hir: Disallow target_feature on constants

[davidtwco]

Fixes #64768.

This PR fixes an ICE when #[target_feature] is applied to constants by disallowing this with the same error as when #[target_feature] is applied to other places it shouldn't be.

I couldn't see anything in the RFC that suggested that #[target_feature] should be applicable to constants or any tests that suggested it should, though I might have missed something - if this is desirable in future, it remains possible to remove this error (but for the time being, I think this error is better than an ICE).

I also added some extra cases to the test for other places where #[target_feature] should not be permitted.

cc @gnzlbg

[rust-highfive]

r? @estebank

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

[gnzlbg]

This LGTM, target feature should only be allowed on functions and functions only - I think we should disallow it for const fns.

[varkor]

Thanks! r=me after a test case for multiple invalid attributes.

[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.

2019-09-26T13:35:15.3525571Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-26T13:35:15.3715437Z ##[command]git config gc.auto 0
2019-09-26T13:35:15.3795716Z ##[command]git config --get-all http.https://github.com/rust-lang/rust.extraheader
2019-09-26T13:35:15.3862150Z ##[command]git config --get-all http.proxy
2019-09-26T13:35:15.4054899Z ##[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/64809/merge:refs/remotes/pull/64809/merge
---
2019-09-26T13:42:19.6401777Z    Compiling serde_json v1.0.40
2019-09-26T13:42:21.1726983Z    Compiling tidy v0.1.0 (/checkout/src/tools/tidy)
2019-09-26T13:42:31.4290540Z     Finished release [optimized] target(s) in 1m 19s
2019-09-26T13:42:31.4360707Z tidy check
2019-09-26T13:42:31.9043977Z tidy error: /checkout/src/librustc/hir/check_attr.rs:138: line longer than 100 chars
2019-09-26T13:42:31.9044140Z tidy error: /checkout/src/librustc/hir/check_attr.rs:168: line longer than 100 chars
2019-09-26T13:42:33.0717758Z some tidy checks failed
2019-09-26T13:42:33.0719556Z 
2019-09-26T13:42:33.0719556Z 
2019-09-26T13:42:33.0720701Z 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"
2019-09-26T13:42:33.0720869Z 
2019-09-26T13:42:33.0720915Z 
2019-09-26T13:42:33.0732887Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
2019-09-26T13:42:33.0732944Z Build completed unsuccessfully in 0:01:22
2019-09-26T13:42:33.0732944Z Build completed unsuccessfully in 0:01:22
2019-09-26T13:42:33.0777125Z == clock drift check ==
2019-09-26T13:42:33.0800453Z   local time: Thu Sep 26 13:42:33 UTC 2019
2019-09-26T13:42:33.3412426Z   network time: Thu, 26 Sep 2019 13:42:33 GMT
2019-09-26T13:42:33.3416864Z == end clock drift check ==
2019-09-26T13:42:34.7539413Z ##[error]Bash exited with code '1'.
2019-09-26T13:42:34.7570935Z ##[section]Starting: Checkout
2019-09-26T13:42:34.7573055Z ==============================================================================
2019-09-26T13:42:34.7573295Z Task         : Get sources
2019-09-26T13:42:34.7573349Z 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]

r=me when tests pass (with or without the final suggestions, which are entirely preferential).

[gnzlbg]

@moonheart08

Of note: The reason I was attempting to use target function on a const

was because the const needed a different value on different targets. For AVX2 targets, it needed a value of 7, for AVX512 targets, a value of 15, and for other targets, a value of 3. Pointers on how to do it right are appreciated. You can use #[cfg(target_feature)] for that.

[moonheart08]

@moonheart08
Of note: The reason I was attempting to use target function on a const
was because the const needed a different value on different targets. For AVX2 targets, it needed a value of 7, for AVX512 targets, a value of 15, and for other targets, a value of 3. Pointers on how to do it right are appreciated. You can use #[cfg(target_feature)] for that.

I had deleted my comment and moved it to the issue that caused this PR, thank you, however!

[estebank]

@bors r=varkor

[bors]

📌 Commit c3368bd has been approved by varkor