Unexpected doc errors when using --nocapture and compile_fail

[gliderkite]

Problem
cargo test -- --nocapture outputs error logs for documentation tests even when specifying the compile_fail argument.

Steps

1. Create a new project with cargo new <project name>
2. Add a lib.rs file to src
3. Add the following method with documentation and code example which is supposed to fail compilation (the section code will contain invalid Rust code):

/// ```compile_fail
/// Input: 123
/// ```
pub fn foo() {
}

4. Run cargo test -- --nocapture

The output will be:

Finished dev [unoptimized + debuginfo] target(s) in 0.03s
Running target/debug/deps/compile_fail-af4564015b61948e

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

 Running target/debug/deps/compile_fail-8c19ede40c69c8ad

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Doc-tests compile-fail

running 1 test
error: expected type, found 123
--> src/lib.rs:2:8
|
3 | Input: 123
| ^^^ expecting a type here because of type ascription

test src/lib.rs - foo (line 1) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Notes

Running cargo test presents a different output with no errors:

Finished dev [unoptimized + debuginfo] target(s) in 0.03s
Running target/debug/deps/compile_fail-af4564015b61948e

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

 Running target/debug/deps/compile_fail-8c19ede40c69c8ad

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Doc-tests compile-fail

running 1 test
test src/lib.rs - foo (line 1) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Output of cargo version:

cargo 1.30.0 (36d96825d 2018-10-24)

[alexcrichton]

Thanks for the report! I've transferred this to the rust-lang/rust repository as this looks like it's a bug with rustdoc rather than Cargo.

This can be reproduced with the following:

/// ```compile_fail
/// Input: 123
/// ```
pub fn foo() {
}

executed with:

$ rustdoc --test foo.rs --test-args --nocapture

running 1 test
error: expected type, found `123`
 --> foo.rs:2:8
  |
3 | Input: 123
  |        ^^^ expecting a type here because of type ascription

test foo.rs - foo (line 1) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[woshilapin]

As I was browsing issues (what a nice hobby!) looking for something else, I went through another issue #26309 that might be related. I'm just pointing to it, it might help to whoever is gonna work on this.