[GHC] #10752: Print which warning-flag controls/enabled an emitted warning

[GHC]

#10752: Print which warning-flag controls/enabled an emitted warning
-------------------------------------+-------------------------------------
              Reporter:  hvr         |             Owner:
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Both `gcc` and `clang` tell which warning flag a reported warning can be
 controlled with, e.g.

 {{{
 $ clang-3.5 -Wall -c hello.c
 hello.c:3:7: warning: unused variable 'x' [-Wunused-variable]
   int x;
       ^
 1 warning generated.
 }}}

 {{{
 $ gcc -Wall -c hello.c
 hello.c: In function ‘main’:
 hello.c:3:7: warning: unused variable ‘x’ [-Wunused-variable]
    int x;
        ^
 hello.c:4:1: warning: control reaches end of non-void function [-Wreturn-
 type]
  }
  ^
 }}}

 With GHC however, we need to lookup the documentation (or memorise all
 warning flags) to find out which `-fno-warn-*` flag we need to use to
 silence a specific warning.

 I propose we augment GHC's warnings in a similar style to how
 `gcc`/`clang` report flags in compile warnings.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10752>
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