Don't allocate for missing or existing members by default

- Missing members are zero in the happy case
- Existing members will always be populated in the happy case, but I
  don't think they improve the error message, so just get rid of it.

compiler/solve/src/ability.rs

@@ -36,7 +36,6 @@ pub enum Unfulfilled {
     Incomplete {
         typ: Symbol,
         ability: Symbol,
-        specialized_members: Vec<Loc<Symbol>>,
         missing_members: Vec<Loc<Symbol>>,
     },
     /// Cannot derive implementation of an ability for a type.
@@ -170,18 +169,16 @@ impl ObligationCache<'_> {
         match typ {
             Obligated::Opaque(typ) => {
                 let members_of_ability = self.abilities_store.members_of_ability(ability).unwrap();
-                let mut specialized_members = Vec::with_capacity(members_of_ability.len());
+                let mut missing_members = Vec::new();
-                let mut missing_members = Vec::with_capacity(members_of_ability.len());
                 for &member in members_of_ability {
-                    match self.abilities_store.get_specialization(member, typ) {
+                    if self
-                        None => {
+                        .abilities_store
+                        .get_specialization(member, typ)
+                        .is_none()
+                    {
                         let root_data = self.abilities_store.member_def(member).unwrap();
                         missing_members.push(Loc::at(root_data.region, member));
                     }
-                        Some(specialization) => {
-                            specialized_members.push(Loc::at(specialization.region, member));
-                        }
-                    }
                 }
 
                 if !missing_members.is_empty() {
@@ -190,7 +187,6 @@ impl ObligationCache<'_> {
                         Unfulfilled::Incomplete {
                             typ,
                             ability,
-                            specialized_members,
                             missing_members,
                         },
                     );

reporting/src/error/type.rs

@@ -232,7 +232,6 @@ fn report_unfulfilled_ability<'a>(
         Unfulfilled::Incomplete {
             typ,
             ability,
-            specialized_members,
             missing_members,
         } => {
             debug_assert!(!missing_members.is_empty());
@@ -254,24 +253,6 @@ fn report_unfulfilled_ability<'a>(
                 stack.push(alloc.region(lines.convert_region(member.region)));
             }
 
-            if !specialized_members.is_empty() {
-                stack.push(alloc.concat([
-                    alloc.note(""),
-                    alloc.symbol_unqualified(typ),
-                    alloc.reflow(" specializes the following members of "),
-                    alloc.symbol_unqualified(ability),
-                    alloc.reflow(":"),
-                ]));
-
-                for spec in specialized_members {
-                    stack.push(alloc.concat([
-                        alloc.symbol_unqualified(spec.value),
-                        alloc.reflow(", specialized here:"),
-                    ]));
-                    stack.push(alloc.region(lines.convert_region(spec.region)));
-                }
-            }
-
             alloc.stack(stack)
         }
         Unfulfilled::Underivable {

reporting/tests/test_reporting.rs

@@ -9379,13 +9379,6 @@ All branches in an `if` must have the same type!
 
                 5│      le : a, a -> Bool | a has Eq
                         ^^
-
-                Note: `Id` specializes the following members of `Eq`:
-
-                `eq`, specialized here:
-
-                9│  eq = \@Id m, @Id n -> m == n
-                    ^^
                 "#
             ),
         )