solver: clean up emission of conflicts

lib/spack/spack/solver/asp.py

@@ -747,13 +747,15 @@ def target_ranges(self, spec, single_target_fn):
         return [fn.node_target_satisfies(spec.name, target)]
 
     def conflict_rules(self, pkg):
+        default_msg = "{0} conflicts for '{1}' and '{2}'"
         for trigger, constraints in pkg.conflicts.items():
             trigger_id = self.condition(spack.spec.Spec(trigger), name=pkg.name)
-            self.gen.fact(fn.conflict_trigger(trigger_id))
 
-            for constraint, _ in constraints:
+            for constraint, msg in constraints:
+                if msg is None:
+                    msg = default_msg.format(pkg.name, trigger, constraint)
                 constraint_id = self.condition(constraint, name=pkg.name)
-                self.gen.fact(fn.conflict(pkg.name, trigger_id, constraint_id))
+                self.gen.fact(fn.conflict(pkg.name, trigger_id, constraint_id, msg))
                 self.gen.newline()
 
     def available_compilers(self):

lib/spack/spack/solver/concretize.lp

@@ -180,13 +180,13 @@ path(Parent, Descendant) :- path(Parent, A), depends_on(A, Descendant).
 % Conflicts
 %-----------------------------------------------------------------------------
 :- node(Package),
-   conflict(Package, TriggerID, ConstraintID),
+   conflict(Package, TriggerID, ConstraintID, Msg),
    condition_holds(TriggerID),
    condition_holds(ConstraintID),
    not external(Package),  % ignore conflicts for externals
    error("A conflict was triggered").
 
-#defined conflict/3.
+#defined conflict/4.
 
 %-----------------------------------------------------------------------------
 % Virtual dependencies