Bug 1315785 - Invoke cargo with --color=always when original stdout is a TTY; r=glandium

Combined with the previous patch that sets MACH_STDOUT_ISATTY, the
practical effect of this patch is that cargo is invoked with
`--color=always` when mach was attached to a TTY and colorized output
is sent to the terminal.

Note: this doesn't work with Rust/Cargo 1.10 for reasons unknown to me.
It appears there was a bug with Rust/Cargo because `--color=never` still
sent colorized output on that version! Cargo/Rust 1.12.1 works fine.

MozReview-Commit-ID: 6uXS3t3413i

--HG--
extra : rebase_source : 72df6e8248cb14c96850964f694576c7fd2f5fbb

config/rules.mk

@@ -921,6 +921,15 @@ cargo_build_flags += --manifest-path $(CARGO_FILE)
 cargo_build_flags += --target=$(RUST_TARGET)
 cargo_build_flags += --verbose
 
+# Enable color output if original stdout was a TTY and color settings
+# aren't already present. This essentially restores the default behavior
+# of cargo when running via `mach`.
+ifdef MACH_STDOUT_ISATTY
+ifeq (,$(findstring --color,$(cargo_build_flags)))
+cargo_build_flags += --color=always
+endif
+endif
+
 # Assume any system libraries rustc links against are already in the target's LIBS.
 #
 # We need to run cargo unconditionally, because cargo is the only thing that