On E0308 involving dyn Trait, mention trait objects

When encountering a type mismatch error involving `dyn Trait`, mention
the existence of boxed trait objects if the other type involved
implements `Trait`.

Partially addresses #102629.

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -294,6 +294,55 @@ fn foo(&self, x: T) -> T { x }
                             );
                         }
                     }
+                    (ty::Dynamic(t, _, ty::DynKind::Dyn), _)
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}` so you could box the found value \
+                                 and coerce it to the trait object `Box<dyn {trait_name}>`, you \
+                                 will have to change the expected type as well",
+                                values.found,
+                            ));
+                        }
+                    }
+                    (_, ty::Dynamic(t, _, ty::DynKind::Dyn))
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.expected, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}` so you could change the expected \
+                                 type to `Box<dyn {trait_name}>`",
+                                values.expected,
+                            ));
+                        }
+                    }
+                    (ty::Dynamic(t, _, ty::DynKind::DynStar), _)
+                        if let Some(def_id) = t.principal_def_id() =>
+                    {
+                        let mut impl_def_ids = vec![];
+                        tcx.for_each_relevant_impl(def_id, values.found, |did| {
+                            impl_def_ids.push(did)
+                        });
+                        if let [_] = &impl_def_ids[..] {
+                            let trait_name = tcx.item_name(def_id);
+                            diag.help(format!(
+                                "`{}` implements `{trait_name}`, `#[feature(dyn_star)]` is likely \
+                                 not enabled; that feature it is currently incomplete",
+                                values.found,
+                            ));
+                        }
+                    }
                     (_, ty::Alias(ty::Opaque, opaque_ty))
                     | (ty::Alias(ty::Opaque, opaque_ty), _) => {
                         if opaque_ty.def_id.is_local()

tests/ui/dst/dst-bad-assign-3.stderr

@@ -8,6 +8,7 @@ LL |     f5.2 = Bar1 {f: 36};
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
+   = help: `Bar1` implements `ToBar` so you could box the found value and coerce it to the trait object `Box<dyn ToBar>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `dyn ToBar` cannot be known at compilation time
   --> $DIR/dst-bad-assign-3.rs:33:5

tests/ui/dst/dst-bad-assign.stderr

@@ -8,6 +8,7 @@ LL |     f5.ptr = Bar1 {f: 36};
    |
    = note: expected trait object `dyn ToBar`
                     found struct `Bar1`
+   = help: `Bar1` implements `ToBar` so you could box the found value and coerce it to the trait object `Box<dyn ToBar>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `dyn ToBar` cannot be known at compilation time
   --> $DIR/dst-bad-assign.rs:35:5

tests/ui/dyn-star/no-implicit-dyn-star.stderr

@@ -8,6 +8,7 @@ LL |     dyn_star_foreign::require_dyn_star_display(1usize);
    |
    = note: expected trait object `(dyn* std::fmt::Display + 'static)`
                       found type `usize`
+   = help: `usize` implements `Display`, `#[feature(dyn_star)]` is likely not enabled; that feature it is currently incomplete
 note: function defined here
   --> $DIR/auxiliary/dyn-star-foreign.rs:6:8
    |

tests/ui/generic-associated-types/issue-79422.extended.stderr

@@ -27,6 +27,7 @@ LL |     type VRefCont<'a> = &'a V where Self: 'a;
    |                         ^^^^^
    = note: expected trait object `(dyn RefCont<'_, u8> + 'static)`
                  found reference `&u8`
+   = help: `&u8` implements `RefCont` so you could box the found value and coerce it to the trait object `Box<dyn RefCont>`, you will have to change the expected type as well
    = note: required for the cast from `Box<BTreeMap<u8, u8>>` to `Box<dyn MapLike<u8, u8, VRefCont = (dyn RefCont<'_, u8> + 'static)>>`
 
 error: aborting due to 2 previous errors

tests/ui/impl-trait/dyn-trait-return-should-be-impl-trait.stderr

@@ -6,6 +6,7 @@ LL | fn fuz() -> (usize, Trait) { (42, Struct) }
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
+   = help: `Struct` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:13
@@ -27,6 +28,7 @@ LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
    |
    = note: expected trait object `(dyn Trait + 'static)`
                     found struct `Struct`
+   = help: `Struct` implements `Trait` so you could box the found value and coerce it to the trait object `Box<dyn Trait>`, you will have to change the expected type as well
 
 error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
   --> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:13

tests/ui/methods/issues/issue-61525.stderr

@@ -27,6 +27,7 @@ LL |         1.query::<dyn ToString>("")
    |
    = note: expected trait object `dyn ToString`
                  found reference `&'static str`
+   = help: `&'static str` implements `ToString` so you could box the found value and coerce it to the trait object `Box<dyn ToString>`, you will have to change the expected type as well
 note: method defined here
   --> $DIR/issue-61525.rs:2:8
    |