Prevent rebuilds when the project file changes

...provided that nothing else changes.

cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs

@@ -111,6 +111,9 @@ import Distribution.Simple.Utils (ordNub)
 import Distribution.Solver.Types.ComponentDeps (ComponentDeps)
 import qualified Distribution.Solver.Types.ComponentDeps as CD
 import Distribution.Solver.Types.OptionalStanza
+import Distribution.Solver.Types.ConstraintSource
+  ( ConstraintSource (..)
+  )
 import Distribution.Solver.Types.WithConstraintSource
   ( WithConstraintSource (..)
   )
@@ -348,7 +351,15 @@ normaliseConfiguredPackage
   -> ElaboratedConfiguredPackage
   -> ElaboratedConfiguredPackage
 normaliseConfiguredPackage ElaboratedSharedConfig{pkgConfigCompilerProgs} pkg =
-  pkg{elabProgramArgs = Map.mapMaybeWithKey lookupFilter (elabProgramArgs pkg)}
+  pkg
+    { elabProgramArgs = Map.mapMaybeWithKey lookupFilter (elabProgramArgs pkg)
+    , -- Wipe the constraint source so that if (e.g.) a project file changes we
+      -- don't necessarily force a rebuild if nothing else changes.
+      elabPkgSourceLocation =
+        (elabPkgSourceLocation pkg)
+          { constraintSource = ConstraintSourceUnknown
+          }
+    }
   where
     knownProgramDb = addKnownPrograms builtinPrograms pkgConfigCompilerProgs
 

cabal-testsuite/PackageTests/ConditionalAndImport/cabal.out

@@ -90,12 +90,7 @@ Configuration is affected by the following files:
 - same-filename/noncyclical-same-filename-b.config
     imported by: noncyclical-same-filename-b.project
 Resolving dependencies...
-Build profile: -w ghc-<GHCVER> -O1
-In order, the following will be built:
- - my-0.1 (lib:my) (configuration changed)
-Configuring my-0.1...
-Preprocessing library for my-0.1...
-Building library for my-0.1...
+Up to date
 # checking that cyclical check catches a same file name that imports itself
 # cabal v2-build
 Error: [Cabal-7090]
@@ -182,12 +177,7 @@ Configuration is affected by the following files:
     imported by: hops/hops-1.config
     imported by: hops-0.project
 Resolving dependencies...
-Build profile: -w ghc-<GHCVER> -O1
-In order, the following will be built:
- - my-0.1 (lib:my) (configuration changed)
-Configuring my-0.1...
-Preprocessing library for my-0.1...
-Building library for my-0.1...
+Up to date
 # checking conflicting constraints skipping into a subfolder and then back out again and again
 # cabal v2-build
 Configuration is affected by the following files:
@@ -391,12 +381,7 @@ Configuration is affected by the following files:
     imported by: yops/yops-1.config
     imported by: yops-0.project
 Resolving dependencies...
-Build profile: -w ghc-<GHCVER> -O1
-In order, the following will be built:
- - my-0.1 (lib:my) (configuration changed)
-Configuring my-0.1...
-Preprocessing library for my-0.1...
-Building library for my-0.1...
+Up to date
 # checking bad conditional
 # cabal v2-build
 Error: [Cabal-7090]