Print which warning-flag controls an emitted warning

Both gcc and clang tell which warning flag a reported warning can be
controlled with, this patch makes ghc do the same. More generally, this
allows for annotated compiler output, where an optional annotation is
displayed in brackets after the severity.

This also adds a new flag `-f(no-)show-warning-groups` to control
whether to show which warning-group (such as `-Wall` or `-Wcompat`)
a warning belongs to. This flag is on by default.

This implements #10752

Reviewed By: quchen, bgamari, hvr

Differential Revision: https://phabricator.haskell.org/D1943

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 24f8039451..3f24f6a6dc 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -102,6 +102,15 @@ The following flags are simple ways to select standard "packages" of warnings:
     Warnings are treated only as warnings, not as errors. This is the
     default, but can be useful to negate a :ghc-flag:`-Werror` flag.
 
+When a warning is emitted, the specific warning flag which controls
+it, as well as the group it belongs to, are shown.
+
+.. ghc-flag:: -fshow-warning-groups
+
+    Name the group a warning flag belongs to.
+
+    This is enabled by default. Disable with ``-fno-show-warning-groups``.
+
 The full set of warning options is described below. To turn off any
 warning, simply give the corresponding ``-Wno-...`` option on the
 command line. For backwards compatibility with GHC versions prior to 8.0,