Use en_US.UTF-8 for pure nix mode

ChangeLog.md

@@ -71,6 +71,9 @@ Other enhancements:
   e.g. `stack unpack packagename`.
 * Git repos are shared across multiple projects. See
   [#3551](https://github.com/commercialhaskell/stack/issues/3551)
+* Use en_US.UTF-8 locale by default in pure Nix mode so programs won't
+  crash because of Unicode in their output
+  [#4095](https://github.com/commercialhaskell/stack/issues/4095)
 
 Bug fixes:
 

src/Stack/Nix.hs

@@ -100,6 +100,8 @@ runShellAndExit mprojectRoot getCompilerVersion getCmdArgs = do
                               ,"LD_LIBRARY_PATH = libPath;"  -- LD_LIBRARY_PATH is set because for now it's
                                -- needed by builds using Template Haskell
                               ,"STACK_IN_NIX_EXTRA_ARGS = stackExtraArgs; "
+                               -- overriding default locale so Unicode output using base won't be broken
+                              ,"LANG=\"en_US.UTF-8\";"
                               ,"} \"\""]]
                     -- glibcLocales is necessary on Linux to avoid warnings about GHC being incapable to set the locale.
          fullArgs = concat [if pureShell then ["--pure"] else []

test/integration/tests/4095-utf8-pure-nix/Main.hs

@@ -0,0 +1,6 @@
+import StackTest
+
+main :: IO ()
+main = do
+  stack ["build", "--nix-pure"]
+  stack ["exec", "--nix-pure", "ShowUnicode"]

test/integration/tests/4095-utf8-pure-nix/files/ShowUnicode.hs

@@ -0,0 +1,5 @@
+import System.IO (stdout)
+import Text.Printf (hPrintf)
+
+main :: IO ()
+main = hPrintf stdout "平和"

test/integration/tests/4095-utf8-pure-nix/files/package.yaml

@@ -0,0 +1,4 @@
+executables:
+ ShowUnicode:
+  dependencies: [base]
+  main: ShowUnicode.hs

test/integration/tests/4095-utf8-pure-nix/files/stack.yaml

@@ -0,0 +1 @@
+resolver: lts-11.19