Point at type that doesn't implement needed trait

```
error[E0277]: `?` couldn't convert the error: `E: std::error::Error` is not satisfied
  --> $DIR/bad-question-mark-on-trait-object.rs:7:13
   |
LL | fn foo() -> Result<(), Box<dyn std::error::Error>> {
   |             -------------------------------------- required `E: std::error::Error` because of this
LL |     Ok(bar()?)
   |        -----^ the trait `std::error::Error` is not implemented for `E`
   |        |
   |        this has type `Result<_, E>`
   |
note: `E` needs to implement `std::error::Error`
  --> $DIR/bad-question-mark-on-trait-object.rs:1:1
   |
LL | struct E;
   | ^^^^^^^^
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = note: required for `Box<dyn std::error::Error>` to implement `From<E>`

error[E0277]: `?` couldn't convert the error to `X`
  --> $DIR/bad-question-mark-on-trait-object.rs:18:13
   |
LL | fn bat() -> Result<(), X> {
   |             ------------- expected `X` because of this
LL |     Ok(bar()?)
   |        -----^ the trait `From<E>` is not implemented for `X`
   |        |
   |        this can't be annotated with `?` because it has type `Result<_, E>`
   |
note: `X` needs to implement `From<E>`
  --> $DIR/bad-question-mark-on-trait-object.rs:4:1
   |
LL | struct X;
   | ^^^^^^^^
note: alternatively, `E` needs to implement `Into<X>`
  --> $DIR/bad-question-mark-on-trait-object.rs:1:1
   |
LL | struct E;
   | ^^^^^^^^
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
```

Author: estebank

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

@@ -966,6 +966,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         };
         let self_ty = trait_pred.skip_binder().self_ty();
         let found_ty = trait_pred.skip_binder().trait_ref.args.get(1).and_then(|a| a.as_type());
+        self.note_missing_impl_for_question_mark(err, self_ty, found_ty, trait_pred);
 
         let mut prev_ty = self.resolve_vars_if_possible(
             typeck.expr_ty_adjusted_opt(expr).unwrap_or(Ty::new_misc_error(self.tcx)),
@@ -1130,6 +1131,56 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         suggested
     }
 
+    fn note_missing_impl_for_question_mark(
+        &self,
+        err: &mut Diag<'_>,
+        self_ty: Ty<'_>,
+        found_ty: Option<Ty<'_>>,
+        trait_pred: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        match (self_ty.kind(), found_ty) {
+            (ty::Adt(def, _), Some(ty))
+                if let ty::Adt(found, _) = ty.kind()
+                    && def.did().is_local()
+                    && found.did().is_local() =>
+            {
+                err.span_note(
+                    self.tcx.def_span(def.did()),
+                    format!("`{self_ty}` needs to implement `From<{ty}>`"),
+                );
+                err.span_note(
+                    self.tcx.def_span(found.did()),
+                    format!("alternatively, `{ty}` needs to implement `Into<{self_ty}>`"),
+                );
+            }
+            (ty::Adt(def, _), None) if def.did().is_local() => {
+                err.span_note(
+                    self.tcx.def_span(def.did()),
+                    format!(
+                        "`{self_ty}` needs to implement `{}`",
+                        trait_pred.skip_binder().trait_ref.print_only_trait_path(),
+                    ),
+                );
+            }
+            (ty::Adt(def, _), Some(ty)) if def.did().is_local() => {
+                err.span_note(
+                    self.tcx.def_span(def.did()),
+                    format!("`{self_ty}` needs to implement `From<{ty}>`"),
+                );
+            }
+            (_, Some(ty))
+                if let ty::Adt(def, _) = ty.kind()
+                    && def.did().is_local() =>
+            {
+                err.span_note(
+                    self.tcx.def_span(def.did()),
+                    format!("`{ty}` needs to implement `Into<{self_ty}>`"),
+                );
+            }
+            _ => {}
+        }
+    }
+
     fn report_const_param_not_wf(
         &self,
         ty: Ty<'tcx>,

tests/ui/try-trait/bad-question-mark-on-trait-object.rs

@@ -1,5 +1,7 @@
 struct E;
-struct X;
+//~^ NOTE `E` needs to implement `std::error::Error`
+//~| NOTE alternatively, `E` needs to implement `Into<X>`
+struct X; //~ NOTE `X` needs to implement `From<E>`
 
 fn foo() -> Result<(), Box<dyn std::error::Error>> { //~ NOTE required `E: std::error::Error` because of this
     Ok(bar()?)

tests/ui/try-trait/bad-question-mark-on-trait-object.stderr

@@ -1,5 +1,5 @@
 error[E0277]: `?` couldn't convert the error: `E: std::error::Error` is not satisfied
-  --> $DIR/bad-question-mark-on-trait-object.rs:5:13
+  --> $DIR/bad-question-mark-on-trait-object.rs:7:13
    |
 LL | fn foo() -> Result<(), Box<dyn std::error::Error>> {
    |             -------------------------------------- required `E: std::error::Error` because of this
@@ -8,11 +8,16 @@ LL |     Ok(bar()?)
    |        |
    |        this has type `Result<_, E>`
    |
+note: `E` needs to implement `std::error::Error`
+  --> $DIR/bad-question-mark-on-trait-object.rs:1:1
+   |
+LL | struct E;
+   | ^^^^^^^^
    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
    = note: required for `Box<dyn std::error::Error>` to implement `From<E>`
 
 error[E0277]: `?` couldn't convert the error to `X`
-  --> $DIR/bad-question-mark-on-trait-object.rs:16:13
+  --> $DIR/bad-question-mark-on-trait-object.rs:18:13
    |
 LL | fn bat() -> Result<(), X> {
    |             ------------- expected `X` because of this
@@ -21,6 +26,16 @@ LL |     Ok(bar()?)
    |        |
    |        this can't be annotated with `?` because it has type `Result<_, E>`
    |
+note: `X` needs to implement `From<E>`
+  --> $DIR/bad-question-mark-on-trait-object.rs:4:1
+   |
+LL | struct X;
+   | ^^^^^^^^
+note: alternatively, `E` needs to implement `Into<X>`
+  --> $DIR/bad-question-mark-on-trait-object.rs:1:1
+   |
+LL | struct E;
+   | ^^^^^^^^
    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
 
 error: aborting due to 2 previous errors