From f19488064a4752703fde8a3596a9d90ca2095534 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Tue, 13 Dec 2022 18:55:00 -0800
Subject: [PATCH] Suggest constraining type parameter with `Clone`

Fix #34896.
---
 .../src/fn_ctxt/suggestions.rs                | 15 ++++++++++-
 ...on-unconstrained-borrowed-type-param.fixed |  8 ++++++
 ...ne-on-unconstrained-borrowed-type-param.rs |  8 ++++++
 ...n-unconstrained-borrowed-type-param.stderr | 25 +++++++++++++++++++
 4 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.fixed
 create mode 100644 src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.rs
 create mode 100644 src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr

diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
index 719f44f9f665a..08ef2484776b2 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
+++ b/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, Ty};
+use rustc_middle::ty::{
+    self, suggest_constraining_type_params, Binder, DefIdTree, IsSuggestable, Ty,
+};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -1293,6 +1295,17 @@ 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(),
+                );
+            }
         }
     }
 
diff --git a/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.fixed b/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.fixed
new file mode 100644
index 0000000000000..3e5ded6738b31
--- /dev/null
+++ b/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);
+}
diff --git a/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.rs b/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.rs
new file mode 100644
index 0000000000000..1a5a38369ec52
--- /dev/null
+++ b/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);
+}
diff --git a/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr b/src/test/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr
new file mode 100644
index 0000000000000..01246955fae8a
--- /dev/null
+++ b/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`.