Suggest borrowing when trying to coerce unsized type into dyn Trait

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -474,6 +474,12 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             err.span_label(span, explanation);
                         }
 
+                        if let ObligationCauseCode::ObjectCastObligation(obj_ty) = obligation.cause.code().peel_derives() &&
+                           let Some(self_ty) = trait_predicate.self_ty().no_bound_vars() &&
+                           Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
+                            self.suggest_borrowing_for_object_cast(&mut err, &obligation, self_ty, *obj_ty);
+                        }
+
                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
                             let non_const_predicate = trait_ref.without_const();
                             let non_const_obligation = Obligation {

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -77,6 +77,14 @@ pub trait InferCtxtExt<'tcx> {
         has_custom_message: bool,
     ) -> bool;
 
+    fn suggest_borrowing_for_object_cast(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        self_ty: Ty<'tcx>,
+        object_ty: Ty<'tcx>,
+    );
+
     fn suggest_remove_reference(
         &self,
         obligation: &PredicateObligation<'tcx>,
@@ -801,6 +809,35 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
     }
 
+    // Suggest borrowing the type
+    fn suggest_borrowing_for_object_cast(
+        &self,
+        err: &mut Diagnostic,
+        obligation: &PredicateObligation<'tcx>,
+        self_ty: Ty<'tcx>,
+        object_ty: Ty<'tcx>,
+    ) {
+        let ty::Dynamic(predicates, _) = object_ty.kind() else { return; };
+        let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
+
+        for predicate in predicates.iter() {
+            if !self.predicate_must_hold_modulo_regions(
+                &obligation.with(predicate.with_self_ty(self.tcx, self_ref_ty)),
+            ) {
+                return;
+            }
+        }
+
+        err.span_suggestion(
+            obligation.cause.span.shrink_to_lo(),
+            &format!(
+                "consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
+            ),
+            "&".to_string(),
+            Applicability::MaybeIncorrect,
+        );
+    }
+
     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
     /// suggest removing these references until we reach a type that implements the trait.
     fn suggest_remove_reference(

src/test/ui/issues/issue-14366.stderr

@@ -6,6 +6,10 @@ LL |     let _x = "test" as &dyn (::std::any::Any);
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn Any`
+help: consider borrowing the value, since `&str` can be coerced into `dyn Any`
+   |
+LL |     let _x = &"test" as &dyn (::std::any::Any);
+   |              +
 
 error: aborting due to previous error
 

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -224,6 +224,10 @@ LL |     let _ = fat_v as *const dyn Foo;
    |
    = help: the trait `Sized` is not implemented for `[u8]`
    = note: required for the cast to the object type `dyn Foo`
+help: consider borrowing the value, since `&[u8]` can be coerced into `dyn Foo`
+   |
+LL |     let _ = &fat_v as *const dyn Foo;
+   |             +
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/cast-rfc0401.rs:62:13
@@ -233,6 +237,10 @@ LL |     let _ = a as *const dyn Foo;
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn Foo`
+help: consider borrowing the value, since `&str` can be coerced into `dyn Foo`
+   |
+LL |     let _ = &a as *const dyn Foo;
+   |             +
 
 error[E0606]: casting `&{float}` as `f32` is invalid
   --> $DIR/cast-rfc0401.rs:71:30

src/test/ui/unsized/unsized-fn-param.stderr

@@ -6,6 +6,10 @@ LL |     foo11("bar", &"baz");
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn AsRef<Path>`
+help: consider borrowing the value, since `&str` can be coerced into `dyn AsRef<Path>`
+   |
+LL |     foo11(&"bar", &"baz");
+   |           +
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/unsized-fn-param.rs:13:19
@@ -15,6 +19,10 @@ LL |     foo12(&"bar", "baz");
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn AsRef<Path>`
+help: consider borrowing the value, since `&str` can be coerced into `dyn AsRef<Path>`
+   |
+LL |     foo12(&"bar", &"baz");
+   |                   +
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/unsized-fn-param.rs:16:11
@@ -24,6 +32,10 @@ LL |     foo21("bar", &"baz");
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn AsRef<str>`
+help: consider borrowing the value, since `&str` can be coerced into `dyn AsRef<str>`
+   |
+LL |     foo21(&"bar", &"baz");
+   |           +
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/unsized-fn-param.rs:18:19
@@ -33,6 +45,10 @@ LL |     foo22(&"bar", "baz");
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: required for the cast to the object type `dyn AsRef<str>`
+help: consider borrowing the value, since `&str` can be coerced into `dyn AsRef<str>`
+   |
+LL |     foo22(&"bar", &"baz");
+   |                   +
 
 error: aborting due to 4 previous errors