Basic type inference and solving for abilities

Note that is still pretty limited. We only permit opaque types to
implement abilities, abilities cannot have type arguments, and also no
other functions may depend on abilities

compiler/constrain/src/pattern.rs

@@ -188,9 +188,23 @@ pub fn constrain_pattern(
             // Erroneous patterns don't add any constraints.
         }
 
-        Identifier(symbol) | Shadowed(_, _, symbol)
-        // TODO(abilities): handle linking the member def to the specialization ident
-        | AbilityMemberSpecialization {
+        Identifier(symbol) | Shadowed(_, _, symbol) => {
+            if could_be_a_tag_union(expected.get_type_ref()) {
+                state
+                    .constraints
+                    .push(constraints.is_open_type(expected.get_type_ref().clone()));
+            }
+
+            state.headers.insert(
+                *symbol,
+                Loc {
+                    region,
+                    value: expected.get_type(),
+                },
+            );
+        }
+
+        AbilityMemberSpecialization {
             ident: symbol,
             specializes: _,
         } => {