hadrian: Fix building from source-dist without alex/happy

This fixes two bugs which were adding dependencies on alex/happy when
building from a source dist.

* When we try to pass `--with-alex` and `--with-happy` to cabal when
  configuring but the builders are not set. This is fixed by making them
  optional.
* When we configure, cabal requires alex/happy because of the
  build-tool-depends fields. These are now made optional with a cabal
  flag (build-tool-depends) for compiler/hpc-bin/genprimopcode.

Fixes #21627

5 files changed, 33 insertions, 3 deletions

diff --git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index 9e61e0e2a2..74d95cdc65 100644
--- a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ -56,6 +56,12 @@ Flag dynamic-system-linker
     Default: True
     Manual: True
 
+-- hadrian disables this flag because the user may be building from a source
+-- distribution where the parser has already been generated.
+Flag build-tool-depends
+    Description: Use build-tool-depends
+    Default: True
+
 Library
     Default-Language: Haskell2010
     Exposed: False
@@ -67,7 +73,8 @@ Library
               FunTypes.h
               ghc-llvm-version.h
 
-    build-tool-depends: alex:alex >= 3.2.6, happy:happy >= 1.20.0, genprimopcode:genprimopcode, deriveConstants:deriveConstants
+    if flag(build-tool-depends)
+      build-tool-depends: alex:alex >= 3.2.6, happy:happy >= 1.20.0, genprimopcode:genprimopcode, deriveConstants:deriveConstants
 
     Build-Depends: base       >= 4.11 && < 4.17,
                    deepseq    >= 1.4 && < 1.5,
diff --git a/hadrian/src/Builder.hs b/hadrian/src/Builder.hs
index 160eb44f7a..b80ac59f8d 100644
--- a/hadrian/src/Builder.hs
+++ b/hadrian/src/Builder.hs
@@ -390,6 +390,10 @@ instance H.Builder Builder where
 isOptional :: Builder -> Bool
 isOptional = \case
     Objdump  -> True
+    -- alex and happy are not required when building source distributions
+    -- and ./configure will complain if they are not available when building in-tree
+    Happy    -> True
+    Alex     -> True
     _        -> False
 
 -- | Determine the location of a system 'Builder'.
diff --git a/hadrian/src/Settings/Packages.hs b/hadrian/src/Settings/Packages.hs
index 85fd0812f6..871e7235f8 100644
--- a/hadrian/src/Settings/Packages.hs
+++ b/hadrian/src/Settings/Packages.hs
@@ -73,6 +73,7 @@ packageArgs = do
           , builder (Cabal Flags) ? mconcat
             [ andM [expr ghcWithInterpreter, notStage0] `cabalFlag` "internal-interpreter"
             , notM cross `cabalFlag` "terminfo"
+            , arg "-build-tool-depends"
             ]
 
           , builder (Haddock BuildPackage) ? arg ("--optghc=-I" ++ path) ]
@@ -202,6 +203,14 @@ packageArgs = do
         , package runGhc ?
           builder Ghc ? input "**/Main.hs" ?
           (\version -> ["-cpp", "-DVERSION=" ++ show version]) <$> getSetting ProjectVersion
+
+        --------------------------------- genprimopcode ------------------------
+        , package genprimopcode
+          ? builder (Cabal Flags) ? arg "-build-tool-depends"
+
+        --------------------------------- hpcBin ----------------------------------
+        , package hpcBin
+          ? builder (Cabal Flags) ? arg "-build-tool-depends"
         ]
 
 ghcBignumArgs :: Args
diff --git a/utils/genprimopcode/genprimopcode.cabal b/utils/genprimopcode/genprimopcode.cabal
index 2dc581dc49..2b92735c2f 100644
--- a/utils/genprimopcode/genprimopcode.cabal
+++ b/utils/genprimopcode/genprimopcode.cabal
@@ -17,6 +17,10 @@ Category: Development
 build-type: Simple
 cabal-version: 2.0
 
+Flag build-tool-depends
+    Description: Use build-tool-depends
+    Default: True
+
 Executable genprimopcode
     Default-Language: Haskell2010
     Main-Is: Main.hs
@@ -26,4 +30,5 @@ Executable genprimopcode
                    Syntax
     Build-Depends: base       >= 4   && < 5,
                    array
-    build-tool-depends: alex:alex >= 3.2.6, happy:happy >= 1.20.0
+    if flag(build-tool-depends)
+      build-tool-depends: alex:alex >= 3.2.6, happy:happy >= 1.20.0
diff --git a/utils/hpc/hpc-bin.cabal b/utils/hpc/hpc-bin.cabal
index 6e344de856..62601cdb07 100644
--- a/utils/hpc/hpc-bin.cabal
+++ b/utils/hpc/hpc-bin.cabal
@@ -12,6 +12,10 @@ Category: Development
 build-type: Simple
 cabal-version: 2.0
 
+Flag build-tool-depends
+    Description: Use build-tool-depends
+    Default: True
+
 Executable hpc
     Default-Language: Haskell2010
     Main-Is: Main.hs
@@ -36,4 +40,5 @@ Executable hpc
                    array      >= 0.1 && < 0.6,
                    hpc        >= 0.6.1 && < 0.7
 
-    build-tool-depends: happy:happy >= 1.20.0
+    if flag(build-tool-depends)
+      build-tool-depends: happy:happy >= 1.20.0