Suggest #[derive(Clone)]

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1278,17 +1278,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             && !results.expr_adjustments(callee_expr).iter().any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(..)))
             // Check that we're in fact trying to clone into the expected type
             && self.can_coerce(*pointee_ty, expected_ty)
+            && let predicate = ty::Binder::dummy(ty::TraitRef {
+                def_id: clone_trait_did,
+                substs: self.tcx.mk_substs([expected_ty.into()].iter()),
+            })
+                .without_const()
+                .to_predicate(self.tcx)
             // And the expected type doesn't implement `Clone`
             && !self.predicate_must_hold_considering_regions(&traits::Obligation {
                 cause: traits::ObligationCause::dummy(),
                 param_env: self.param_env,
                 recursion_depth: 0,
-                predicate: ty::Binder::dummy(ty::TraitRef {
-                    def_id: clone_trait_did,
-                    substs: self.tcx.mk_substs([expected_ty.into()].iter()),
-                })
-                .without_const()
-                .to_predicate(self.tcx),
+                predicate,
             })
         {
             diag.span_note(
@@ -1307,6 +1308,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     diag,
                     vec![(param.name.as_str(), "Clone", Some(clone_trait_did))].into_iter(),
                 );
+            } else {
+                self.suggest_derive(diag, &[(predicate, None, None)]);
             }
         }
     }

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -1843,7 +1843,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.suggest_derive(err, &preds);
     }
 
-    fn suggest_derive(
+    pub fn suggest_derive(
         &self,
         err: &mut Diagnostic,
         unsatisfied_predicates: &[(

src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.fixed

@@ -3,6 +3,14 @@ fn wat<T: Clone>(t: &T) -> T {
     t.clone() //~ ERROR E0308
 }
 
+#[derive(Clone)]
+struct Foo;
+
+fn wut(t: &Foo) -> Foo {
+    t.clone() //~ ERROR E0308
+}
+
 fn main() {
     wat(&42);
+    wut(&Foo);
 }

src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.rs

@@ -3,6 +3,13 @@ fn wat<T>(t: &T) -> T {
     t.clone() //~ ERROR E0308
 }
 
+struct Foo;
+
+fn wut(t: &Foo) -> Foo {
+    t.clone() //~ ERROR E0308
+}
+
 fn main() {
     wat(&42);
+    wut(&Foo);
 }

src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr

@@ -20,6 +20,24 @@ help: consider restricting type parameter `T`
 LL | fn wat<T: Clone>(t: &T) -> T {
    |         +++++++
 
-error: aborting due to previous error
+error[E0308]: mismatched types
+  --> $DIR/clone-on-unconstrained-borrowed-type-param.rs:9:5
+   |
+LL | fn wut(t: &Foo) -> Foo {
+   |                    --- expected `Foo` because of return type
+LL |     t.clone()
+   |     ^^^^^^^^^ expected struct `Foo`, found `&Foo`
+   |
+note: `Foo` does not implement `Clone`, so `&Foo` was cloned instead
+  --> $DIR/clone-on-unconstrained-borrowed-type-param.rs:9:5
+   |
+LL |     t.clone()
+   |     ^
+help: consider annotating `Foo` with `#[derive(Clone)]`
+   |
+LL | #[derive(Clone)]
+   |
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

src/test/ui/typeck/explain_clone_autoref.stderr

@@ -12,6 +12,10 @@ note: `NotClone` does not implement `Clone`, so `&NotClone` was cloned instead
    |
 LL |     nc.clone()
    |     ^^
+help: consider annotating `NotClone` with `#[derive(Clone)]`
+   |
+LL | #[derive(Clone)]
+   |
 
 error: aborting due to previous error