Enforce stability of const fn in promoteds

[oli-obk]

r? @eddyb

fixes #50901

what's going on here? Why do we have two promoted computation algorithms?

[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (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.

[00:05:32] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:05:33] tidy error: /checkout/src/test/ui/const-eval/dont_promote_unstable_const_fn.rs: missing trailing newline
[00:05:33] tidy error: /checkout/src/librustc_mir/transform/qualify_consts.rs:962: trailing whitespace
[00:05:34] some tidy checks failed
[00:05:34] 
[00:05:34] 
[00:05:34] 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" "--quiet"
[00:05:34] 
[00:05:34] 
[00:05:34] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:05:34] Build completed unsuccessfully in 0:02:08
[00:05:34] Build completed unsuccessfully in 0:02:08
[00:05:34] Makefile:79: recipe for target 'tidy' failed
[00:05:34] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0d5190c8
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[eddyb]

Why is this inside the if? IMO the flags should be as general as possible, so we can make sure less problems show up again. Also, shouldn't this be something like self.add(Qualif::NOT_CONST)? (EDIT: ah, no, below you can see it's also that; the overwrite is because the argument qualifications and the function call result are unrelated and the former have to be cleared anyway)

[oli-obk]

Doesn't make much difference as the other branch errors, but seems more consistent indeed

[eddyb]

@bors try

(we'll likely need a check-only crater run on this)

[bors]

⌛ Trying commit 329fd90 with merge f0ca7e1...

[oli-obk]

In case of regressions we could just stabilize the relevant const fns together with fixing the stability hole

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[oli-obk]

Ping @rust-lang/infra can you start a check-only crater run?

[pietroalbini]

Crater run (check-only) started.

[pietroalbini]

Hi @oli-obk (crater requester), @eddyb (PR reviewer)! Crater results are at: http://cargobomb-reports.s3.amazonaws.com/pr-50909/index.html. 'Blacklisted' crates (spurious failures etc) can be found here. If you see any spurious failures not on the list, please make a PR against that file.

(interested observers: Crater is a tool for testing the impact of changes on the crates.io ecosystem. You can find out more at the repo if you're curious)

[eddyb]

Looks clean! (spurious regression)

r=me after rebase

[oli-obk]

@bors r=eddyb

[bors]

📌 Commit a11f785 has been approved by eddyb

[pietroalbini]

@bors p=1 (backporting to beta)

[bors]

⌛ Testing commit a11f785 with merge 0746522...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 0746522 to master...