Add werror function to Flavour.hs

This function makes it easy to turn on `-Werror` in the correct manner
to mimic how CI turns on -Werror.

3 files changed, 22 insertions, 1 deletions

diff --git a/hadrian/doc/user-settings.md b/hadrian/doc/user-settings.md
index 49e560cc7e..b997cd977f 100644
--- a/hadrian/doc/user-settings.md
+++ b/hadrian/doc/user-settings.md
@@ -102,6 +102,17 @@ patterns such as `"//Prelude.*"` can be used when matching input and output file
 where `//` matches an arbitrary number of path components and `*` matches an entire
 path component, excluding any separators.
 
+### Enabling -Werror
+
+It is useful to enable `-Werror` when building GHC as this setting is
+used in the CI to ensure a warning free build. The `werror` function can be
+used to easily modify a flavour to turn this setting on.
+
+```
+devel2WerrorFlavour :: Flavour
+devel2WerrorFlavour = werror (developmentFlavour Stage2)
+```
+
 ## Packages
 
 Users can add and remove packages from particular build stages. As an example,
diff --git a/hadrian/src/Flavour.hs b/hadrian/src/Flavour.hs
index e95e0d7e1f..23e2aa8a81 100644
--- a/hadrian/src/Flavour.hs
+++ b/hadrian/src/Flavour.hs
@@ -1,4 +1,4 @@
-module Flavour (Flavour (..)) where
+module Flavour (Flavour (..), werror) where
 
 import Expression
 
@@ -32,3 +32,9 @@ data Flavour = Flavour {
     ghcProfiled :: Bool,
     -- | Build GHC with debug information.
     ghcDebugged :: Bool }
+
+
+-- | Turn on -Werror for packages built with the stage1 compiler.
+-- It mimics the CI settings so is useful to turn on when developing.
+werror :: Flavour -> Flavour
+werror fl = fl { args = args fl <> (builder Ghc ? notStage0 ? arg "-Werror") }
diff --git a/hadrian/src/UserSettings.hs b/hadrian/src/UserSettings.hs
index 8e0b4fc769..c92dd11d44 100644
--- a/hadrian/src/UserSettings.hs
+++ b/hadrian/src/UserSettings.hs
@@ -2,6 +2,10 @@
 -- hadrian/src/UserSettings.hs to hadrian/UserSettings.hs and edit your copy.
 -- If you don't copy the file your changes will be tracked by git and you can
 -- accidentally commit them.
+--
+-- See doc/user-settings.md for instructions, and src/Flavour.hs for auxiliary
+-- functions for manipulating flavours.
+-- Please update doc/user-settings.md when committing changes to this file.
 module UserSettings (
     userFlavours, userPackages, userDefaultFlavour,
     verboseCommand, buildProgressColour, successColour, finalStage