Suggest binding type variables to an ability when they're unsatisfied

crates/compiler/solve/src/ability.rs

@@ -502,7 +502,7 @@ trait DerivableVisitor {
         if abilities != [Self::ABILITY] {
             Err(NotDerivable {
                 var,
-                context: NotDerivableContext::NoContext,
+                context: NotDerivableContext::UnboundVar,
             })
         } else {
             Ok(())
@@ -634,7 +634,7 @@ trait DerivableVisitor {
                 RigidVar(_) => {
                     return Err(NotDerivable {
                         var,
-                        context: NotDerivableContext::NoContext,
+                        context: NotDerivableContext::UnboundVar,
                     })
                 }
                 FlexAbleVar(opt_name, abilities) => {

crates/reporting/tests/test_reporting.rs

@@ -12446,4 +12446,37 @@ All branches in an `if` must have the same type!
     to be more general?
     "###
     );
+
+    test_report!(
+        suggest_binding_rigid_var_to_ability,
+        indoc!(
+            r#"
+            app "test" provides [f] to "./p"
+
+            f : List e -> List e
+            f = \l -> if l == l then l else l
+            "#
+        ),
+    @r###"
+    ── TYPE MISMATCH ───────────────────────────────────────── /code/proj/Main.roc ─
+
+    This expression has a type that does not implement the abilities it's expected to:
+
+    4│  f = \l -> if l == l then l else l
+                     ^
+
+    I can't generate an implementation of the `Eq` ability for
+
+        List e
+
+    In particular, an implementation for
+
+        e
+
+    cannot be generated.
+
+    Tip: This type variable is not bound to `Eq`. Consider adding a `has`
+    clause to bind the type variable, like `| e has Bool.Eq`
+    "###
+    );
 }