fix matches! and assert_matches! on edition 2021

[lukas-code]

Previously this code failed to compile on edition 2021. (Playground)

fn main() {
    matches!((), ());
}

   Compiling playground v0.0.1 (/playground)
error: `$pattern:pat` may be followed by `|`, which is not allowed for `pat` fragments
    |
    = note: allowed there are: `=>`, `,`, `=`, `if` or `in`

error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.

[rust-highfive]

r? @dtolnay

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[dtolnay]

Thanks! Is it possible to get a test for this? I believe the ui test harness supports setting the edition that tests are built with.

rust/src/test/ui/hello2021.rs

Lines 2 to 3 in fbf1b1a

	// edition:2021
	// compile-flags: -Zunstable-options

[lukas-code]

I added a regression test.

[ehuss]

I haven't seen any discussion about proceeding with this solution for #84429. As of this morning, there is work going on to fix the hygiene problem. Was there some discussion on this?

[petrochenkov]

@ehuss

I haven't seen any discussion about proceeding with this solution for #84429.

#84452 (comment) ?

[ehuss]

Ah, forgot about that! I do believe they should be equivalent, even when std is migrated to 2021, so I don't think it matters much one way or the other.

Just a forewarning, I think this may conflict with #85272, so the two PRs will be racing one another.

[dtolnay]

@bors r+

[bors]

📌 Commit 824c743 has been approved by dtolnay

[Aaron1011]

Note that this PR only fixes the problem for the specific matches! macro in the standard library. Any 2018 edition crate which defines a similar macro will have the same problem when invoked from a 2021-edition crate. This PR is still fine to land, since we're just explicitly indicating (via :pat_param) the behavior that we should be getting from :pat.

PR #85709 fixes this for all macro definitions.