Capture output from threads spawned in tests

[tmandry]

Fixes #42474.

r? @dtolnay since you expressed interest in this, but feel free to redirect if you aren't the right person anymore.

[dtolnay]

Looks good!

[dtolnay]

Er, CI failed though:

--- [ui] ui/panic-while-printing.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit code: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/panic-while-printing.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/panic-while-printing/a" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/panic-while-printing/auxiliary"
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
error[E0277]: the trait bound `std::vec::Vec<_>: std::io::LocalOutput` is not satisfied
  --> /checkout/src/test/ui/panic-while-printing.rs:19:20
   |
LL |     set_panic(Some(Box::new(Vec::new())));
   |                    ^^^^^^^^^^^^^^^^^^^^ the trait `std::io::LocalOutput` is not implemented for `std::vec::Vec<_>`
   |
   = note: required for the cast to the object type `dyn std::io::LocalOutput`



--- [ui] ui/threads-sendsync/task-stderr.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit code: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/threads-sendsync/task-stderr.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/threads-sendsync/task-stderr/a" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/threads-sendsync/task-stderr/auxiliary"
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
error[E0277]: the trait bound `Sink: std::io::LocalOutput` is not satisfied
  --> /checkout/src/test/ui/threads-sendsync/task-stderr.rs:24:28
   |
LL |         io::set_panic(Some(Box::new(sink)));
   |                            ^^^^^^^^^^^^^^ the trait `std::io::LocalOutput` is not implemented for `Sink`
   |
   = note: required for the cast to the object type `dyn std::io::LocalOutput`

[tmandry]

Tests fixed!

[dtolnay]

error[E0277]: the trait bound `dyn realstd::io::LocalOutput: io::Write` is not satisfied
   --> library/std/src/panicking.rs:216:15
    |
216 |         write(&mut local);
    |               ^^^^^^^^^^ the trait `io::Write` is not implemented for `dyn realstd::io::LocalOutput`
    |
    = note: required because of the requirements on the impl of `io::Write` for `realstd::boxed::Box<dyn realstd::io::LocalOutput>`
    = note: required for the cast to the object type `dyn io::Write`

[tmandry]

Yep, working on this now. Sorry for the noise.

[tmandry]

CI passed.

[dtolnay]

@bors r+

[bors]

📌 Commit 38b920d has been approved by dtolnay

[tmandry]

@bors rollup=iffy

I'm trying to track down the cause of #75684 (comment) and while I don't think this can cause it (nor can I reproduce locally), nothing else in that change seems related.

[bors]

⌛ Testing commit 38b920d with merge 7b86e1eeec51cc8ce5e8edf130f2df638605da97...

[tmandry]

@bors retry
Yielding to rollup

[bors]

⌛ Testing commit 38b920d with merge 049ea2e42ed0a109e27a83fa7636690d19e3bfc8...

[tmandry]

@bors retry

[bors]

⌛ Testing commit 38b920d with merge ea7c21aedf7c1bbab87df466b39bb79968f6804f...

[bors]

💔 Test failed - checks-actions

[tmandry]

Same failure. Seems to only happen on the dist-i586-gnu-i586-i686-musl bot, I'll investigate more tomorrow.

[bors]

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

[crlf0710]

Triage: There's merge conflicts now.

[m-ou-se]

This PR partially breaks the effect of std::io::stdio::LOCAL_STREAMS:

rust/library/std/src/io/stdio.rs

Lines 33 to 37 in 981346f

	/// Flag to indicate LOCAL_STDOUT and/or LOCAL_STDERR is used.
	///
	/// If both are None and were never set on any thread, this flag is set to
	/// false, and both LOCAL_STDOUT and LOCAL_STDOUT can be safely ignored on all
	/// threads, saving some time and memory registering an unused thread local.

clone_io should first check LOCAL_STREAMS.load(Relaxed), and return (None, None) if it was false. Otherwise, spawning a thread in non-test program will needlessly register thread local destructors for LOCAL_STDOUT and LOCAL_STDERR.

[dtolnay]

Closing in favor of #78227 which includes a rebase of this commit.