[Fix #11850] Build dependencies of build tool on disk too

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

@@ -3766,6 +3766,11 @@ pruneInstallPlanPass2 pkgs =
               elabBuildTargets elab
                 ++ libTargetsRequiredForRevDeps
                 ++ exeTargetsRequiredForRevDeps
+        , elabBuildStyle =
+            if installedUnitId elab `Set.member` mustBuildOnDisk
+              && elabBuildStyle elab == BuildInplaceOnly InMemory
+              then BuildInplaceOnly OnDisk
+              else elabBuildStyle elab
         , elabPkgOrComp =
             case elabPkgOrComp elab of
               ElabPackage pkg ->
@@ -3802,10 +3807,13 @@ pruneInstallPlanPass2 pkgs =
 
         libTargetsRequiredForRevDeps =
           [ c
-          | installedUnitId elab `Set.member` hasReverseLibDeps
+          | installedUnitId elab `Set.member` libDeps
           , let c = ComponentTarget (CLibName Cabal.defaultLibName) WholeComponent
-          , -- Don't enable building for anything which is being build in memory
+          , -- Don't enable building for anything which is being built in memory,
+          -- unless it is a (transitive) library dependency of an exe build tool,
+          -- in which case it must be compiled to disk so the exe can link against it.
           elabBuildStyle elab /= BuildInplaceOnly InMemory
+            || installedUnitId elab `Set.member` mustBuildOnDisk
           ]
         exeTargetsRequiredForRevDeps =
           -- TODO: allow requesting executable with different name
@@ -3817,35 +3825,73 @@ pruneInstallPlanPass2 pkgs =
                     elabPkgSourceId elab
             )
             WholeComponent
-          | installedUnitId elab `Set.member` hasReverseExeDeps
+          | installedUnitId elab `Set.member` exeDeps
           ]
 
     availablePkgs :: Set UnitId
     availablePkgs = Set.fromList (map installedUnitId pkgs)
 
     inMemoryTargets :: Set ConfiguredId
-    inMemoryTargets = do
+    inMemoryTargets =
       Set.fromList
         [ configuredId pkg
         | InstallPlan.Configured pkg <- pkgs
+        , -- Exclude packages that must be built on disk (for exe build tools).
+        -- Their dependents will receive a real -package-id, not a promise.
+        installedUnitId pkg `Set.notMember` mustBuildOnDisk
         , BuildInplaceOnly InMemory <- [elabBuildStyle pkg]
         ]
 
-    hasReverseLibDeps :: Set UnitId
-    hasReverseLibDeps =
-      Set.fromList
-        [ depid
-        | InstallPlan.Configured pkg <- pkgs
-        , depid <- elabOrderLibDependencies pkg
-        ]
+    -- Packages that must be built on disk because they are (transitively)
+    -- needed as library dependencies by an exe build-tool.  Even if such a
+    -- package was originally marked InMemory for the multi-repl session, it
+    -- must also produce real on-disk artifacts so the exe can link against it.
+    -- The repl phase still runs for these packages, so GHCi loads them from
+    -- source under the same package-id, preserving type identity.
+    mustBuildOnDisk :: Set UnitId
+    mustBuildOnDisk = go exeDeps Set.empty
+      where
+        go frontier visited
+          | Set.null frontier = visited
+          | otherwise =
+              let newVisited = visited <> frontier
+                  newFrontier =
+                    Set.fromList
+                      [ dep
+                      | uid <- Set.toList frontier
+                      , dep <- Map.findWithDefault [] uid planLibDepMap
+                      ]
+                      Set.\\ newVisited
+               in go newFrontier newVisited
+
+    -- Lib and exe deps computed once per in-plan package, shared below.
+    -- Note: these must be the order-dependency UnitIds, which match the
+    -- 'installedUnitId' of the units in the plan; in particular for
+    -- Backpack-instantiated units they include the instantiation, unlike
+    -- the ComponentIds from 'elabLibDependencies'.
+    perPkgDeps :: [(UnitId, [UnitId], [UnitId])]
+    perPkgDeps =
+      [ ( installedUnitId pkg
+        , elabOrderLibDependencies pkg
+        , elabOrderExeDependencies pkg
+        )
+      | InstallPlan.Configured pkg <- pkgs
+      ]
 
-    hasReverseExeDeps :: Set UnitId
-    hasReverseExeDeps =
-      Set.fromList
-        [ depid
-        | InstallPlan.Configured pkg <- pkgs
-        , depid <- elabOrderExeDependencies pkg
-        ]
+    -- Library-dependency adjacency for in-plan packages only.
+    planLibDepMap :: Map UnitId [UnitId]
+    planLibDepMap =
+      Map.fromList [(uid, ls) | (uid, ls, _) <- perPkgDeps]
+
+    -- All packages that appear as a library dep of any in-plan package.
+    libDeps :: Set UnitId
+    libDeps =
+      Set.fromList [d | (_, ls, _) <- perPkgDeps, d <- ls]
+
+    -- All packages that appear as an exe dep of any in-plan package.
+    exeDeps :: Set UnitId
+    exeDeps =
+      Set.fromList [d | (_, _, es) <- perPkgDeps, d <- es]
 
 mapConfiguredPackage
   :: (srcpkg -> srcpkg')

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/app/Main.hs

@@ -0,0 +1,6 @@
+module Main (main) where
+
+import MyLib (mylib)
+
+main :: IO ()
+main = print mylib

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/bar/Main.hs

@@ -0,0 +1,6 @@
+module Main (main) where
+
+import MyLib (mylib)
+
+main :: IO ()
+main = print (mylib + 1)

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/cabal.all.out

@@ -0,0 +1,19 @@
+# cabal v2-repl
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - foo-0.1.0.0 (lib) (first run)
+ - foo-0.1.0.0 (exe:foo) (first run)
+ - foo-0.1.0.0 (interactive) (exe:bar) (first run)
+Configuring library for foo-0.1.0.0...
+Preprocessing library for foo-0.1.0.0...
+Building library for foo-0.1.0.0...
+Preprocessing library for foo-0.1.0.0...
+Configuring executable 'foo' for foo-0.1.0.0...
+Preprocessing executable 'foo' for foo-0.1.0.0...
+Building executable 'foo' for foo-0.1.0.0...
+Preprocessing executable 'foo' for foo-0.1.0.0...
+Configuring executable 'bar' for foo-0.1.0.0...
+Preprocessing executable 'bar' for foo-0.1.0.0...
+# foo foo
+3735929054

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/cabal.exes.out

@@ -0,0 +1,18 @@
+# cabal v2-repl
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - foo-0.1.0.0 (lib) (first run)
+ - foo-0.1.0.0 (exe:foo) (first run)
+ - foo-0.1.0.0 (interactive) (exe:bar) (first run)
+Configuring library for foo-0.1.0.0...
+Preprocessing library for foo-0.1.0.0...
+Building library for foo-0.1.0.0...
+Configuring executable 'foo' for foo-0.1.0.0...
+Preprocessing executable 'foo' for foo-0.1.0.0...
+Building executable 'foo' for foo-0.1.0.0...
+Preprocessing executable 'foo' for foo-0.1.0.0...
+Configuring executable 'bar' for foo-0.1.0.0...
+Preprocessing executable 'bar' for foo-0.1.0.0...
+# foo foo
+3735929054

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/cabal.project

@@ -0,0 +1 @@
+packages: .

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/cabal.test.hs

@@ -0,0 +1,36 @@
+import Test.Cabal.Prelude
+
+-- Test for issue #11850: in a multi-repl session, an executable which is a
+-- build-tool-depends of another repl target must be compiled to disk together
+-- with its library dependencies (not just loaded in memory), so the build
+-- tool can be linked and run.
+main = do
+  -- The library is itself a repl target here, as in the original report of
+  -- issue #11850: before the fix it was loaded in memory only, so building
+  -- the build tool on disk failed with
+  -- "cannot satisfy -package-id foo-0.1.0.0-inplace".
+  cabalTest' "all" $ do
+    skipUnlessGhcVersion ">= 9.4"
+    res <- cabalWithStdin "v2-repl" ["--enable-multi-repl", "all"] "print 0xdeadbeef"
+    -- The tool and its library dep are built on disk, not merely loaded in memory
+    assertOutputContains "Building executable 'foo' for foo-0.1.0.0" res
+    -- The multi-repl session is functional (0xdeadbeef = 3735928559)
+    assertOutputContains "3735928559" res
+    -- The tool binary exists on disk and was linked against the on-disk library
+    -- (0xdeadc0de = 3735929054)
+    withPlan $ do
+      r <- runPlanExe' "foo" "foo" []
+      assertOutputContains "3735929054" r
+
+  -- The repl targets are the two executables; the library is not a repl
+  -- target but is still pulled onto disk as a dependency of the build tool.
+  cabalTest' "exes" $ do
+    skipUnlessGhcVersion ">= 9.4"
+    res <- cabalWithStdin "v2-repl" ["--enable-multi-repl", "exe:foo", "exe:bar"] "print 0xdeadbeef"
+    assertOutputContains "Building executable 'foo' for foo-0.1.0.0" res
+    -- Both executables are loaded into the multi-repl session
+    assertOutputContains "Ok, two modules loaded." res
+    assertOutputContains "3735928559" res
+    withPlan $ do
+      r <- runPlanExe' "foo" "foo" []
+      assertOutputContains "3735929054" r

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/foo.cabal

@@ -0,0 +1,23 @@
+cabal-version: 3.0
+name: foo
+version: 0.1.0.0
+build-type: Simple
+
+library
+  default-language: Haskell2010
+  hs-source-dirs: src
+  exposed-modules: MyLib
+  build-depends: base
+
+executable foo
+  default-language: Haskell2010
+  hs-source-dirs: app
+  main-is: Main.hs
+  build-depends: base, foo
+
+executable bar
+  default-language: Haskell2010
+  hs-source-dirs: bar
+  main-is: Main.hs
+  build-depends: base, foo
+  build-tool-depends: foo:foo

cabal-testsuite/PackageTests/MultiRepl/BuildToolDepends/src/MyLib.hs

@@ -0,0 +1,4 @@
+module MyLib (mylib) where
+
+mylib :: Integer
+mylib = 0xdeadc0de

changelog.d/11850.md

@@ -0,0 +1,13 @@
+---
+synopsis: "Build `build-tool-depends` executables on disk in multi-repl sessions"
+packages: [cabal-install]
+prs: 11850
+issues: 11850
+---
+
+`cabal repl --enable-multi-repl` failed when one repl target had a
+`build-tool-depends` on an executable of the same project: the executable and
+its library dependencies were only loaded in memory and never compiled to
+disk, so the build tool could not be built. The build tool and its transitive
+library dependencies are now built on disk, while still being loaded from
+source in the repl session.