Improve the error message when no gem is found due to platform mismat…

…ches

lib/rubygems/dependency_resolver.rb

@@ -118,14 +118,6 @@ def find_conflict_state conflict, states # :nodoc:
     states.replace rejected
   end
 
-  ##
-  # Extracts the specifications that may be able to fulfill +dependency+
-
-  def find_possible dependency # :nodoc:
-    possible = @set.find_all dependency
-    select_local_platforms possible
-  end
-
   def handle_conflict(dep, existing)
     # There is a conflict! We return the conflict object which will be seen by
     # the caller and be handled at the right level.
@@ -189,11 +181,12 @@ def resolve_for needed, specs
         next
       end
 
-      possible = find_possible dep
+      all_possible = @set.find_all dep
+      possible = select_local_platforms all_possible
 
       case possible.size
       when 0
-        resolve_for_zero dep
+        resolve_for_zero dep, all_possible
       when 1
         needed, specs =
           resolve_for_single needed, specs, dep, possible
@@ -282,11 +275,11 @@ def resolve_for_single needed, specs, dep, possible # :nodoc:
   ##
   # When there are no possible specifications for +dep+ our work is done.
 
-  def resolve_for_zero dep # :nodoc:
+  def resolve_for_zero dep, platform_mismatch # :nodoc:
     @missing << dep
 
     unless @soft_missing
-      raise Gem::UnsatisfiableDependencyError, dep
+      raise Gem::UnsatisfiableDependencyError.new(dep, platform_mismatch)
     end
   end
 

lib/rubygems/exceptions.rb

@@ -226,10 +226,15 @@ class Gem::UnsatisfiableDependencyError < Gem::Exception
   # Creates a new UnsatisfiableDepedencyError for the unsatisfiable
   # Gem::DependencyResolver::DependencyRequest +dep+
 
-  def initialize dep
-    requester = dep.requester ? dep.requester.request : '(unknown)'
-
-    super "Unable to resolve dependency: #{requester} requires #{dep}"
+  def initialize dep, platform_mismatch=nil
+    if platform_mismatch and !platform_mismatch.empty?
+      plats = platform_mismatch.map { |x| x.platform.to_s }.sort.uniq
+      super "Unable to resolve dependency: No match for '#{dep}' on this platform. Found: #{plats.join(', ')}"
+    else
+      requester = dep.requester ? dep.requester.request : '(unknown)'
+
+      super "Unable to resolve dependency: #{requester} requires #{dep}"
+    end
 
     @dependency = dep
   end

test/rubygems/test_gem_dependency_resolver.rb

@@ -408,5 +408,21 @@ def test_select_local_platforms
     assert_equal [a1, a1_p1], selected
   end
 
+  def test_raises_and_explains_when_platform_prevents_install
+    a1 = util_spec "a", "1" do |s|
+      s.platform = Gem::Platform.new %w[c p 1]
+    end
+
+    ad = make_dep "a", "= 1"
+
+    r = Gem::DependencyResolver.new([ad], set(a1))
+
+    e = assert_raises Gem::UnsatisfiableDepedencyError do
+      r.resolve
+    end
+
+    assert_match /No match for 'a \(= 1\)' on this platform. Found: c-p-1/, e.message
+  end
+
 end