Hadrian: Fix untracked dependencies

This is a preparation for #16295: https://ghc.haskell.org/trac/ghc/ticket/16295

This commit mostly focuses on getting rid of untracked dependencies,
which prevent Shake's new `--shared` feature from appropriately caching
build rules.

There are three different solutions to untracked dependencies:

* Track them! This is the obvious and the best approach, but in some
  situations we cannot use it, for example, because a build rule creates
  files whose names are not known statically and hence cannot be
  specified as the rule's outputs.

* Use Shake's `produces` to record outputs dynamically, within the rule.

* Use Shake's `historyDisable` to disable caching for a particular build
  rule. We currently use this approach only for `ghc-pkg` which mutates
  the package database and the file `package.cache`.

These two tickets are fixed as the result:

Ticket #16271: ​https://ghc.haskell.org/trac/ghc/ticket/16271

Ticket #16272: ​https://ghc.haskell.org/trac/ghc/ticket/16272 (this one
is fixed only partially: we correctly record the dependency, but we
still copy files into the RTS build tree).

1 files changed, 8 insertions, 1 deletions

diff --git a/hadrian/src/Oracles/ModuleFiles.hs b/hadrian/src/Oracles/ModuleFiles.hs
index 1e508c0090..d2f0299563 100644
--- a/hadrian/src/Oracles/ModuleFiles.hs
+++ b/hadrian/src/Oracles/ModuleFiles.hs
@@ -81,7 +81,9 @@ findGenerator Context {..} file = do
 -- | Find all Haskell source files for a given 'Context'.
 hsSources :: Context -> Action [FilePath]
 hsSources context = do
-    let modFile (m, Nothing   ) = generatedFile context m
+    let modFile (m, Nothing)
+            | "Paths_" `isPrefixOf` m = autogenFile context m
+            | otherwise               = generatedFile context m
         modFile (m, Just file )
             | takeExtension file `elem` haskellExtensions = return file
             | otherwise = generatedFile context m
@@ -99,6 +101,10 @@ hsObjects context = do
 generatedFile :: Context -> ModuleName -> Action FilePath
 generatedFile context moduleName = buildPath context <&> (-/- moduleSource moduleName)
 
+-- | Generated module files live in the 'Context' specific build directory.
+autogenFile :: Context -> ModuleName -> Action FilePath
+autogenFile context modName = autogenPath context <&> (-/- moduleSource modName)
+
 -- | Turn a module name (e.g. @Data.Functor@) to a path (e.g. @Data/Functor.hs@).
 moduleSource :: ModuleName -> FilePath
 moduleSource moduleName = replaceEq '.' '/' moduleName <.> "hs"
@@ -125,6 +131,7 @@ moduleFilesOracle :: Rules ()
 moduleFilesOracle = void $ do
     void . addOracleCache $ \(ModuleFiles (stage, package)) -> do
         let context = vanillaContext stage package
+        ensureConfigured context
         srcDirs <- interpretInContext context (getContextData PD.srcDirs)
         mainIs  <- interpretInContext context (getContextData PD.mainIs)
         let removeMain = case mainIs of