#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler