Suggest constraining type parameter with Clone

Fix #34896.

Author: estebank

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -13,7 +13,9 @@ use rustc_hir_analysis::astconv::AstConv;
 use rustc_infer::infer;
 use rustc_infer::traits::{self, StatementAsExpression};
 use rustc_middle::lint::in_external_macro;
-use rustc_middle::ty::{self, Binder, DefIdTree, IsSuggestable, ToPredicate, Ty};
+use rustc_middle::ty::{
+    self, suggest_constraining_type_params, Binder, DefIdTree, IsSuggestable, ToPredicate, Ty,
+};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -1295,6 +1297,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     "`{expected_ty}` does not implement `Clone`, so `{found_ty}` was cloned instead"
                 ),
             );
+            let owner = self.tcx.hir().enclosing_body_owner(expr.hir_id);
+            if let ty::Param(param) = expected_ty.kind()
+                && let Some(generics) = self.tcx.hir().get_generics(owner)
+            {
+                suggest_constraining_type_params(
+                    self.tcx,
+                    generics,
+                    diag,
+                    vec![(param.name.as_str(), "Clone", Some(clone_trait_did))].into_iter(),
+                );
+            }
         }
     }
 

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

@@ -0,0 +1,8 @@
+// run-rustfix
+fn wat<T: Clone>(t: &T) -> T {
+    t.clone() //~ ERROR E0308
+}
+
+fn main() {
+    wat(&42);
+}

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

@@ -0,0 +1,8 @@
+// run-rustfix
+fn wat<T>(t: &T) -> T {
+    t.clone() //~ ERROR E0308
+}
+
+fn main() {
+    wat(&42);
+}

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

@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/clone-on-unconstrained-borrowed-type-param.rs:3:5
+   |
+LL | fn wat<T>(t: &T) -> T {
+   |        -            - expected `T` because of return type
+   |        |
+   |        this type parameter
+LL |     t.clone()
+   |     ^^^^^^^^^ expected type parameter `T`, found `&T`
+   |
+   = note: expected type parameter `T`
+                   found reference `&T`
+note: `T` does not implement `Clone`, so `&T` was cloned instead
+  --> $DIR/clone-on-unconstrained-borrowed-type-param.rs:3:5
+   |
+LL |     t.clone()
+   |     ^
+help: consider restricting type parameter `T`
+   |
+LL | fn wat<T: Clone>(t: &T) -> T {
+   |         +++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.