compiletest --nocapture still delays prints

add the following snippet to the top of any run-make test's main function:

   #![allow(unreachable_code)]
    println!("infinite loop");
    loop {}

then run COMPILETEST_FORCE_STAGE0=1 ./x test --stage 0 tests/run-make/TEST_NAME/ -- --nocapture

this should print "infinite loop" and then hang, but it does not print anything.

running the same piece of code on the playground works properly.

flushing stdout doesn't help either.

@rustbot

@rustbot claim

This occurs because the output of the child(executor) process in which the test runs is piped to parent (compiletest) process. So it's output cannot be read and printed till the process is complete. (in src/tools/compiletest/src/runtest/run_make.rs).

let mut cmd = Command::new(&recipe_bin);
       cmd.current_dir(&rmake_out_dir)
           .stdout(Stdio::piped())
           .stderr(Stdio::piped())

But this is not required in the case where we want to run a test with --no-capture.
So a fix would be to use Stdio:inherit() and not reading the stdout of the executor process using self.read2_abbreviated(proc); when using the --no-capture flag .

if self.config.capture {
            cmd.stdout(Stdio::piped()).stderr(Stdio::piped());
        } else {
            cmd.stdout(Stdio::inherit()).stderr(Stdio::inherit());
        }

Rollup merge of rust-lang#162714 - Ashutosh1910:fix_compiletest_issue…

Unrolled build for #162714 in rollup 163404

Rollup merge of rust-lang#163989 - jieyouxu:revert-stream-output, r=lqd

Unrolled build for #163989 in rollup 164005

Rollup merge of #163989 - jieyouxu:revert-stream-output, r=lqd

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compiletest --nocapture still delays prints #128565

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compiletest --nocapture still delays prints #128565

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Ashutosh1910 commented on Sep 3, 2026

Ashutosh1910 commented on Sep 12, 2026

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