Require issue annotations for #[unstable] features

[alexcrichton]

This commit turns #[unstable] attributes missing an issue annotation into a hard error. This will require the libs team to ensure that there's a tracking issue for all unstable features in the standard library.

All existing unstable features have had issues created and they've all been updated. Yay!

Closes #26868

[rust-highfive]

r? @pcwalton

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[alexcrichton]

cc @rust-lang/libs, @huonw

r? @aturon

[alexcrichton]

This is broken into separate commits per library, and the massive removal count is because this is rebased on #27684 so I didn't have to tag any of those unstable items.

[huonw]

(Travis failed due to a missing issue on an item, but I guess that's somewhat expected.)

[bors]

☔ The latest upstream changes (presumably #27684) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #27696) made this pull request unmergeable. Please resolve the merge conflicts.

[aturon]

Sorry for the reviewing delay. LGTM!

@bors: r+

[bors]

📌 Commit 1539f1b has been approved by aturon

[bors]

⌛ Testing commit 1539f1b with merge 6b5e0e7...

[bors]

💔 Test failed - auto-win-msvc-64-opt

[Gankra]

../src/libstd\sys/windows\ext\fs.rs:24:1: 49:2 error: non-deprecated unstable items need to point to an issue with `issue = "NNN"`
../src/libstd\sys/windows\ext\fs.rs:24 pub trait OpenOptionsExt {
../src/libstd\sys/windows\ext\fs.rs:25     /// Overrides the `dwDesiredAccess` argument to the call to `CreateFile`
../src/libstd\sys/windows\ext\fs.rs:26     /// with the specified value.
../src/libstd\sys/windows\ext\fs.rs:27     fn desired_access(&mut self, access: u32) -> &mut Self;
../src/libstd\sys/windows\ext\fs.rs:28 
../src/libstd\sys/windows\ext\fs.rs:29     /// Overrides the `dwCreationDisposition` argument to the call to
                                       ...

[alexcrichton]

@bors: r=aturon 5101876

[bors]

⌛ Testing commit 5101876 with merge df572f0...

[bors]

💔 Test failed - auto-mac-32-opt

[alexcrichton]

@bors: r=aturon ed37f05

[bors]

⌛ Testing commit ed37f05 with merge 80905c4...

[bors]

💔 Test failed - auto-mac-32-opt

[bluss]

new failure

maketest: alloc-extern-crates
maketest: allow-non-lint-warnings-cmdline
maketest: allow-warnings-cmdline-stability
----- /Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/src/test/run-make/allow-warnings-cmdline-stability/ --------------------
------ stdout ---------------------------------------------
DYLD_LIBRARY_PATH="/Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/obj/i686-apple-darwin/test/run-make/allow-warnings-cmdline-stability:/Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/obj/i686-apple-darwin/stage2/lib:" /Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/obj/i686-apple-darwin/stage2/bin/rustc --out-dir /Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/obj/i686-apple-darwin/test/run-make/allow-warnings-cmdline-stability -L /Users/rustbuild/src/rust-buildbot/slave/auto-mac-32-opt/build/obj/i686-apple-darwin/test/run-make/allow-warnings-cmdline-stability bar.rs

------ stderr ---------------------------------------------
bar.rs:11:1: 16:16 error: non-deprecated unstable items need to point to an issue with `issue = "NNN"`
bar.rs:11 #![crate_type = "lib"]
bar.rs:12 #![feature(staged_api)]
bar.rs:13 #![staged_api]
bar.rs:14 #![unstable(feature = "test_feature")]
bar.rs:15 
bar.rs:16 pub fn baz() { }
error: aborting due to previous error
make[1]: *** [bar] Error 101

------        ---------------------------------------------

[alexcrichton]

@bors: r=aturon 8ef1e3b

[bors]

⌛ Testing commit 8ef1e3b with merge 9165a4e...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt