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, 12 insertions, 2 deletions

diff --git a/hadrian/src/Settings.hs b/hadrian/src/Settings.hs
index 519d1fc80c..fdbef1c359 100755
--- a/hadrian/src/Settings.hs
+++ b/hadrian/src/Settings.hs
@@ -1,7 +1,7 @@
 module Settings (
     getArgs, getLibraryWays, getRtsWays, flavour, knownPackages,
-    findPackageByName, isLibrary, stagePackages, programContext,
-    getIntegerPackage
+    findPackageByName, unsafeFindPackageByName, unsafeFindPackageByPath,
+    isLibrary, stagePackages, programContext, getIntegerPackage
     ) where
 
 import CommandLine
@@ -66,3 +66,13 @@ knownPackages = sort $ ghcPackages ++ userPackages
 -- Note: this is slow but we keep it simple as there are just ~50 packages
 findPackageByName :: PackageName -> Maybe Package
 findPackageByName name = find (\pkg -> pkgName pkg == name) knownPackages
+
+unsafeFindPackageByName :: PackageName -> Package
+unsafeFindPackageByName name = fromMaybe (error msg) $ findPackageByName name
+  where
+    msg = "unsafeFindPackageByName: No package with name " ++ name
+
+unsafeFindPackageByPath :: FilePath -> Package
+unsafeFindPackageByPath path = err $ find (\pkg -> pkgPath pkg == path) knownPackages
+  where
+    err = fromMaybe $ error ("findPackageByPath: No package for path " ++ path)