Rollup merge of #137232 - estebank:from-residual-note, r=petrochenkov

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>`
```

Author: matthiaskrgr

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

@@ -3289,6 +3289,14 @@ 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();

tests/ui/traits/question-mark-result-err-mismatch.rs

@@ -1,3 +1,4 @@
+#![feature(try_trait_v2)]
 fn foo() -> Result<String, String> { //~ NOTE expected `String` because of this
     let test = String::from("one,two");
     let x = test

tests/ui/traits/question-mark-result-err-mismatch.stderr

@@ -1,5 +1,5 @@
 error[E0277]: `?` couldn't convert the error to `String`
-  --> $DIR/question-mark-result-err-mismatch.rs:14:22
+  --> $DIR/question-mark-result-err-mismatch.rs:15:22
    |
 LL |   fn foo() -> Result<String, String> {
    |               ---------------------- expected `String` because of this
@@ -17,7 +17,7 @@ LL |           .map(|()| "")?;
    = note: required for `Result<String, String>` to implement `FromResidual<Result<Infallible, ()>>`
 
 error[E0277]: `?` couldn't convert the error to `String`
-  --> $DIR/question-mark-result-err-mismatch.rs:28:25
+  --> $DIR/question-mark-result-err-mismatch.rs:29:25
    |
 LL | fn bar() -> Result<(), String> {
    |             ------------------ expected `String` because of this
@@ -40,7 +40,7 @@ LL |         .map_err(|_| ())?;
    = note: required for `Result<(), String>` to implement `FromResidual<Result<Infallible, ()>>`
 
 error[E0277]: `?` couldn't convert the error to `String`
-  --> $DIR/question-mark-result-err-mismatch.rs:48:11
+  --> $DIR/question-mark-result-err-mismatch.rs:49:11
    |
 LL |   fn baz() -> Result<String, String> {
    |               ---------------------- expected `String` because of this

tests/ui/try-block/try-block-bad-type.stderr

@@ -10,7 +10,6 @@ LL |         Err("")?;
    = help: the trait `From<&str>` is not implemented for `TryFromSliceError`
            but trait `From<Infallible>` is implemented for it
    = help: for that trait implementation, expected `Infallible`, found `&str`
-   = note: required for `Result<u32, TryFromSliceError>` to implement `FromResidual<Result<Infallible, &str>>`
 
 error[E0271]: type mismatch resolving `<Result<i32, i32> as Try>::Output == &str`
   --> $DIR/try-block-bad-type.rs:12:9

tests/ui/try-trait/bad-interconversion.stderr

@@ -12,7 +12,6 @@ LL |     Ok(Err(123_i32)?)
    = help: the following other types implement trait `From<T>`:
              `u8` implements `From<Char>`
              `u8` implements `From<bool>`
-   = note: required for `Result<u64, u8>` to implement `FromResidual<Result<Infallible, i32>>`
 
 error[E0277]: the `?` operator can only be used on `Result`s, not `Option`s, in a function that returns `Result`
   --> $DIR/bad-interconversion.rs:9:12

tests/ui/try-trait/issue-32709.stderr

@@ -19,7 +19,6 @@ LL |     Err(5)?;
              `(T, T, T, T, T, T, T, T)` implements `From<[T; 8]>`
              `(T, T, T, T, T, T, T, T, T)` implements `From<[T; 9]>`
            and 4 others
-   = note: required for `Result<i32, ()>` to implement `FromResidual<Result<Infallible, {integer}>>`
 
 error: aborting due to 1 previous error