MM: Identify module source for extensions

This is to resolve the issue of merging extensions that provide
protocol compliance -- in this case there may be no members to annotate
and show the source module.

A bit messy and not entirely complete because of the way the code is
structured -- really we should rework 'declaration' to be a list of
stuff for the templating layer but it is already tangled up with html
because of Rouge.

Good enough though, covers everything, and in reality these are unusual
cases.

lib/jazzy/sourcekitten.rb

@@ -895,17 +895,40 @@ def self.move_merged_extension_marks(decls)
     # declaration: public protocol conformances and, for top-level extensions,
     # further conditional extensions of the same type.
     def self.merge_code_declaration(decls)
-      first = decls.first
-
       declarations = decls[1..].select do |decl|
         decl.type.swift_extension? &&
           (decl.other_inherited_types?(@inaccessible_protocols) ||
-            (first.type.swift_extension? && decl.constrained_extension?))
-      end.map(&:declaration)
-
-      unless declarations.empty?
-        first.declaration = declarations.prepend(first.declaration).uniq.join
+            (decls.first.type.swift_extension? && decl.constrained_extension?))
+      end.prepend(decls.first)
+
+      html_declaration = ''
+      until declarations.empty?
+        module_decls, declarations = next_doc_module_group(declarations)
+        first = module_decls.first
+        if need_doc_module_note?(first, html_declaration)
+          html_declaration += "<span class='declaration-note'>From #{first.doc_module_name}:</span>"
+        end
+        html_declaration += module_decls.map(&:declaration).uniq.join
       end
+
+      # Must preserve `nil` for edge cases
+      decls.first.declaration = html_declaration unless html_declaration.empty?
+    end
+
+    # Grab all the extensions from the same doc module
+    def self.next_doc_module_group(decls)
+      decls.partition { _1.doc_module_name == decls.first.doc_module_name }
+    end
+
+    # Does this extension/type need a note explaining which doc module it is from?
+    # Only for extensions, if there actually are multiple modules.
+    # Last condition avoids it for simple 'extension Array'.
+    def self.need_doc_module_note?(decl, html_declaration)
+      Config.instance.multiple_modules? &&
+        decl.type.swift_extension? &&
+        !(html_declaration.empty? &&
+          !decl.constrained_extension? &&
+          !decl.inherited_types?)
     end
 
     #

lib/jazzy/themes/apple/assets/css/jazzy.css.scss

@@ -414,11 +414,12 @@ header {
   .discouraged {
     text-decoration: line-through;
   }
-  .declaration-note {
-    font-size: .85em;
-    color: rgba(128,128,128,1);
-    font-style: italic;
-  }
+}
+
+.declaration-note {
+  font-size: .85em;
+  color: rgba(128,128,128,1);
+  font-style: italic;
 }
 
 .pointer-container {

lib/jazzy/themes/fullwidth/assets/css/jazzy.css.scss

@@ -454,12 +454,12 @@ pre code {
     margin-left: 20px;
     font-size: 1rem;
   }
+}
 
-  .declaration-note {
-    font-size: .85em;
-    color: #808080;
-    font-style: italic;
-  }
+.declaration-note {
+  font-size: .85em;
+  color: #808080;
+  font-style: italic;
 }
 
 .pointer-container {

lib/jazzy/themes/jony/assets/css/jazzy.css.scss

@@ -450,11 +450,12 @@ header {
   .token-open {
     margin-left: 45px;
   }
-  .declaration-note {
-    font-size: .85em;
-    color: rgba(128,128,128,1);
-    font-style: italic;
-  }
+}
+
+.declaration-note {
+  font-size: .85em;
+  color: rgba(128,128,128,1);
+  font-style: italic;
 }
 
 .pointer-container {