Respect GHC_CHARENC environment variable #10762

Only supports UTF-8 as a value right now. I expect some discussion to go
on around the naming of this variable and whether it's valid to backport
it to GHC 7.10 (which would be my preference). The motivation here is
that, when capturing the output of GHC to a file, we often want to
ensure that the output is UTF-8, regardless of the actual character
encoding of the terminal/console.

On the other hand, we don't want to necessary change the
terminal/console encoding. The reason being:

* On Windows, this requires a global-esque change to the console
  codepage, which adversely affects other processes in the same console

* On all OSes, this can break features like smart quote auto-detection.

Test Plan:
Set LANG to C, GHC_CHARENC to UTF-8, and compile a Haskell source
file with a non-ASCII warning produced. The output who include the UTF-8
sequence instead of replacing it with ?.

Reviewers: austin, rwbarton, bgamari

Reviewed By: bgamari

Subscribers: hsyl20, thomie

Differential Revision: https://phabricator.haskell.org/D1167

GHC Trac Issues: #10762

1 files changed, 12 insertions, 2 deletions

diff --git a/ghc/Main.hs b/ghc/Main.hs
index ed2ac67fa6..a1a4eccb12 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -80,8 +80,18 @@ main = do
    initGCStatistics -- See Note [-Bsymbolic and hooks]
    hSetBuffering stdout LineBuffering
    hSetBuffering stderr LineBuffering
-   hSetTranslit stdout
-   hSetTranslit stderr
+
+   -- Handle GHC-specific character encoding flags, allowing us to control how
+   -- GHC produces output regardless of OS.
+   env <- getEnvironment
+   case lookup "GHC_CHARENC" env of
+    Just "UTF-8" -> do
+     hSetEncoding stdout utf8
+     hSetEncoding stderr utf8
+    _ -> do
+     -- Avoid GHC erroring out when trying to display unhandled characters
+     hSetTranslit stdout
+     hSetTranslit stderr
 
    GHC.defaultErrorHandler defaultFatalMessager defaultFlushOut $ do
     -- 1. extract the -B flag from the args