support issue = "none" in unstable attributes

[rossmacarthur]

This works towards fixing #41260.

This PR allows the use of issue = "none" in unstable attributes and makes changes to internally store the issue number as an Option<NonZeroU32>. For example:

#[unstable(feature = "unstable_test_feature", issue = "none")]
fn unstable_issue_none() {}

It was not made optional because feedback seen here #60860 suggested that people might forget the issue field if it was optional.

I could not remove the current uses of issue = "0" (of which there are a lot) because the stage 0 compiler expects the old syntax. Once this is available in the stage 0 compiler we can replace all uses of "0" with "none" and no longer allow "0". This is my first time contributing, so I'm not sure what the protocol is with two-part things like this, so some guidance would be appreciated.

r? @varkor

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed (pretty log, 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.

2019-11-11T17:11:13.0212284Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-11-11T17:11:13.0417256Z ##[command]git config gc.auto 0
2019-11-11T17:11:13.0496068Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-11-11T17:11:13.0560363Z ##[command]git config --get-all http.proxy
2019-11-11T17:11:13.0705152Z ##[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/66299/merge:refs/remotes/pull/66299/merge
---
2019-11-11T17:17:58.2175658Z     Finished release [optimized] target(s) in 1m 23s
2019-11-11T17:17:58.2256507Z tidy check
2019-11-11T17:17:59.5934682Z * 588 error codes
2019-11-11T17:17:59.5934962Z * highest error code: E0743
2019-11-11T17:17:59.6288340Z tidy error: /checkout/src/libsyntax/feature_gate/active.rs:213: no tracking issue for feature test_2018_feature
2019-11-11T17:18:00.7868868Z Found 485 error codes
2019-11-11T17:18:00.7868952Z Found 0 error codes with no tests
2019-11-11T17:18:00.7869018Z Done!
2019-11-11T17:18:00.7869221Z some tidy checks failed
2019-11-11T17:18:00.7869221Z some tidy checks failed
2019-11-11T17:18:00.7869249Z 
2019-11-11T17:18:00.7869270Z 
2019-11-11T17:18:00.7870096Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor"
2019-11-11T17:18:00.7870185Z 
2019-11-11T17:18:00.7870207Z 
2019-11-11T17:18:00.7880382Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-11-11T17:18:00.7880491Z Build completed unsuccessfully in 0:01:27
2019-11-11T17:18:00.7880491Z Build completed unsuccessfully in 0:01:27
2019-11-11T17:18:00.7936384Z == clock drift check ==
2019-11-11T17:18:00.7946476Z   local time: Mon Nov 11 17:18:00 UTC 2019
2019-11-11T17:18:00.8349496Z   network time: Mon, 11 Nov 2019 17:18:00 GMT
2019-11-11T17:18:00.8354009Z == end clock drift check ==
2019-11-11T17:18:02.3527728Z 
2019-11-11T17:18:02.3623705Z ##[error]Bash exited with code '1'.
2019-11-11T17:18:02.3655491Z ##[section]Starting: Checkout
2019-11-11T17:18:02.3656970Z ==============================================================================
2019-11-11T17:18:02.3657014Z Task         : Get sources
2019-11-11T17:18:02.3657069Z 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)

[varkor]

This looks good, thanks!

@bors r+

This is my first time contributing, so I'm not sure what the protocol is with two-part things like this, so some guidance would be appreciated.

We could use #[cfg(bootstrap)] to switch between issue = "0" and issue = "none" depending on whether we were bootstrapping or not, but that just creates a lot of churn pointlessly. Let's just replace them all as soon as this becomes the bootstrapping compiler.

[bors]

📌 Commit 3ba8257 has been approved by varkor