Add compiler flag to force color output

[hannobraun]

This pull request fixes #12881.

Two caveats:

1. As explained in the commit message, this doesn't include a regression test. If this is unacceptable, please let me know, I'll see what I can do.
2. I'm getting some test failures on make check, all from debuginfo. I suspect this is due to Not all debuginfo tests work on GDB 7.2 #13680 and not related to my changes (I have GDB 7.7). This is the list of failed tests:

[debuginfo-gdb] debuginfo/basic-types-globals.rs
[debuginfo-gdb] debuginfo/basic-types-mut-globals.rs
[debuginfo-gdb] debuginfo/basic-types.rs
[debuginfo-gdb] debuginfo/borrowed-basic.rs
[debuginfo-gdb] debuginfo/borrowed-managed-basic.rs
[debuginfo-gdb] debuginfo/borrowed-struct.rs
[debuginfo-gdb] debuginfo/borrowed-unique-basic.rs
[debuginfo-gdb] debuginfo/box.rs
[debuginfo-gdb] debuginfo/by-value-non-immediate-argument.rs
[debuginfo-gdb] debuginfo/by-value-self-argument-in-trait-impl.rs
[debuginfo-gdb] debuginfo/closure-in-generic-function.rs
[debuginfo-gdb] debuginfo/evec-in-struct.rs
[debuginfo-gdb] debuginfo/function-arg-initialization.rs
[debuginfo-gdb] debuginfo/function-prologue-stepping-no-split-stack.rs
[debuginfo-gdb] debuginfo/generic-function.rs
[debuginfo-gdb] debuginfo/generic-functions-nested.rs
[debuginfo-gdb] debuginfo/generic-method-on-generic-struct.rs
[debuginfo-gdb] debuginfo/generic-static-method-on-struct-and-enum.rs
[debuginfo-gdb] debuginfo/generic-struct.rs
[debuginfo-gdb] debuginfo/lexical-scope-in-stack-closure.rs
[debuginfo-gdb] debuginfo/lexical-scope-in-unique-closure.rs
[debuginfo-gdb] debuginfo/method-on-generic-struct.rs
[debuginfo-gdb] debuginfo/method-on-tuple-struct.rs
[debuginfo-gdb] debuginfo/name-shadowing-and-scope-nesting.rs
[debuginfo-gdb] debuginfo/recursive-struct.rs
[debuginfo-gdb] debuginfo/self-in-generic-default-method.rs
[debuginfo-gdb] debuginfo/shadowed-argument.rs
[debuginfo-gdb] debuginfo/shadowed-variable.rs
[debuginfo-gdb] debuginfo/simd.rs
[debuginfo-gdb] debuginfo/simple-lexical-scope.rs
[debuginfo-gdb] debuginfo/simple-struct.rs
[debuginfo-gdb] debuginfo/simple-tuple.rs
[debuginfo-gdb] debuginfo/static-method-on-struct-and-enum.rs
[debuginfo-gdb] debuginfo/tuple-struct.rs
[debuginfo-gdb] debuginfo/var-captured-in-nested-closure.rs
[debuginfo-gdb] debuginfo/var-captured-in-sendable-closure.rs
[debuginfo-gdb] debuginfo/var-captured-in-stack-closure.rs

I can provide the full output on request.

[alexcrichton]

When I've seen a flag like this previously, it's normally --color or --color=never or something like that. Perhaps the flag could be --color and turning it off could be --no-color? We don't currently handle --color[=foo] well right now because if you do something like rustc --color foo.rs it'll end up interpreting "foo.rs" as the argument to color.

[hannobraun]

Sounds good to me. If I understand you correctly, you want the default behaviour to stay the same? That is, if none of the flags are there, the code should check if stderr is a tty?

[alexcrichton]

That sounds good to me!

[sfackler]

As one example, ag uses --color to force coloring, --nocolor to force no coloring and the default is to color if stdout is a tty.

[hannobraun]

Great! I've renamed the argument to --color and will push that later today (I need to get going now and make will take a while on my machine).

Do you want me to send a separate PR for --no-color or should I update this one? Either way is fine with me, but it might take a week or two until I can find the time to work on this again.

[alexcrichton]

Feel free to add it to this PR, it's a small enough change.

[hannobraun]

Okay, will do.

[hannobraun]

I've changed the argument from --force-color-output to --color. I'll implement --no-color as soon as I find some time.

[huonw]

Another option is --color=always ...=auto and ...=never.

[alexcrichton]

Hm, I think I like @huonw's suggestion because it only adds one flag, and --color=always probably won't be too, too common.

[hannobraun]

Yes, that sounds even better.

[hannobraun]

I've implemented --color=auto|always|never as per @huonw's suggestion. Please let me know, if there any further objections against merging this.