Intermingled JSON output from concurrent compilers

[Boddlnagg]

I don't know whether this is the right way to report this, since I found two related issues (#32486 and #30559), but my case is a bit different. It might also be a cargo issue.

In order to get JSON output from cargo, I passed --error-format json through RUSTFLAGS. When multiple crates are compiled in parallel, sometimes their build output is intermingled:

   Compiling library1 v0.1.0 (file:///D:/Code/Rust/dependency_test/library1)
   Compiling library2 v0.1.0 (file:///D:/Code/Rust/dependency_test/library2)
{"message":"function is never used: `foobar2`, #[warn(dead_code)] on by default","{"message":"function is never used: `foobar`, #[warn(dead_code)] on by default","code":codenull,"level":"warning"":,"spans":[null,"level":"warning{"file_name":"D:\\Code","\\Rust\\dependency_test\\library1\\spans":[{"file_name"src\\lib.rs":"D:,"byte_start":0,"byte_end"\\Code\\Rust\\dependency_test\\library2\\src\\lib.rs",:30,"line_start":1,"byte_start":"line_end":4,0,"byte_end"column_start":1,"":31,column_end":2,"is_primary""line_start":1,"line_end:true,"text"":4,"column_start":1,"column_end":2,"is_primary":true:[{"text":"fn foobar()","highlight_start":1,"highlight_end":,"text":12},{"text":"{","highlight_start":1,"highlight_end":2},[{"text{"text":"    let x = 2;","highlight_start":1,"highlight_end":15},{"text":"}","highlight_start":":"fn foobar2()","highlight_start":1,"highlight_end":131,"highlight_end":2}],"label":null,"suggested_replacement":null,"expansion":},{"text":"{","highlight_start":1,"highlight_end":2},{"nulltext":"    let y = 2;","highlight_start"}],"children":[:1,"highlight_end":15],"rendered":null}
},{"text":"}",{"message":"unused variable: `x`, #[warn(unused_variables)] on by default","code":null,"level":"warning","spans":[{"file_name":"D:\\Code\\Rust\\dependency_test\\library1\\src\\lib.rs","byte_start":22,"byte_end":23,"line_start""highlight_start"::1,"highlight_end":32}],"label":null,"line_end":3,"column_start","suggested_replacement":null,"expansion":null}],"children":[:9,"column_end":10,"is_primary":true,"text":[{"text":"    let x = 2;","highlight_start],"rendered":null}
":9,"highlight_end":10{"message":"unused variable: `y`, #[warn(unused_variables)] on by default","code":null,"level":"warning","spans":[{"file_name":"D:\\Code\\Rust\\dependency_test\\}],"label":null,"suggested_replacement":null,"expansion":null}library2],"children"\\src\\lib.rs","byte_start":23,"byte_end"::[],"rendered":null}
24,"line_start":3,"line_end":3,"column_start":9,"column_end":10,"is_primary":true,"text":[{"text":"    let y = 2;","highlight_start":9,"highlight_end":10}],"label":null,"suggested_replacement":null,"expansion":null}],"children":[],"rendered":null}
   Compiling testbin v0.1.0 (file:///D:/Code/Rust/dependency_test/binary)
{"message":"incorrect close delimiter: `}`","code":null,"level":"error","spans":[{"file_name":"src\\main.rs","byte_start":41,"byte_end":42,"line_start":3,"line_end":3,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"}","highlight_start":1,"highlight_end":2}],"label":null,"suggested_replacement":null,"expansion":null}],"children":[{"message":"unclosed delimiter","code":null,"level":"note","spans":[{"file_name":"src\\main.rs","byte_start":24,"byte_end":25,"line_start":2,"line_end":2,"column_start":13,"column_end":14,"is_primary":true,"text":[{"text":"    println!(\"Hello, world!\"","highlight_start":13,"highlight_end":14}],"label":null,"suggested_replacement":null,"expansion":null}],"children":[],"rendered":null}],"rendered":null}
{"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":null}
error: Could not compile `testbin`.

To learn more, run the command again with --verbose.

This also happens when redirecting stderr (where Cargo prints its output) to a file, so I won't be able to parse those messages correctly.

I'm using the latest nightlies:

D:\Code\Rust\dependency_test\binary>cargo -V
cargo 0.13.0-nightly (a713da1 2016-10-05)

D:\Code\Rust\dependency_test\binary>rustc -V
rustc 1.14.0-nightly (3210fd5c2 2016-10-05)

[alexcrichton]

cc @jonathandturner

You may also be interested in the --message-format json flag that just landed in Cargo which will get release tonight hopefully. I think that fixes this problem?

[Boddlnagg]

Oh, yes, I was looking for such a flag but totally missed that PR. I think this will fix the problem for me 👍

[steveklabnik]

So, is this not a bug, then?

[Boddlnagg]

Just tried the latest nightly and that's exactly what I need (for cargo support in VisualRust), so it's a least not a bug that needs to be fixed urgently. Maybe it should be documented somewhere (in the cargo documentation) that using --error-format json via RUSTFLAGS is not supported.

[sophiajt]

That sounds like a bug if setting RUSTFLAGS doesn't change the output format...

[Boddlnagg]

It does change the output format, but it's useless, because the output might be intermingled.

[Mark-Simulacrum]

If this is a bug, I think this is a Cargo bug -- rustc itself just prints to stdout/stderr -- therefore I'm going to close.