[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:  barrucadu
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1934
       Wiki Page:  Design/Warnings   |
-------------------------------------+-------------------------------------

@@ -30,0 +30,23 @@
+
+ -----
+
+ As an obvious extension, (which maybe should be controlled by some `-f`
+ flag, e.g. `-f(no-)show-warning-groups`), GHC could then also show by
+ which warning-group a given warning is implied, e.g.
+
+
+ {{{
+ WCompatWarningsOn.hs:16:1: warning: [-Wsemigroup (in -Wcompat)]
+     Local definition of ‘<>’ clashes with a future Prelude name.
+     This will become an error in a future release.
+
+ WCompatWarningsOn.hs:22:3: warning: [-Wnoncanonical-monoid-instances (in
+ -Wcompat)]
+     Noncanonical ‘(<>) = mappend’ definition detected
+     in the instance declaration for ‘Semi.Semigroup S’.
+     Move definition from ‘mappend’ to ‘(<>)’
+ }}}
+
+ For the warning-sets which form a superset chain (Weverything > Wextra >
+ Wall > Wdefault) we'd only mention the smallest one (and omit the trivial
+ `-Weverything` set altogether) to keep the output concise.

New description:

 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.

 -----

 As an obvious extension, (which maybe should be controlled by some `-f`
 flag, e.g. `-f(no-)show-warning-groups`), GHC could then also show by
 which warning-group a given warning is implied, e.g.


 {{{
 WCompatWarningsOn.hs:16:1: warning: [-Wsemigroup (in -Wcompat)]
     Local definition of ‘<>’ clashes with a future Prelude name.
     This will become an error in a future release.

 WCompatWarningsOn.hs:22:3: warning: [-Wnoncanonical-monoid-instances (in
 -Wcompat)]
     Noncanonical ‘(<>) = mappend’ definition detected
     in the instance declaration for ‘Semi.Semigroup S’.
     Move definition from ‘mappend’ to ‘(<>)’
 }}}

 For the warning-sets which form a superset chain (Weverything > Wextra >
 Wall > Wdefault) we'd only mention the smallest one (and omit the trivial
 `-Weverything` set altogether) to keep the output concise.

--

Comment (by hvr):

 @barrucadu This looks very promising... but have you talked to David
 (`quchen`) before you took the ticket from him? He had already started
 working on a warning refactoring in his Github repo afaik (see .e.g.
 https://github.com/quchen/ghc/commits/warn-refactoring)...

 PS: I've extended the ticket description with a bonus task

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10752#comment:12>
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