hadrian: Always specify flag values explicitly

Previously we would often allow cabal flags to default, making it harder
than necessary to reason about the effective build configuration.

(cherry picked from commit dd3c9602)

hadrian/src/Expression.hs

+{-# LANGUAGE FlexibleContexts #-}
+
 module Expression (
     -- * Expressions
     Expr, Predicate, Args, Ways,
 
     -- ** Construction and modification
-    expr, exprIO, arg, remove,
+    expr, exprIO, arg, remove, cabalFlag,
 
     -- ** Predicates
     (?), stage, stage0, stage1, stage2, notStage0, threadedBootstrapper,
@@ -131,3 +133,11 @@ notPackage = notM . package
 -- | Is a library package currently being built?
 libraryPackage :: Predicate
 libraryPackage = isLibrary <$> getPackage
+
+-- | Either @-flagName@ or @flagName@, depending upon a predicate.
+-- For use in @Cabal Flags@ argument lists.
+cabalFlag :: ToPredicate p Context Builder => p -> String -> Args
+cabalFlag pred flagName = do
+    ifM (toPredicate pred) (arg flagName) (arg $ "-"<>flagName)
+
+infixr 3 `cabalFlag`

hadrian/src/Hadrian/Expression.hs

@@ -8,6 +8,7 @@ module Hadrian.Expression (
 
     -- ** Predicates
     (?), input, inputs, output, outputs, VerboseCommand (..), verboseCommand,
+    ToPredicate(..),
 
     -- ** Evaluation
     interpret, interpretInContext,

hadrian/src/Settings/Packages.hs

@@ -25,7 +25,7 @@ packageArgs = do
     mconcat
         --------------------------------- base ---------------------------------
         [ package base ? mconcat
-          [ builder (Cabal Flags) ? notStage0 ? arg (pkgName ghcBignum)
+          [ builder (Cabal Flags) ? notStage0 `cabalFlag` (pkgName ghcBignum)
 
           -- This fixes the 'unknown symbol stat' issue.
           -- See: https://github.com/snowleopard/hadrian/issues/259.
@@ -65,8 +65,8 @@ packageArgs = do
               notStage0 ? arg "--ghc-pkg-option=--force" ]
 
           , builder (Cabal Flags) ? mconcat
-            [ ghcWithInterpreter ? notStage0 ? arg "internal-interpreter"
-            , cross ? arg "-terminfo"
+            [ andM [expr ghcWithInterpreter, notStage0] `cabalFlag` "internal-interpreter"
+            , notM cross `cabalFlag` "terminfo"
             ]
 
           , builder (Haddock BuildPackage) ? arg ("--optghc=-I" ++ path) ]
@@ -76,25 +76,22 @@ packageArgs = do
           [ builder Ghc ? arg ("-I" ++ compilerPath)
 
           , builder (Cabal Flags) ? mconcat
-            [ ghcWithInterpreter ? notStage0 ? arg "internal-interpreter"
-            , cross ? arg "-terminfo"
+            [ andM [expr ghcWithInterpreter, notStage0] `cabalFlag` "internal-interpreter"
+            , notM cross `cabalFlag` "terminfo"
             , ifM stage0
                   -- We build a threaded stage 1 if the bootstrapping compiler
                   -- supports it.
-                  (ifM threadedBootstrapper
-                       (arg "threaded")
-                       (arg "-threaded"))
+                  (threadedBootstrapper `cabalFlag` "threaded")
+
                   -- We build a threaded stage N, N>1 if the configuration calls
                   -- for it.
-                  (ifM (ghcThreaded <$> expr flavour)
-                       (arg "threaded")
-                       (arg "-threaded"))
+                  ((ghcThreaded <$> expr flavour) `cabalFlag` "threaded")
             ]
           ]
 
         -------------------------------- ghcPkg --------------------------------
         , package ghcPkg ?
-          builder (Cabal Flags) ? cross ? arg "-terminfo"
+          builder (Cabal Flags) ? notM cross `cabalFlag` "terminfo"
 
         -------------------------------- ghcPrim -------------------------------
         , package ghcPrim ? mconcat
@@ -105,8 +102,7 @@ packageArgs = do
 
         --------------------------------- ghci ---------------------------------
         , package ghci ? mconcat
-          [ notStage0 ? builder (Cabal Flags) ? arg "internal-interpreter"
-
+          [
           -- The use case here is that we want to build @iserv-proxy@ for the
           -- cross compiler. That one needs to be compiled by the bootstrap
           -- compiler as it needs to run on the host. Hence @libiserv@ needs
@@ -133,7 +129,9 @@ packageArgs = do
           -- the Stage1 libraries, as we already know that the bootstrap
           -- compiler comes with the same versions as the one we are building.
           --
-          , cross ? stage0 ? bootCross ? builder (Cabal Flags) ? arg "internal-interpreter"
+            builder (Cabal Flags) ? ifM stage0
+              (andM [cross, bootCross] `cabalFlag` "internal-interpreter")
+              (arg "internal-interpreter")
 
           ]
 
@@ -163,7 +161,7 @@ packageArgs = do
         -- dependencies.
         -- TODO: Perhaps the user should rather be responsible for this?
         , package haskeline ?
-          builder (Cabal Flags) ? cross ? arg "-terminfo"
+          builder (Cabal Flags) ? notM cross `cabalFlag` "terminfo"
 
         -------------------------------- hsc2hs --------------------------------
         , package hsc2hs ?
@@ -195,7 +193,7 @@ ghcBignumArgs = package ghcBignum ? do
             builder (Cabal Flags) ? arg backend
 
           , -- check the selected backend against native backend
-            builder (Cabal Flags) ? check ? arg "check"
+            builder (Cabal Flags) ? check `cabalFlag` "check"
 
             -- backend specific
           , case backend of
@@ -358,12 +356,12 @@ rtsPackageArgs = package rts ? do
 
     mconcat
         [ builder (Cabal Flags) ? mconcat
-          [ any (wayUnit Profiling) rtsWays ? arg "profiling"
-          , any (wayUnit Debug) rtsWays     ? arg "debug"
-          , any (wayUnit Logging) rtsWays   ? arg "logging"
-          , any (wayUnit Dynamic) rtsWays   ? arg "dynamic"
-          , useLibffiForAdjustors           ? arg "libffi-adjustors"
-          , Debug `wayUnit` way             ? arg "find-ptr"
+          [ any (wayUnit Profiling) rtsWays `cabalFlag` "profiling"
+          , any (wayUnit Debug) rtsWays     `cabalFlag` "debug"
+          , any (wayUnit Logging) rtsWays   `cabalFlag` "logging"
+          , any (wayUnit Dynamic) rtsWays   `cabalFlag` "dynamic"
+          , useLibffiForAdjustors           `cabalFlag` "libffi-adjustors"
+          , Debug `wayUnit` way             `cabalFlag` "find-ptr"
           ]
         , builder (Cabal Setup) ? mconcat
           [ if not (null libdwLibraryDir) then arg ("--extra-lib-dirs="++libdwLibraryDir) else mempty