Don't suggest dereferencing to unsized type

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1239,21 +1239,18 @@ pub trait PrettyPrinter<'tcx>:
                         .generics_of(principal.def_id)
                         .own_args_no_defaults(cx.tcx(), principal.args);
 
-                    let mut projections = predicates.projection_bounds();
-
-                    let mut args = args.iter().cloned();
-                    let arg0 = args.next();
-                    let projection0 = projections.next();
-                    if arg0.is_some() || projection0.is_some() {
-                        let args = arg0.into_iter().chain(args);
-                        let projections = projection0.into_iter().chain(projections);
+                    let mut projections: Vec<_> = predicates.projection_bounds().collect();
+                    projections.sort_by_cached_key(|proj| {
+                        cx.tcx().item_name(proj.item_def_id()).to_string()
+                    });
 
+                    if !args.is_empty() || !projections.is_empty() {
                         p!(generic_delimiters(|mut cx| {
-                            cx = cx.comma_sep(args)?;
-                            if arg0.is_some() && projection0.is_some() {
+                            cx = cx.comma_sep(args.iter().copied())?;
+                            if !args.is_empty() && !projections.is_empty() {
                                 write!(cx, ", ")?;
                             }
-                            cx.comma_sep(projections)
+                            cx.comma_sep(projections.iter().copied())
                         }));
                     }
                 }

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

@@ -838,7 +838,20 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         obligation.param_env,
                         real_trait_pred_and_base_ty,
                     );
-                    if self.predicate_may_hold(&obligation) {
+                    let sized_obligation = Obligation::new(
+                        self.tcx,
+                        obligation.cause.clone(),
+                        obligation.param_env,
+                        ty::TraitRef::from_lang_item(
+                            self.tcx,
+                            hir::LangItem::Sized,
+                            obligation.cause.span,
+                            [base_ty],
+                        ),
+                    );
+                    if self.predicate_may_hold(&obligation)
+                        && self.predicate_must_hold_modulo_regions(&sized_obligation)
+                    {
                         let call_node = self.tcx.hir().get(*call_hir_id);
                         let msg = "consider dereferencing here";
                         let is_receiver = matches!(

tests/ui/traits/object/enforce-supertrait-projection.rs

@@ -7,7 +7,7 @@ trait Trait: SuperTrait<A = <Self as SuperTrait>::B> {}
 
 fn transmute<A, B>(x: A) -> B {
     foo::<A, B, dyn Trait<A = A, B = B>>(x)
-    //~^ ERROR type mismatch resolving `<dyn Trait<B = B, A = A> as SuperTrait>::A == B`
+    //~^ ERROR type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
 }
 
 fn foo<A, B, T: ?Sized>(x: T::A) -> B

tests/ui/traits/object/enforce-supertrait-projection.stderr

@@ -1,4 +1,4 @@
-error[E0271]: type mismatch resolving `<dyn Trait<B = B, A = A> as SuperTrait>::A == B`
+error[E0271]: type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
   --> $DIR/enforce-supertrait-projection.rs:9:17
    |
 LL | fn transmute<A, B>(x: A) -> B {

tests/ui/traits/suggest-dereferences/dont-suggest-unsize-deref.rs

@@ -0,0 +1,15 @@
+fn use_iterator<I>(itr: I)
+where
+    I: IntoIterator<Item = i32>,
+{
+}
+
+fn pass_iterator<I>(i: &dyn IntoIterator<Item = i32, IntoIter = I>)
+where
+    I: Iterator<Item = i32>,
+{
+    use_iterator(i);
+    //~^ ERROR `&dyn IntoIterator<IntoIter = I, Item = i32>` is not an iterator
+}
+
+fn main() {}

tests/ui/traits/suggest-dereferences/dont-suggest-unsize-deref.stderr

@@ -0,0 +1,22 @@
+error[E0277]: `&dyn IntoIterator<IntoIter = I, Item = i32>` is not an iterator
+  --> $DIR/dont-suggest-unsize-deref.rs:11:18
+   |
+LL |     use_iterator(i);
+   |     ------------ ^ `&dyn IntoIterator<IntoIter = I, Item = i32>` is not an iterator
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Iterator` is not implemented for `&dyn IntoIterator<IntoIter = I, Item = i32>`
+   = note: required for `&dyn IntoIterator<IntoIter = I, Item = i32>` to implement `IntoIterator`
+note: required by a bound in `use_iterator`
+  --> $DIR/dont-suggest-unsize-deref.rs:3:8
+   |
+LL | fn use_iterator<I>(itr: I)
+   |    ------------ required by a bound in this function
+LL | where
+LL |     I: IntoIterator<Item = i32>,
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `use_iterator`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.