Allow assignments in const contexts

[oli-obk]

fixes#54098
fixes#51251
fixes#52613

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

r? @RalfJung

[RalfJung]

r=me with tidy happy:

[00:03:46] tidy error: /checkout/src/test/ui/consts/promote_const_let.rs: missing trailing newline
[00:03:47] some tidy checks failed

[oli-obk]

@bors r=RalfJung

[bors]

📌 Commit 37fcd5c has been approved by RalfJung

[eddyb]

This doesn't seem correct - if you assign something with less qualifications to a field of a value with more, you turn those bits off.

[eddyb]

In the future, please cc me on qualify_consts changes.

@bors r-

[oli-obk]

Continuing with

the mindset when dealing with [const qualif] should be "how do I break this?"

Projections would include Deref, which isn't a problem because we can't have mutable references at all inside const contexts. Still something to think about.

Downcast also can't happen, because we'd need match for that.

Maybe we should just whitelist field projections for now.

[eddyb]

The problem is something like this:

constFOO:&(Cell<usize>,bool) = &{letmut foo = (Cell::new(0),false);
foo.1 = true;// resets `qualif(foo)` to `qualif(true)`
foo
};

I suspect that will actually compile with this PR, although I'd like a confirmation.

We could add this as a testcase, but you might be able to come up with something simpler/more general.

[oli-obk]

This indeed compiles successfully with this PR

[eddyb]

My suggestion is field (or projections in general) assignment should combine qualifications (i.e. |=) instead of replacing them.

[oli-obk]

I now reuse the existing projection checks for reading values. These checks replace the qualification with NOT_CONST, so there's no real use in combining I think?

[RalfJung]

In the future, please cc me on qualify_consts changes.

I'll happily hand them over to you. ;)

r? @eddyb

[rust-highfive]

The job x86_64-gnu-llvm-5.0 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.

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travis_time:end:065e7ce8:start=1542793454701620808,finish=1542793512762261299,duration=58060640491
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout
Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
---
[00:04:52] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:52] tidy error: /checkout/src/test/ui/consts/projection_qualif.rs: missing trailing newline
[00:04:52] tidy error: /checkout/src/test/ui/consts/partial_qualif.rs: missing trailing newline
[00:04:53] some tidy checks failed
[00:04:53] [00:04:53] [00:04:53] 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"
[00:04:53] [00:04:53] [00:04:53] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:53] Build completed unsuccessfully in 0:01:02
[00:04:53] Build completed unsuccessfully in 0:01:02
[00:04:53] Makefile:79: recipe for target 'tidy' failed
[00:04:53] make: *** [tidy] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:2218e2ff
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Wed Nov 21 09:50:16 UTC 2018
---
travis_time:end:22c719fc:start=1542793817090665327,finish=1542793817095606984,duration=4941657
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travis_time:start:0497bbba
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
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travis_time:start:00f12be8
travis_time:start:00f12be8
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
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travis_time:start:0ef37a6f
$ dmesg | grep -i kill

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[oli-obk]

@bors r=eddyb

[bors]

📌 Commit 6db8c6c has been approved by eddyb

[bors]

⌛ Testing commit 6db8c6c with merge b51632e...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing b51632e to master...