Don't mention `FromResidual` on bad `?`

Unless `try_trait_v2` is enabled, don't mention that `FromResidual` isn't implemented for a specific type when the implicit `From` conversion of a `?` fails. For the end user on stable, `?` might as well be a compiler intrinsic, so we remove that note to avoid further confusion and allowing other parts of the error to be more prominent.

```
error[E0277]: `?` couldn't convert the error to `u8`
  --> $DIR/bad-interconversion.rs:4:20
   |
LL | fn result_to_result() -> Result<u64, u8> {
   |                          --------------- expected `u8` because of this
LL |     Ok(Err(123_i32)?)
   |        ------------^ the trait `From<i32>` is not implemented for `u8`
   |        |
   |        this can't be annotated with `?` because it has type `Result<_, i32>`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the following other types implement trait `From<T>`:
             `u8` implements `From<Char>`
             `u8` implements `From<bool>`
```

1 files changed, 8 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 527d2e54e43..94d652a0e98 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -3293,6 +3293,14 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 let mut parent_trait_pred =
                     self.resolve_vars_if_possible(data.derived.parent_trait_pred);
                 let parent_def_id = parent_trait_pred.def_id();
+                if tcx.is_diagnostic_item(sym::FromResidual, parent_def_id)
+                    && !tcx.features().enabled(sym::try_trait_v2)
+                {
+                    // If `#![feature(try_trait_v2)]` is not enabled, then there's no point on
+                    // talking about `FromResidual<Result<A, B>>`, as the end user has nothing they
+                    // can do about it. As far as they are concerned, `?` is compiler magic.
+                    return;
+                }
                 let self_ty_str =
                     tcx.short_string(parent_trait_pred.skip_binder().self_ty(), err.long_ty_path());
                 let trait_name = parent_trait_pred.print_modifiers_and_trait_path().to_string();