Explain why &T is cloned when T is not Clone

compiler/rustc_typeck/src/check/demand.rs

@@ -40,6 +40,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty);
         self.suggest_missing_parentheses(err, expr);
         self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected);
+        self.note_type_is_not_clone(err, expected, expr_ty, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);
         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
         self.report_closure_inferred_return_type(err, expected);
@@ -630,7 +631,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     Applicability::MachineApplicable,
                                     true,
                                 ));
-
                     }
                 }
                 _ => {}

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -2,8 +2,6 @@ use super::FnCtxt;
 use crate::astconv::AstConv;
 
 use rustc_ast::util::parser::ExprPrecedence;
-use rustc_span::{self, Span};
-
 use rustc_errors::{Applicability, Diagnostic, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind};
@@ -13,12 +11,14 @@ use rustc_hir::{
     WherePredicate,
 };
 use rustc_infer::infer::{self, TyCtxtInferExt};
-
+use rustc_infer::traits;
 use rustc_middle::lint::in_external_macro;
-use rustc_middle::ty::{self, Binder, Ty};
-use rustc_span::symbol::{kw, sym};
-
 use rustc_middle::ty::subst::GenericArgKind;
+use rustc_middle::ty::{self, Binder, ToPredicate, Ty};
+use rustc_span::symbol::{kw, sym};
+use rustc_span::Span;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
+
 use std::iter;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -846,4 +846,53 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let node = self.tcx.hir().get(id);
         matches!(node, Node::Stmt(Stmt { kind: StmtKind::Local(..), .. }))
     }
+
+    /// Suggest that `&T` was cloned instead of `T` because `T` does not implement `Clone`,
+    /// which is a side-effect of autoref.
+    pub(crate) fn note_type_is_not_clone(
+        &self,
+        diag: &mut Diagnostic,
+        expected_ty: Ty<'tcx>,
+        found_ty: Ty<'tcx>,
+        expr: &hir::Expr<'_>,
+    ) {
+        let hir::ExprKind::MethodCall(segment, &[ref callee_expr], _) = expr.kind else { return; };
+        let Some(clone_trait_did) = self.tcx.lang_items().clone_trait() else { return; };
+        let ty::Ref(_, pointee_ty, _) = found_ty.kind() else { return };
+        let results = self.typeck_results.borrow();
+        // First, look for a `Clone::clone` call
+        if segment.ident.name == sym::clone
+            && results.type_dependent_def_id(expr.hir_id).map_or(
+                false,
+                |did| {
+                    self.tcx.associated_item(did).container
+                        == ty::AssocItemContainer::TraitContainer(clone_trait_did)
+                },
+            )
+            // If that clone call hasn't already dereferenced the self type (i.e. don't give this
+            // diagnostic in cases where we have `(&&T).clone()` and we expect `T`).
+            && !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)
+            // 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),
+            })
+        {
+            diag.span_note(
+                callee_expr.span,
+                &format!(
+                    "`{expected_ty}` does not implement `Clone`, so `{found_ty}` was cloned instead"
+                ),
+            );
+        }
+    }
 }

src/test/ui/typeck/explain_clone_autoref.rs

@@ -0,0 +1,13 @@
+struct NotClone;
+
+fn main() {
+    clone_thing(&NotClone);
+}
+
+fn clone_thing(nc: &NotClone) -> NotClone {
+    //~^ NOTE expected `NotClone` because of return type
+    nc.clone()
+    //~^ ERROR mismatched type
+    //~| NOTE `NotClone` does not implement `Clone`, so `&NotClone` was cloned instead
+    //~| NOTE expected struct `NotClone`, found `&NotClone`
+}

src/test/ui/typeck/explain_clone_autoref.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/explain_clone_autoref.rs:9:5
+   |
+LL | fn clone_thing(nc: &NotClone) -> NotClone {
+   |                                  -------- expected `NotClone` because of return type
+LL |
+LL |     nc.clone()
+   |     ^^^^^^^^^^ expected struct `NotClone`, found `&NotClone`
+   |
+note: `NotClone` does not implement `Clone`, so `&NotClone` was cloned instead
+  --> $DIR/explain_clone_autoref.rs:9:5
+   |
+LL |     nc.clone()
+   |     ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.