Support shorthand ability implementation syntax

crates/compiler/can/src/def.rs

@@ -436,7 +436,7 @@ fn canonicalize_claimed_ability_impl<'a>(
             let label_str = label.value;
             let region = label.region;
 
-            let _member_symbol =
+            let member_symbol =
                 match env.qualified_lookup_with_module_id(scope, ability_home, label_str, region) {
                     Ok(symbol) => symbol,
                     Err(_) => {
@@ -449,21 +449,18 @@ fn canonicalize_claimed_ability_impl<'a>(
                     }
                 };
 
-            todo!();
-            // match scope.lookup(&label_str.into(), label.region) {
-            //     Ok(symbol) => {
-            //         return Ok((member_symbol, symbol));
-            //     }
-            //     Err(_) => {
-            //         // [Eq { eq }]
-
-            //         env.problem(Problem::ImplementationNotFound {
-            //             ability,
-            //             member: scope.lookup(),
-            //             region: label.region,
-            //         });
-            //     }
-            // }
+            match scope.lookup_ability_member_shadow(member_symbol) {
+                Some(symbol) => {
+                    return Ok((member_symbol, symbol));
+                }
+                None => {
+                    env.problem(Problem::ImplementationNotFound {
+                        member: member_symbol,
+                        region: label.region,
+                    });
+                    Err(())
+                }
+            }
         }
         AssignedField::RequiredValue(label, _spaces, value) => {
             let impl_ident = match value.value {

crates/compiler/can/src/scope.rs

@@ -30,6 +30,12 @@ pub struct Scope {
     /// Identifiers that are imported (and introduced in the header)
     imports: Vec<(Ident, Symbol, Region)>,
 
+    /// Shadows of an ability member, for example a local specialization of `eq` for the ability
+    /// member `Eq has eq : a, a -> Bool` gets a shadow symbol it can use for its implementation.
+    ///
+    /// Only one shadow of an ability member is permitted per scope.
+    shadows: VecMap<Symbol, Loc<Symbol>>,
+
     /// Identifiers that are in scope, and defined in the current module
     pub locals: ScopedIdentIds,
 }
@@ -51,6 +57,7 @@ impl Scope {
             locals: ScopedIdentIds::from_ident_ids(home, initial_ident_ids),
             aliases: VecMap::default(),
             abilities_store: starting_abilities_store,
+            shadows: VecMap::default(),
             imports,
         }
     }
@@ -59,6 +66,10 @@ impl Scope {
         self.lookup_str(ident.as_str(), region)
     }
 
+    pub fn lookup_ability_member_shadow(&self, member: Symbol) -> Option<Symbol> {
+        self.shadows.get(&member).map(|loc_shadow| loc_shadow.value)
+    }
+
     pub fn add_docs_imports(&mut self) {
         self.imports
             .push(("Dict".into(), Symbol::DICT_DICT, Region::zero()));
@@ -306,11 +317,26 @@ impl Scope {
                         .iter()
                         .any(|(_, members)| members.iter().any(|m| *m == original_symbol))
                 {
-                    // TODO: remove register_specializing_symbol
-                    self.abilities_store
-                        .register_specializing_symbol(shadow_symbol, original_symbol);
+                    match self.shadows.get(&original_symbol) {
+                        Some(loc_original_shadow) => {
+                            // Duplicate shadow of an ability members; that's illegal.
+                            let shadow = Loc {
+                                value: ident.clone(),
+                                region,
+                            };
+                            Err((loc_original_shadow.region, shadow, shadow_symbol))
+                        }
+                        None => {
+                            // TODO: remove register_specializing_symbol
+                            self.abilities_store
+                                .register_specializing_symbol(shadow_symbol, original_symbol);
 
-                    Ok((shadow_symbol, Some(original_symbol)))
+                            self.shadows
+                                .insert(original_symbol, Loc::at(region, shadow_symbol));
+
+                            Ok((shadow_symbol, Some(original_symbol)))
+                        }
+                    }
                 } else {
                     // This is an illegal shadow.
                     let shadow = Loc {

crates/compiler/problem/src/can.rs

@@ -131,6 +131,10 @@ pub enum Problem {
     AbilityUsedAsType(Lowercase, Symbol, Region),
     NestedSpecialization(Symbol, Region),
     IllegalClaimedAbility(Region),
+    ImplementationNotFound {
+        member: Symbol,
+        region: Region,
+    },
     NotAnAbilityMember {
         ability: Symbol,
         name: String,

crates/reporting/src/error/canonicalize.rs

@@ -48,6 +48,7 @@ const ABILITY_NOT_ON_TOPLEVEL: &str = "ABILITY NOT ON TOP-LEVEL";
 const SPECIALIZATION_NOT_ON_TOPLEVEL: &str = "SPECIALIZATION NOT ON TOP-LEVEL";
 const ABILITY_USED_AS_TYPE: &str = "ABILITY USED AS TYPE";
 const ILLEGAL_DERIVE: &str = "ILLEGAL DERIVE";
+const IMPLEMENTATION_NOT_FOUND: &str = "IMPLEMENTATION NOT FOUND";
 const NOT_AN_ABILITY_MEMBER: &str = "NOT AN ABILITY MEMBER";
 const OPTIONAL_ABILITY_IMPLEMENTATION: &str = "OPTIONAL ABILITY IMPLEMENTATION";
 const QUALIFIED_ABILITY_IMPLEMENTATION: &str = "QUALIFIED ABILITY IMPLEMENTATION";
@@ -769,6 +770,18 @@ pub fn can_problem<'b>(
             title = NOT_AN_ABILITY_MEMBER.to_string();
             severity = Severity::RuntimeError;
         }
+        Problem::ImplementationNotFound { member, region } => {
+            let member_str = member.as_str(alloc.interns);
+            doc = alloc.stack([
+                alloc.concat([
+                    alloc.reflow("An implementation of "), alloc.symbol_unqualified(member), alloc.reflow(" could not be found in this scope:"),
+                ]),
+                alloc.region(lines.convert_region(region)),
+                alloc.tip().append(alloc.concat([alloc.reflow("consider adding a value of name "), alloc.symbol_unqualified(member), alloc.reflow(" in this scope, or using another variable that implements this ability member, like "), alloc.type_str(&format!("{{ {}: my{} }}", member_str, member_str))]))
+            ]);
+            title = IMPLEMENTATION_NOT_FOUND.to_string();
+            severity = Severity::RuntimeError;
+        }
         Problem::OptionalAbilityImpl { ability, region } => {
             let hint = if ability.is_builtin() {
                 alloc.hint("").append(

crates/reporting/tests/test_reporting.rs

@@ -9134,11 +9134,10 @@ All branches in an `if` must have the same type!
     );
 
     test_report!(
-        #[ignore="TODO"]
         opaque_ability_impl_not_found_shorthand_syntax,
         indoc!(
             r#"
-            app "test" provides [] to "./platform"
+            app "test" provides [A] to "./platform"
 
             Eq has eq : a, a -> U64 | a has Eq
 
@@ -9146,6 +9145,16 @@ All branches in an `if` must have the same type!
             "#
         ),
         @r###"
+    ── IMPLEMENTATION NOT FOUND ────────────────────────────── /code/proj/Main.roc ─
+
+    An implementation of `eq` could not be found in this scope:
+
+    5│  A := U8 has [Eq {eq}]
+                         ^^
+
+    Tip: consider adding a value of name `eq` in this scope, or using
+    another variable that implements this ability member, like
+    { eq: myeq }
     "###
     );