concretizer: simplify and suppress warnings for variant handling

We are relying on default logic in the variant handling in that we set a
default value if we never see `variant_set(P, V, X)`.

- Move the logic for this into `concretize.lp` instead of generating it
  for every package.

- For programs that don't have explicit variant settings, clingo warns
  that variant_set(P, V, X) doesn't appear in any rule head, because a
  setting is never generated.

  - Specifically suppress this warning.

lib/spack/spack/solver/asp.py

@@ -196,14 +196,17 @@ def pkg_rules(self, pkg):
             single = fn.variant_single_value(pkg.name, name)
             if single_value:
                 self.rule(single, fn.node(pkg.name))
-                self.rule(fn.variant_value(pkg.name, name, variant.default),
-                          self._not(fn.variant_set(pkg.name, name)))
+                self.rule(
+                    fn.variant_default_value(pkg.name, name, variant.default),
+                    fn.node(pkg.name))
             else:
                 self.rule(self._not(single), fn.node(pkg.name))
                 defaults = variant.default.split(',')
                 for val in defaults:
-                    self.rule(fn.variant_value(pkg.name, name, val),
-                              self._not(fn.variant_set(pkg.name, name)))
+                    self.rule(
+                        fn.variant_default_value(pkg.name, name, val),
+                        fn.node(pkg.name))
+            self.out.write('\n')
 
         # dependencies
         for name, conditions in pkg.dependencies.items():
@@ -231,7 +234,7 @@ def spec_rules(self, spec):
 
         # variants
         for vname, variant in spec.variants.items():
-            self.fact(fn.variant_value(spec.name, vname, variant.value))
+            self.fact(fn.variant_set(spec.name, vname, variant.value))
 
         # TODO
         # dependencies
@@ -265,9 +268,10 @@ def generate_asp_program(self, specs):
 
         self.title('Spec Constraints')
         for spec in specs:
-            self.section('Spec: %s' % pkg)
-            self.spec_rules(spec)
-            self.out.write('\n')
+            for dep in spec.traverse():
+                self.section('Spec: %s' % str(dep))
+                self.spec_rules(dep)
+                self.out.write('\n')
 
         self.out.write(pkgutil.get_data('spack.solver', 'display.lp'))
         self.out.write('\n')

lib/spack/spack/solver/concretize.lp

@@ -25,6 +25,13 @@ arch_target(D, A) :- node(D), depends_on(P, D), arch_target(P, A).
 % if a variant is set to anything, it is considered 'set'.
 variant_set(P, V) :- variant_set(P, V, _).
 
+% suppress wranings about this atom being unset.  It's only set if some
+% spec or some package sets it, and without this, clingo will give
+% warnings like 'info: atom does not occur in any rule head'.
+#defined variant_set/3.
+
 % variant_set is an explicitly set variant value. If it's not 'set',
 % we revert to the default value. If it is set, we force the set value
 variant_value(P, V, X) :- node(P), variant(P, V), variant_set(P, V, X).
+variant_value(P, V, X) :- node(P), variant(P, V), not variant_set(P, V),
+                 variant_default_value(P, V, X).