Changes the error message from:

```
Error:
    The following packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used.
planned package consumer-0.1.0.0 is broken due to missing package
  framework-0.1.0.0+95RTb42ZWxa9J13cUStM0q
````

To

```
Error:
    The following packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used.
      planned package consumer-0.1.0.0
          is broken due to missing package
            framework-0.1.0.0 (has unfilled signature: App)
              The package is installed as indefinite.
              To use it, rebuild it in the same cabal project as the consumer so cabal can fill the signatures.
```

Hopefully it gives more guidence on what's going on to the user. I encountered this when trying to use backpack in a nix setup (for example jappeace-sloth/prrrrrrrrr#2) It keeps on wanting to build everything locally and I couldn't understand why. I still don't fully understand why it can't have partially filled modules upstream. It claims it's because of nix but I don't understand it.

Author: jappeace-sloth

Cabal/src/Distribution/Backpack/Configure.hs

@@ -31,6 +31,7 @@ import qualified Distribution.Compat.Graph as Graph
 import Distribution.InstalledPackageInfo
   ( InstalledPackageInfo
   , emptyInstalledPackageInfo
+  , requiredSignatures
   )
 import qualified Distribution.InstalledPackageInfo as Installed
 import Distribution.ModuleName
@@ -261,6 +262,17 @@ toComponentLocalBuildInfos
       packageDependsIndex = PackageIndex.fromList (lefts local_graph)
       fullIndex = Graph.fromDistinctList local_graph
 
+    let -- Map from dependency UnitId to its PackageId, built from includes
+        -- of all ready components.  Used to resolve opaque hashed UnitIds
+        -- in broken-package error messages.
+        depPkgMap :: Map UnitId PackageId
+        depPkgMap = Map.fromList
+          [ (unDefUnitId (ci_id ci), ci_pkgid ci)
+          | rc <- graph
+          , Right instc <- [rc_i rc]
+          , ci <- instc_includes instc
+          ]
+
     case Graph.broken fullIndex of
       [] -> return ()
       -- If there are promised dependencies, we don't know what the dependencies
@@ -270,26 +282,25 @@ toComponentLocalBuildInfos
       broken
         | not (null promisedPkgDeps) -> return ()
         | otherwise ->
-            -- TODO: ppr this
-            dieProgress . text $
-              "The following packages are broken because other"
-                ++ " packages they depend on are missing. These broken "
-                ++ "packages must be rebuilt before they can be used.\n"
-                -- TODO: Undupe.
-                ++ unlines
-                  [ "installed package "
-                    ++ prettyShow (packageId pkg)
-                    ++ " is broken due to missing package "
-                    ++ intercalate ", " (map prettyShow deps)
+            dieProgress $
+              text "The following packages are broken because other"
+                <+> text "packages they depend on are missing. These broken"
+                <+> text "packages must be rebuilt before they can be used."
+                $$ nest 2 (vcat $
+                  [ hang (text "installed package" <+> pretty (packageId pkg)) 4
+                      (text "is broken due to missing package"
+                        <+> hsep (punctuate comma (map pretty deps)))
                   | (Left pkg, deps) <- broken
                   ]
-                ++ unlines
-                  [ "planned package "
-                    ++ prettyShow (packageId pkg)
-                    ++ " is broken due to missing package "
-                    ++ intercalate ", " (map prettyShow deps)
+                  ++
+                  [ hang (text "planned package" <+> pretty (packageId pkg)) 4
+                      (vcat $
+                        text "is broken due to missing package"
+                        : [ nest 2 (dispMissingDep installedPackageSet depPkgMap dep)
+                          | dep <- deps
+                          ])
                   | (Right pkg, deps) <- broken
-                  ]
+                  ])
 
     -- In this section, we'd like to look at the 'packageDependsIndex'
     -- and see if we've picked multiple versions of the same
@@ -338,6 +349,39 @@ toComponentLocalBuildInfos
     -- forM clbis $ \(clbi,deps) -> info verbosity $ "UNIT" ++ hashUnitId (componentUnitId clbi) ++ "\n" ++ intercalate "\n" (map hashUnitId deps)
     return (clbis, packageDependsIndex)
 
+-- | Pretty-print a missing dependency, resolving opaque hashed 'UnitId's
+-- to their human-readable package id and signature info when possible.
+--
+-- When an indefinite Backpack package is installed separately (e.g. via
+-- nix callCabal2nix), only the indefinite variant (with unfilled signatures)
+-- exists in the package DB.  The consumer needs an instantiated variant
+-- which was never built.  The fix is to add both packages to the same
+-- cabal project so cabal can fill the signatures.
+dispMissingDep
+  :: InstalledPackageIndex  -- ^ all installed packages
+  -> Map UnitId PackageId  -- ^ dep UnitId to its PackageId (from includes)
+  -> UnitId  -- ^ the missing dependency
+  -> Doc
+dispMissingDep installedPkgSet depPkgMap uid =
+  case Map.lookup uid depPkgMap of
+    Just pkgid ->
+      let ipiSigs = [ sigs
+                     | ipi <- PackageIndex.lookupSourcePackageId installedPkgSet pkgid
+                     , let sigs = requiredSignatures ipi
+                     , not (Set.null sigs)
+                     ]
+      in case ipiSigs of
+        (sigs : _) ->
+          pretty pkgid
+            <+> parens (text "has unfilled"
+              <+> (if Set.size sigs > 1 then text "signatures:" else text "signature:")
+              <+> hsep (punctuate comma (map pretty (Set.toList sigs))))
+            $$ nest 2 (text "The package is installed as indefinite."
+              $$ text "To use it, rebuild it in the same cabal project as the"
+              <+> text "consumer so cabal can fill the signatures.")
+        [] -> pretty pkgid <+> parens (pretty uid)
+    Nothing -> pretty uid
+
 -- Build ComponentLocalBuildInfo for each component we are going
 -- to build.
 --

cabal-testsuite/PackageTests/Backpack/Fail5/Fail5.cabal

@@ -0,0 +1,24 @@
+cabal-version: 3.0
+name:          Fail5
+version:       0.1.0.0
+build-type:    Simple
+
+library framework
+  signatures:       App
+  hs-source-dirs:   repo/framework-0.1.0.0/src-sig
+  build-depends:    base
+  default-language: Haskell2010
+  visibility:       public
+
+library app-impl
+  exposed-modules:  App
+  hs-source-dirs:   repo/app-impl-0.1.0.0
+  build-depends:    base
+  default-language: Haskell2010
+  visibility:       public
+
+executable consumer
+  main-is:          Main.hs
+  build-depends:    base, Fail5:framework, Fail5:app-impl
+  hs-source-dirs:   repo/consumer-0.1.0.0
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/app-impl-0.1.0.0/App.hs

@@ -0,0 +1,3 @@
+module App where
+appMessage :: String
+appMessage = "Hello from app-impl"

cabal-testsuite/PackageTests/Backpack/Fail5/repo/app-impl-0.1.0.0/app-impl.cabal

@@ -0,0 +1,9 @@
+cabal-version: 3.0
+name:          app-impl
+version:       0.1.0.0
+build-type:    Simple
+
+library
+  exposed-modules:  App
+  build-depends:    base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/consumer-0.1.0.0/Main.hs

@@ -0,0 +1,3 @@
+import App (appMessage)
+main :: IO ()
+main = putStrLn appMessage

cabal-testsuite/PackageTests/Backpack/Fail5/repo/consumer-0.1.0.0/consumer.cabal

@@ -0,0 +1,9 @@
+cabal-version: 3.0
+name:          consumer
+version:       0.1.0.0
+build-type:    Simple
+
+executable consumer
+  main-is:          Main.hs
+  build-depends:    base, framework, app-impl
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/framework.cabal

@@ -0,0 +1,10 @@
+cabal-version: 3.0
+name:          framework
+version:       0.1.0.0
+build-type:    Simple
+
+library
+  signatures:       App
+  hs-source-dirs:   src-sig
+  build-depends:    base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/Backpack/Fail5/repo/framework-0.1.0.0/src-sig/App.hsig

@@ -0,0 +1,2 @@
+signature App where
+appMessage :: String

cabal-testsuite/PackageTests/Backpack/Fail5/setup-external-fail.out

@@ -0,0 +1,9 @@
+# Setup configure
+Configuring consumer-0.1.0.0...
+Error:
+    The following packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used.
+      planned package consumer-0.1.0.0
+          is broken due to missing package
+            framework-0.1.0.0 (has unfilled signature: App)
+              The package is installed as indefinite.
+              To use it, rebuild it in the same cabal project as the consumer so cabal can fill the signatures.

cabal-testsuite/PackageTests/Backpack/Fail5/setup-external-fail.test.hs

@@ -0,0 +1,16 @@
+import Test.Cabal.Prelude
+main = setupAndCabalTest $ do
+    skipUnlessGhcVersion ">= 8.1"
+    withPackageDb $ do
+      -- Install framework WITHOUT instantiation (simulates nix callCabal2nix)
+      recordMode DoNotRecord $
+        withDirectory "repo/framework-0.1.0.0" $ setup_install []
+      -- Install the App implementation package separately
+      recordMode DoNotRecord $
+        withDirectory "repo/app-impl-0.1.0.0" $ setup_install []
+      -- Configure consumer — should fail because the instantiated
+      -- framework (with App=app-impl:App) was never built.
+      -- The exact error message is checked via the .out file.
+      withDirectory "repo/consumer-0.1.0.0" $ do
+        fails $ setup' "configure" []
+        return ()