Add a constraint scope to constrain a given package in any setup script.

I used the new constraint scope to enforce the minimum Cabal version in setup
scripts that lack a 'custom-setup' stanza.  It isn't exposed to users yet.

Author: sebright

cabal-install/Distribution/Client/Dependency.hs

@@ -492,16 +492,14 @@ addDefaultSetupDependencies defaultSetupDeps params =
         pkgdesc  = PD.packageDescription gpkgdesc
 
 -- | If a package has a custom setup then we need to add a setup-depends
--- on Cabal. For now it's easier to add this unconditionally.  Once
--- qualified constraints land we can turn this into a custom setup
--- only constraint.
+-- on Cabal.
 --
 addSetupCabalMinVersionConstraint :: Version
                                   -> DepResolverParams -> DepResolverParams
 addSetupCabalMinVersionConstraint minVersion =
     addConstraints
       [ LabeledPackageConstraint
-          (PackageConstraint (scopeToplevel cabalPkgname)
+          (PackageConstraint (ScopeAnySetupQualifier cabalPkgname)
                              (PackagePropertyVersion $ orLaterVersion minVersion))
           ConstraintSetupCabalMinVersion
       ]

cabal-install/Distribution/Solver/Types/PackageConstraint.hs

@@ -38,6 +38,9 @@ import qualified Text.PrettyPrint as Disp
 data ConstraintScope
      -- | The package with the specified name and qualifier.
    = ScopeQualified Qualifier PackageName
+     -- | The package with the specified name when it has a
+     -- setup qualifier.
+   | ScopeAnySetupQualifier PackageName
      -- | The package with the specified name regardless of
      -- qualifier.
    | ScopeAnyQualifier PackageName
@@ -52,11 +55,13 @@ scopeToplevel = ScopeQualified QualToplevel
 -- | Returns the package name associated with a constraint scope.
 scopeToPackageName :: ConstraintScope -> PackageName
 scopeToPackageName (ScopeQualified _ pn) = pn
+scopeToPackageName (ScopeAnySetupQualifier pn) = pn
 scopeToPackageName (ScopeAnyQualifier pn) = pn
 
 -- | Pretty-prints a constraint scope.
 dispConstraintScope :: ConstraintScope -> Disp.Doc
 dispConstraintScope (ScopeQualified q pn) = dispQualifier q <<>> disp pn
+dispConstraintScope (ScopeAnySetupQualifier pn) = Disp.text "setup." <<>> disp pn
 dispConstraintScope (ScopeAnyQualifier pn) = Disp.text "any." <<>> disp pn
 
 -- | A package property is a logical predicate on packages.