |_| Some(2) => |_| Some(3),
        |_| _ => unreachable!(),
    };
    assert!(matches!(x(..), |_| Some(4)));

(borrowed from weird-exprs.rs)

Could be written as:

    let (x | x) = match x(..) {
        _ | Some(2) => |_| Some(3),
        _ | _ => unreachable!(),
    };
    assert!(matches!(x(..), _ | Some(4)));

[LeoniePhiline]

@rustbot claim

[LeoniePhiline]

Covered by rustfmt.

Should be closed?

[Alexendoo]

Yeah, no need for a lint if rustfmt has it covered

[llogiq]

I think we still might lint this in a matches!(..) macro invocation, because rustfmt doesn't cover macros. We'd still need to check if the pattern comes from another expansion, but otherwise it would be an OK lint. I think the category should still be pedantic or style rather than suspicious. Also we may try not to lint if the pattern spans multiple lines where each line starts with a pipe symbol (something I hear Haskell programmers are quite fond of).

So I think we may reopen this, but it's certainly no longer an easy issue.

[Alexendoo]

There's an open PR on the rustfmt side to fix the matches! case - rust-lang/rustfmt#5554

[llogiq]

Ah, I had overlooked that. In that case, it's probably not a good idea to invest in this.