Provide force coloring option for rustc

[bollu]

I often use tup / ninja as build systems which pipe input into an internal buffer and only later send it to stdout,

Rustc disables coloring when it's piped into any other source. Hence, I lose all coloring when using rust with tup.

Clang has the same behaviour by default, but provides a compiler flag -f-color-diagnostics which forces colored output to be emitted. This would be really useful to have in rustc. It makes debugging and in general compiling far more pleasant.

Thanks!

[hannobraun]

I've started working on this. Here's my progress so far: https://github.com/hannobraun/rust/commit/2f2ac5de204e1abf3f3371b4c772123d018742e2

I haven't tested this yet and haven't even run a full make check so far. Unfortunately I don't seem to have enough RAM in this machine to work while running the Rust build in the background, so I can't finish today.

I'll get back to it and send a pull request as soon as I can! (might be a week or two though, lots to do)

[jonhoo]

Shouldn't this also pass --color always to rustc? It seems unfortunate for the only way to actually force color to be

$ cargo rustc --color always -- --color always