make CStr::from_bytes_with_nul_unchecked() a const fn

[abonander]

closes #54678

[rust-highfive]

r? @dtolnay

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

[abonander]

Not sure if this needs a test to confirm the ability to use this in a const, I guess as a regression test if this were to be removed.

[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.

Click to expand the log.

[00:04:58]    Compiling alloc v0.0.0 (/checkout/src/liballoc)
[00:04:59]    Compiling alloc_system v0.0.0 (/checkout/src/liballoc_system)
[00:04:59]    Compiling panic_abort v0.0.0 (/checkout/src/libpanic_abort)
[00:05:03]    Compiling panic_unwind v0.0.0 (/checkout/src/libpanic_unwind)
[00:05:08] error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
[00:05:08]      |
[00:05:08]      |
[00:05:08] 1044 |         mem::transmute(bytes)
[00:05:08]      |         ^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
[00:05:08]      |
[00:05:08]      = note: consult the function's documentation for information on how to avoid undefined behavior
[00:05:08] error: aborting due to previous error
[00:05:08] 
[00:05:08] error: Could not compile `std`.
[00:05:08] 
[00:05:08] 
[00:05:08] To learn more, run the command again with --verbose.
[00:05:08] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "--message-format" "json"
[00:05:08] expected success, got: exit code: 101
[00:05:08] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1112:9
[00:05:08] travis_fold:end:stage0-std

[00:05:08] travis_time:end:stage0-std:start=1538477459977074430,finish=1538477497042180674,duration=37065106244

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)

[abonander]

@oli-obk little snag with const_transmute here

[oli-obk]

Oh. I should have checked the original source :D You should be able to just add #![feature(const_raw_ptr_deref)] and keep the original source ;)

[oli-obk]

Not sure if this needs a test to confirm the ability to use this in a const, I guess as a regression test if this were to be removed.

A test would be great, yes.

[abonander]

r? @oli-obk

[abonander]

@oli-obk preferred ordering of the attributes? shortest to longest or doesn't really matter?

[oli-obk]

I don't think it matters. I ususally put the const unstable attribute below the stable attribute

[abonander]

Done, and added it to the crate feature flags as well.

[oli-obk]

you need to specify a feature name like in

rust/src/test/ui/feature-gates/feature-gate-rustc_const_unstable.rs

Line 18 in f45f525

#[rustc_const_unstable(feature="fzzzzzt")] //~ERROR internal feature

[abonander]

Should I just make one up or what?

[oli-obk]

jup :D that's how feature names work

[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.

Click to expand the log.

[00:04:52]    Compiling alloc v0.0.0 (/checkout/src/liballoc)
[00:04:53]    Compiling alloc_system v0.0.0 (/checkout/src/liballoc_system)
[00:04:53]    Compiling panic_abort v0.0.0 (/checkout/src/libpanic_abort)
[00:04:57]    Compiling panic_unwind v0.0.0 (/checkout/src/libpanic_unwind)
[00:04:59] error[E0548]: incorrect stability attribute type
[00:04:59]      |
[00:04:59] 1043 |     #[rustc_const_unstable]
[00:04:59]      |     ^^^^^^^^^^^^^^^^^^^^^^^
[00:04:59] 
---
[00:05:01] 
[00:05:01] To learn more, run the command again with --verbose.
[00:05:01] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "--message-format" "json"
[00:05:01] expected success, got: exit code: 101
[00:05:01] thread 'main' panicked at 'cargo must succeed', bootstrap/compile.rs:1112:9
[00:05:01] travis_fold:end:stage0-std

[00:05:01] travis_time:end:stage0-std:start=1538479266326394735,finish=1538479303466363965,duration=37139969230


[00:05:01] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:05:01] Build completed unsuccessfully in 0:00:38
[00:05:01] make: *** [all] Error 1
[00:05:01] Makefile:28: recipe for target 'all' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:08ccbdf5
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:26a7a514:start=1538479304164944651,finish=1538479304170228523,duration=5283872
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:072432a7
$ 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
travis_fold:start:after_failure.5
travis_time:start:03aefab2
travis_time:start:03aefab2
$ 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
travis_fold:start:after_failure.6
travis_time:start:0965d8e0
$ dmesg | grep -i kill

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)

[oli-obk]

@bors r+

[bors]

📌 Commit e0caaec has been approved by oli-obk