Make diagnostics slightly more colorful

This is a preliminary commit to add colors to diagnostics (warning and
error messages).  The aesthetic changes are:

  - 'warning', 'error', and 'fatal' are all colored magenta, red, and
    red respectively.
  - The warning annotation [-Wsomething] shares the same color.
  - Warnings and errors are also bolded (this is consistent with what
    other compilers do).

A new flag has been added to control the behavior:

    -fdiagnostics-color=(always|auto|never)

This flag is 'auto' by default.  However, auto-detection is not
implemented yet, so it effectively it defaults to off.

Test Plan: validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2716

GHC Trac Issues: #8809

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/users_guide/using.rst b/docs/users_guide/using.rst
index 1d7f52c8e6..f5761a4434 100644
--- a/docs/users_guide/using.rst
+++ b/docs/users_guide/using.rst
@@ -786,6 +786,15 @@ messages and in GHCi:
                   in a’
         or by using the flag -fno-warn-unused-do-bind
 
+.. ghc-flag:: -fdiagnostics-color=(always|auto|never)
+
+    Causes GHC to display error messages with colors.  To do this, the
+    terminal must have support for ANSI color codes, or else garbled text will
+    appear.  The default value is `auto`, which means GHC will make an attempt
+    to detect whether terminal supports colors and choose accordingly.  (Note:
+    the detection mechanism is not yet implemented, so colors are off by
+    default on all platforms.)
+
 .. ghc-flag:: -ferror-spans
 
     Causes GHC to emit the full source span of the syntactic entity