Allow leading | anywhere we allow or-patterns

[joshtriplett]

See #54883 (comment) for details.

Filing this issue to check lang-team consensus on this: do we want to allow leading | everywhere we allow or-patterns?

[joshtriplett]

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[scottmcm]

Somehow to me they feel a little weirder in something like matches!(foo, | Bar(_) | Baz(_)), but regardless I think allowing them is the right thing to do for consistency and the overall grammar.

[joshtriplett]

@scottmcm In that use case, they'd likely look like this:

    matches!(
        some_long_expression,
        | SomeLongPattern
        | SomeOtherLongPattern
    )

[nikomatsakis]

@rfcbot reviewed

[joshtriplett]

Checking box for boats per rust-lang/team#526 .

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[mark-i-m]

This is what the grammar would look like after this change: https://github.com/rust-lang/reference/pull/957/files?short_path=cc629f1#diff-cc629f15712821139bc706c63b3845ab59a008e2a998e08ffad42e3aebcbcbe2

[mark-i-m]

Implementation: #81869

[lukechu10]

I think rustfmt should add a leading | when the pattern is on multiple lines but remove it when it's on a single line:

matches!(foo, Bar | Baz);

matches!(
    some_really_long_expression,
    | Bar
    | Baz
)

match foo {
    Bar | Baz => {},
    _ => {}
}

match foo {
    | Bar
    | Baz => {},
    _ => {}
}

Just for consistency