Implement built-in await syntax

[cramertj]

Adds support for .await under the existing async_await feature gate.
Moves macro-like await! syntax to the await_macro feature gate.
Removes support for await as a non-keyword under the async_await
feature.

This new syntax is not final, but is the consensus solution proposed by the lang team, as explained in https://boats.gitlab.io/blog/post/await-decision/

Fix#51719
Fix#51751
Fix#60016

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

@highfive: run-doc-ui d5072b54ac4342f97688391fede9e2caff6b133a

[rust-highfive]

r? @oli-obk

(rust_highfive has picked a reviewer for you, use r? to override)

[eddyb]

LGTM, modulo everyone else's comments.

[Centril]

@bors p=50

This is a priority for the project and we want to facilitate writing of tests.

[oli-obk]

r=me with @Centril happy. The rusfixable suggestion part should happen fast or not at all, since there's no point in creating it for just a week or two.On 7 May 2019 21:38, Taylor Cramer <notifications@github.com> wrote:@cramertj commented on this pull request. In src/libsyntax/ast.rs:

@@ -1186,6 +1187,9 @@ pub enum ExprKind {

/// created during lowering cannot be made the parent of any other /// preexisting defs. Async(CaptureBy, NodeId, P<Block>), + /// An await expression (`my_future.await`) Done. —You are receiving this because you were mentioned.Reply to this email directly, view it on GitHub, or mute the thread.

[cramertj]

@oli-obk Okay, in that case I'd like to go ahead and land this without in order to land this soon and unblock testing.

[oli-obk]

Oh definitely. That's what I meant. Do it in a followup PR soon (or not), but land this PR with the `desugar` -> `lower` changeOn 7 May 2019 22:06, Taylor Cramer <notifications@github.com> wrote:@oli-obk Okay, in that case I'd like to go ahead and land this without in order to land this soon and unblock testing. —You are receiving this because you were mentioned.Reply to this email directly, view it on GitHub, or mute the thread.

[estebank]

👀

[cramertj]

Opened #60615

[cramertj]

@bors r=oli-obk

[bors]

📌 Commit fe8760c has been approved by oli-obk

[bors]

⌛ Testing commit fe8760c with merge 33cde4a...

[Centril]

🎉 This is so exciting! 🎈

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing 33cde4a to master...

[rust-highfive]

📣 Toolstate changed by #60586!

Tested on commit 33cde4a.
Direct link to PR: #60586

💔 clippy-driver on windows: test-pass → build-fail (cc @Manishearth@llogiq@mcarton@oli-obk@phansch, @rust-lang/infra).
💔 clippy-driver on linux: test-pass → build-fail (cc @Manishearth@llogiq@mcarton@oli-obk@phansch, @rust-lang/infra).
💔 rls on windows: test-pass → build-fail (cc @Xanewok, @rust-lang/infra).
💔 rls on linux: test-pass → build-fail (cc @Xanewok, @rust-lang/infra).

[KSXGitHub]

I just came from a Reddit post and I don't like postfix syntax at all. And it seems that the majority of Reddit don't like it either. I already commented on #50547 (comment) but I also write this comment to make my argument more visible.

I hope this syntax isn't final (i.e. You guys are willing to break postfix syntax for every nightly user in favor of a sane preference)

[Manishearth]

@KSXGitHub

This is not the place to express disagreement with the language design choices made. Nor is the tracking issue: the language design team has made an explicit post on Discourse soliciting input.

If you plan to comment there, please bear in mind that the discussion has already played out quite a bit: make sure you have something substantial to say, and make sure it's not been said before in the thread.