cabal-install: justify why legacy-fallback is used

This commit makes it so that cabal-install can explain the reason why
it used the legacy fallback, instead of building per-component.

cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs

@@ -740,9 +740,13 @@ buildAndInstallUnpackedPackage
 
       dispname :: String
       dispname = case elabPkgOrComp pkg of
-        ElabPackage _ ->
+        -- Packages built altogether, instead of per component
+        ElabPackage ElaboratedPackage{pkgWhyNotPerComponent} ->
           prettyShow pkgid
-            ++ " (all, legacy fallback)"
+            ++ " (all, legacy fallback: "
+            ++ unwords (map whyNotPerComponent $ NE.toList pkgWhyNotPerComponent)
+            ++ ")"
+        -- Packages built per component
         ElabComponent comp ->
           prettyShow pkgid
             ++ " ("

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

@@ -1613,13 +1613,13 @@ elaborateInstallPlan
                 buildComponent
                 (Map.empty, Map.empty, Map.empty)
                 (map fst src_comps)
-            let not_per_component_reasons = why_not_per_component src_comps
-            if null not_per_component_reasons
-              then return comps
-              else do
-                checkPerPackageOk comps not_per_component_reasons
+            let whyNotPerComp = why_not_per_component src_comps
+            case NE.nonEmpty whyNotPerComp of
+              Nothing -> return comps
+              Just notPerCompReasons -> do
+                checkPerPackageOk comps notPerCompReasons
                 return
-                  [ elaborateSolverToPackage spkg g $
+                  [ elaborateSolverToPackage notPerCompReasons spkg g $
                       comps ++ maybeToList setupComponent
                   ]
           Left cns ->
@@ -1633,7 +1633,6 @@ elaborateInstallPlan
           why_not_per_component g =
             cuz_buildtype ++ cuz_spec ++ cuz_length ++ cuz_flag
             where
-              cuz reason = [text reason]
               -- We have to disable per-component for now with
               -- Configure-type scripts in order to prevent parallel
               -- invocation of the same `./configure` script.
@@ -1646,28 +1645,29 @@ elaborateInstallPlan
               -- Once you've implemented this, swap it for the code below.
               cuz_buildtype =
                 case PD.buildType (elabPkgDescription elab0) of
-                  PD.Configure -> cuz "build-type is Configure"
-                  PD.Custom -> cuz "build-type is Custom"
+                  PD.Configure -> [CuzBuildType CuzConfigureBuildType]
+                  PD.Custom -> [CuzBuildType CuzCustomBuildType]
+                  PD.Make -> [CuzBuildType CuzMakeBuildType]
                   _ -> []
               -- cabal-format versions prior to 1.8 have different build-depends semantics
               -- for now it's easier to just fallback to legacy-mode when specVersion < 1.8
               -- see, https://github.com/haskell/cabal/issues/4121
               cuz_spec
                 | PD.specVersion pd >= CabalSpecV1_8 = []
-                | otherwise = cuz "cabal-version is less than 1.8"
+                | otherwise = [CuzCabalSpecVersion]
               -- In the odd corner case that a package has no components at all
               -- then keep it as a whole package, since otherwise it turns into
               -- 0 component graph nodes and effectively vanishes. We want to
               -- keep it around at least for error reporting purposes.
               cuz_length
                 | length g > 0 = []
-                | otherwise = cuz "there are no buildable components"
+                | otherwise = [CuzNoBuildableComponents]
               -- For ease of testing, we let per-component builds be toggled
               -- at the top level
               cuz_flag
                 | fromFlagOrDefault True (projectConfigPerComponent sharedPackageConfig) =
                     []
-                | otherwise = cuz "you passed --disable-per-component"
+                | otherwise = [CuzDisablePerComponent]
 
           -- \| Sometimes a package may make use of features which are only
           -- supported in per-package mode.  If this is the case, we should
@@ -1679,7 +1679,7 @@ elaborateInstallPlan
               dieProgress $
                 text "Internal libraries only supported with per-component builds."
                   $$ text "Per-component builds were disabled because"
-                  <+> fsep (punctuate comma reasons)
+                  <+> fsep (punctuate comma $ map (text . whyNotPerComponent) $ toList reasons)
           -- TODO: Maybe exclude Backpack too
 
           elab0 = elaborateSolverToCommon spkg
@@ -1974,11 +1974,13 @@ elaborateInstallPlan
               <$> executables
 
       elaborateSolverToPackage
-        :: SolverPackage UnresolvedPkgLoc
+        :: NE.NonEmpty NotPerComponentReason
+        -> SolverPackage UnresolvedPkgLoc
         -> ComponentsGraph
         -> [ElaboratedConfiguredPackage]
         -> ElaboratedConfiguredPackage
       elaborateSolverToPackage
+        pkgWhyNotPerComponent
         pkg@( SolverPackage
                 (SourcePackage pkgid _gpd _srcloc _descOverride)
                 _flags

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

@@ -2,6 +2,7 @@
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE TypeFamilies #-}
@@ -42,6 +43,9 @@ module Distribution.Client.ProjectPlanning.Types
   , MemoryOrDisk (..)
   , isInplaceBuildStyle
   , CabalFileText
+  , NotPerComponentReason (..)
+  , NotPerComponentBuildType (..)
+  , whyNotPerComponent
 
     -- * Build targets
   , ComponentTarget (..)
@@ -117,6 +121,7 @@ import qualified Distribution.Solver.Types.ComponentDeps as CD
 import Distribution.Solver.Types.OptionalStanza
 
 import qualified Data.ByteString.Lazy as LBS
+import qualified Data.List.NonEmpty as NE
 import qualified Data.Map as Map
 import qualified Data.Monoid as Mon
 import System.FilePath ((</>))
@@ -724,12 +729,53 @@ data ElaboratedPackage = ElaboratedPackage
   , pkgStanzasEnabled :: OptionalStanzaSet
   -- ^ Which optional stanzas (ie testsuites, benchmarks) will actually
   -- be enabled during the package configure step.
+  , pkgWhyNotPerComponent :: NE.NonEmpty NotPerComponentReason
+  -- ^ Why is this not a per-component build?
   }
   deriving (Eq, Show, Generic)
 
 instance Binary ElaboratedPackage
 instance Structured ElaboratedPackage
 
+-- | Why did we fall-back to a per-package build, instead of using
+-- a per-component build?
+data NotPerComponentReason
+  = -- | The build-type does not support per-component builds.
+    CuzBuildType !NotPerComponentBuildType
+  | -- | The Cabal spec version is too old for per-component builds.
+    CuzCabalSpecVersion
+  | -- | There are no buildable components, so we fall-back to a per-package
+    -- build for error-reporting purposes.
+    CuzNoBuildableComponents
+  | -- | The user passed @--disable-per-component@.
+    CuzDisablePerComponent
+  deriving (Eq, Show, Generic)
+
+data NotPerComponentBuildType
+  = CuzConfigureBuildType
+  | CuzCustomBuildType
+  | CuzMakeBuildType
+  deriving (Eq, Show, Generic)
+
+instance Binary NotPerComponentBuildType
+instance Structured NotPerComponentBuildType
+
+instance Binary NotPerComponentReason
+instance Structured NotPerComponentReason
+
+-- | Display the reason we had to fall-back to a per-package build instead
+-- of a per-component build.
+whyNotPerComponent :: NotPerComponentReason -> String
+whyNotPerComponent = \case
+  CuzBuildType bt ->
+    "build-type is " ++ case bt of
+      CuzConfigureBuildType -> "Configure"
+      CuzCustomBuildType -> "Custom"
+      CuzMakeBuildType -> "Make"
+  CuzCabalSpecVersion -> "cabal-version is less than 1.8"
+  CuzNoBuildableComponents -> "there are no buildable components"
+  CuzDisablePerComponent -> "you passed --disable-per-component"
+
 -- | See 'elabOrderDependencies'.  This gives the unflattened version,
 -- which can be useful in some circumstances.
 pkgOrderDependencies :: ElaboratedPackage -> ComponentDeps [UnitId]