[commit: ghc] master: Add some more comments to UsageFile. (b6a572b)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/b6a572b4272274efe117d5e04f4b9d49e124009c/ghc

>---------------------------------------------------------------

commit b6a572b4272274efe117d5e04f4b9d49e124009c
Author: Austin Seipp <aseipp at pobox.com>
Date:   Thu Aug 22 17:32:27 2013 -0500

    Add some more comments to UsageFile.
    
    This brings them up to date with the changes in #8144.
    
    Signed-off-by: Austin Seipp <aseipp at pobox.com>


>---------------------------------------------------------------

b6a572b4272274efe117d5e04f4b9d49e124009c
 compiler/main/HscTypes.lhs |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/compiler/main/HscTypes.lhs b/compiler/main/HscTypes.lhs
index a65437e..2603770 100644
--- a/compiler/main/HscTypes.lhs
+++ b/compiler/main/HscTypes.lhs
@@ -1794,10 +1794,17 @@ data Usage
         usg_safe :: IsSafeImport
             -- ^ Was this module imported as a safe import
     }                                           -- ^ Module from the current package
+  -- | A file upon which the module depends, e.g. a CPP #include, or using TH's
+  -- 'addDependentFile'
   | UsageFile {
         usg_file_path  :: FilePath,
+        -- ^ External file dependency. From a CPP #include or TH
+        -- addDependentFile. Should be absolute.
         usg_file_hash  :: Fingerprint
-        -- ^ External file dependency. From a CPP #include or TH addDependentFile. Should be absolute.
+        -- ^ 'Fingerprint' of the file contents. We don't consider
+        -- things like modification timestamps here, because there's
+        -- no reason to recompile if the actual contents don't change.
+        -- This previously lead to odd recompilation behaviors; see #8114
   }
     deriving( Eq )
         -- The export list field is (Just v) if we depend on the export list: