Added custom messages for conflicts directive. fixes #4965 (#5083)

Users can now add an optional custom message to the conflicts directive.
Layout on screen has been changed to improve readability and the long
spec is shown in tree format. Two conflicts in `espresso` have been
modified to showcase the feature.

lib/spack/spack/directives.py

@@ -263,7 +263,7 @@ def _depends_on(pkg, spec, when=None, type=None):
 
 
 @directive('conflicts')
-def conflicts(conflict_spec, when=None):
+def conflicts(conflict_spec, when=None, msg=None):
     """Allows a package to define a conflict.
 
     Currently, a "conflict" is a concretized configuration that is known
@@ -280,14 +280,16 @@ def conflicts(conflict_spec, when=None):
     Args:
         conflict_spec (Spec): constraint defining the known conflict
         when (Spec): optional constraint that triggers the conflict
+        msg (str): optional user defined message
     """
     def _execute(pkg):
         # If when is not specified the conflict always holds
         condition = pkg.name if when is None else when
         when_spec = parse_anonymous_spec(condition, pkg.name)
 
+        # Save in a list the conflicts and the associated custom messages
         when_spec_list = pkg.conflicts.setdefault(conflict_spec, [])
-        when_spec_list.append(when_spec)
+        when_spec_list.append((when_spec, msg))
     return _execute
 
 

lib/spack/spack/spec.py

@@ -1798,9 +1798,9 @@ def concretize(self):
         for x in self.traverse():
             for conflict_spec, when_list in x.package.conflicts.items():
                 if x.satisfies(conflict_spec):
-                    for when_spec in when_list:
+                    for when_spec, msg in when_list:
                         if x.satisfies(when_spec):
-                            matches.append((x, conflict_spec, when_spec))
+                            matches.append((x, conflict_spec, when_spec, msg))
         if matches:
             raise ConflictsInSpecError(self, matches)
 
@@ -3447,8 +3447,24 @@ def __init__(self, spec, matches):
         message = 'Conflicts in concretized spec "{0}"\n'.format(
             spec.short_spec
         )
-        long_message = 'List of matching conflicts:\n\n'
+
-        match_fmt = '{0}. "{1}" conflicts with "{2}" in spec "{3}"\n'
+        visited = set()
-        for idx, (s, c, w) in enumerate(matches):
+
-            long_message += match_fmt.format(idx + 1, c, w, s)
+        long_message = ''
+
+        match_fmt_default = '{0}. "{1}" conflicts with "{2}"\n'
+        match_fmt_custom = '{0}. "{1}" conflicts with "{2}" [{3}]\n'
+
+        for idx, (s, c, w, msg) in enumerate(matches):
+
+            if s not in visited:
+                visited.add(s)
+                long_message += 'List of matching conflicts for spec:\n\n'
+                long_message += s.tree(indent=4) + '\n'
+
+            if msg is None:
+                long_message += match_fmt_default.format(idx + 1, c, w)
+            else:
+                long_message += match_fmt_custom.format(idx + 1, c, w, msg)
+
         super(ConflictsInSpecError, self).__init__(message, long_message)

var/spack/repos/builtin/packages/espresso/package.py

@@ -75,8 +75,17 @@ class Espresso(Package):
     patch('dspev_drv_elpa.patch', when='@6.1 ^elpa@2016.05.003')
 
     # We can't ask for scalapack or elpa if we don't want MPI
-    conflicts('+scalapack', when='~mpi')
+    conflicts(
-    conflicts('+elpa', when='~mpi')
+        '+scalapack',
+        when='~mpi',
+        msg='scalapack is a parallel library and needs MPI support'
+    )
+
+    conflicts(
+        '+elpa',
+        when='~mpi',
+        msg='elpa is a parallel library and needs MPI support'
+    )
 
     # Elpa is formally supported by @:5.4.0, but QE configure searches
     # for it in the wrong folders (or tries to download it within