Suggest async {} for async || {}

[rokob]

Fixes #76011

This adds support for adding help diagnostics to the feature gating checks and
then uses it for the async_closure gate to add the extra bit of help
information as described in the issue.

[rust-highfive]

r? @Mark-Simulacrum

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[jyn514]

r? @estebank

[estebank]

Can we add a test? It should live somewhere in src/test/ui/** and after the .rs file is created (and test check comments are added in the right place) you can run ./x.py test src/test/ui --stage 1 --bless to autogenerate the .stderr files.

[estebank]

Because we have a single span, I think this should be a bare help call (without a span) so the output isn't too verbose/redundant. I would not close #76011, as that is asking for a structured suggestion, which we might be able to provide in the future (but this current change is a great mitigation).

[rokob]

I updated the src/test/ui/async-await/feature-async-closure UI test. I also made a change to compiler/rustc_parse/src/parser/expr.rs because otherwise this help would show up in this test src/test/ui/parser/block-no-opening-brace.

I can backout that change if you want and instead have this help show up in that case as well. But looking at that example, it didn't seem right to me that this code:

fn f5() {
    async
        let x = 0;
}

would produce an error about async closures being unstable. You don't know that this is trying to be a closure, it could very well have been an async block which is not unstable. The help message I am adding makes this part even more confusing because it suggests removing the || and replacing it with a { but there is no ||. I think moving the feature gate until after the parsing of the rest of the closure has no semantic impact but makes more sense as you shouldn't assume you have a closure if you don't have |, or ||, or {.

[jyn514]

ping @estebank

[jyn514]

ping @estebank - is there a better reviewer for this?

[camelid]

I think estebank is catching up on reviews now.

[estebank]

@bors r+

Noticed the following output

error: expected one of `move`, `|`, or `||`, found keyword `let`
  --> $DIR/block-no-opening-brace.rs:30:9
   |
LL |     async
   |          - expected one of `move`, `|`, or `||`
LL |         let x = 0;
   |         ^^^ unexpected token

should also include { as an expected token, but that's independent of this. Edit: Created #80931.

[bors]

📌 Commit 5b475a4 has been approved by estebank

[bors]

⌛ Testing commit 5b475a4 with merge 467f5e9...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing 467f5e9 to master...