Parse default async fn & Pre-expansion-gate default

[Centril]

• Simplify the overall pre-expansion gating "experience".
• Pre-expansion gate default instead of post-expansion.
• Parse default async fn. Fixes [bug] async specialization is not parsed #63716.

r? @petrochenkov

[petrochenkov]

Suggested change

	gate_all!(async_closure, "async closures are unstable");
	gate_all!(async_closures, "async closures are unstable");

Nit: everything else is plural or uncountable.

[Centril]

Yeah, you are right, but fixing it is not worth the churn of people switching to #![feature(async_closures)]?

[petrochenkov]

r=me with or without addressing the comment

[Centril]

@bors r=petrochenkov

[bors]

📌 Commit b1a9e9e has been approved by petrochenkov

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

2019-08-20T16:25:42.7444085Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-08-20T16:25:42.7624033Z ##[command]git config gc.auto 0
2019-08-20T16:25:42.7702809Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-08-20T16:25:42.7759026Z ##[command]git config --get-all http.proxy
2019-08-20T16:25:42.7888184Z ##[command]git -c http.extraheader="AUTHORIZATION: basic ***" fetch --force --tags --prune --progress --no-recurse-submodules --depth=2 origin +refs/heads/*:refs/remotes/origin/* +refs/pull/63749/merge:refs/remotes/pull/63749/merge
---
2019-08-20T16:26:18.6713740Z do so (now or later) by using -b with the checkout command again. Example:
2019-08-20T16:26:18.6713994Z 
2019-08-20T16:26:18.6714478Z   git checkout -b <new-branch-name>
2019-08-20T16:26:18.6714707Z 
2019-08-20T16:26:18.6714912Z HEAD is now at 041058cbd Merge b1a9e9edb1004f725d253dd3bf444fa61b74d4ca into 51879c3abaedb926739095d19a2af638ee6a07d8
2019-08-20T16:26:18.6876822Z ##[section]Starting: Collect CPU-usage statistics in the background
2019-08-20T16:26:18.6880097Z ==============================================================================
2019-08-20T16:26:18.6880167Z Task         : Bash
2019-08-20T16:26:18.6880219Z Description  : Run a Bash script on macOS, Linux, or Windows
---
2019-08-20T16:58:03.6430192Z    Compiling smallvec v0.6.10
2019-08-20T16:58:03.7093301Z error[E0658]: specialization is unstable
2019-08-20T16:58:03.7094768Z     --> /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/smallvec-0.6.10/lib.rs:1259:5
2019-08-20T16:58:03.7096485Z      |
2019-08-20T16:58:03.7097355Z 1259 |     default fn spec_from(slice: &'a [A::Item]) -> SmallVec<A> {
2019-08-20T16:58:03.7098357Z      |
2019-08-20T16:58:03.7098357Z      |
2019-08-20T16:58:03.7099059Z      = note: for more information, see ***/issues/31844
2019-08-20T16:58:03.7099693Z      = help: add `#![feature(specialization)]` to the crate attributes to enable
2019-08-20T16:58:03.9582354Z error: aborting due to previous error
2019-08-20T16:58:03.9586415Z 
2019-08-20T16:58:03.9595528Z For more information about this error, try `rustc --explain E0658`.
2019-08-20T16:58:03.9595528Z For more information about this error, try `rustc --explain E0658`.
2019-08-20T16:58:03.9707882Z error: Could not compile `smallvec`.
2019-08-20T16:58:04.3932754Z error: build failed
2019-08-20T16:58:04.3944379Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "2" "--release" "--locked" "--color" "always" "--features" "" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json-render-diagnostics"
2019-08-20T16:58:04.3944581Z expected success, got: exit code: 101
2019-08-20T16:58:04.3958627Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
2019-08-20T16:58:04.3958627Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
2019-08-20T16:58:04.3958737Z Build completed unsuccessfully in 0:25:08
2019-08-20T16:58:04.4018117Z == clock drift check ==
2019-08-20T16:58:04.4034516Z   local time: Tue Aug 20 16:58:04 UTC 2019
2019-08-20T16:58:04.5616731Z   network time: Tue, 20 Aug 2019 16:58:04 GMT
2019-08-20T16:58:04.5617469Z == end clock drift check ==
2019-08-20T16:58:05.5193879Z ##[error]Bash exited with code '1'.
2019-08-20T16:58:05.5239608Z ##[section]Starting: Checkout
2019-08-20T16:58:05.5241836Z ==============================================================================
2019-08-20T16:58:05.5241901Z Task         : Get sources
2019-08-20T16:58:05.5241977Z Description  : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[Centril]

Argh... smallvec is doing things it shouldn't... :(

@bors r-

[Centril]

Cherry picked out the 1st and the 3rd commits respectively for now.

[petrochenkov]

Not sure what is this blocked on, #63759 was merged, marking as waiting on author.

[Centril]

@petrochenkov The pre-expansion gating for default fn is the only remaining bit at the moment but it seems smallvec relied on that sadly. I made a fix in servo/rust-smallvec#161 but it seems a new version of smallvec hasn't been made yet. Perhaps we should close this for now and I can try to pre-expansion-gate some other things instead?

[petrochenkov]

Ok.