highlight: rust: recognise more enum variants

Now we recognise any PascalCase::PascalCase.method() as being
Enum::Variant.method(). We recognise PascalCase::PascalCase.field too,
but I don't think that's legal anywhere as the lhs of the dot can only be
a unit enum variant or an associated type, neither of which can have
fields.

runtime/queries/rust/highlights.scm

@@ -232,6 +232,22 @@
     ])
   (identifier) @type.enum.variant (#match? @type.enum.variant "^[A-Z]")])
 
+; ---
+; PascalCase identifiers under a path which is also PascalCase
+; are assumed to be enum variants if they have methods or fields.
+; ---
+
+(field_expression
+  value: (scoped_identifier
+    path: [
+      (identifier) @type.enum
+      (scoped_identifier
+        name: (identifier) @type.enum)
+    ]
+    name: (identifier) @type.enum.variant
+      (#match? @type.enum "^[A-Z]")
+      (#match? @type.enum.variant "^[A-Z]")))
+
 ; ---
 ; Other PascalCase identifiers are assumed to be structs.
 ; ---