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Add a note when suggesting to use Impl Trait #112088

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0bba4e5
Add a note when suggesting to use Impl Trait
sladyn98 May 29, 2023
f863353
Updated tests
sladyn98 May 30, 2023
c4d04e7
Fix typo
sladyn98 May 30, 2023
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1 change: 1 addition & 0 deletions compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
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Expand Up @@ -856,6 +856,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
format!("impl {}", all_bounds_str),
Applicability::MaybeIncorrect,
);
err.note(format!("the type of `{}` is chosen by the caller", expected_ty_as_param.name));
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@compiler-errors compiler-errors May 30, 2023 •
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I'm not sure if this note adds much value in this context. Not exactly sure why, but something about the ordering or placement of this note feels to me like it's missing context.

In the tests/ui/return/return-impl-trait-bad.stderr stderr below:

LL | fn used_in_trait<T>() -> T
   |                  -       -
   |                  |       |
   |                  |       expected `T` because of return type
   |                  |       help: consider using an impl return type: `impl Send`
   |                  this type parameter

My sense would be that "this type parameter" label could be replaced with the text above... Not sure how hard that is though.

Alternatively, "the type of {} is chosen by the caller" should probably be extended to explain its significance. Maybe something along the lines of "and may be a type that is different than {(the other type mentioned)}"...

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@sladyn98 sladyn98 May 30, 2023

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I updated the message to explain its significance. However finding exactly where "this type parameter" is derived from and put into the message here is a bit hard. Do let me know if this is sufficient, or if not some more context on how I can find the construction of the above "this type parameter"

}
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@sladyn98 sladyn98 Jun 1, 2023

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@compiler-errors Here found_ty_as_param should return the parameter found right, or am I understanding something wrong

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@compiler-errors compiler-errors Jun 5, 2023

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You don't need found_ty_as_param -- the found ty might be any type, and you can just render that to string.

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@sladyn98 sladyn98 Jun 8, 2023

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    err.note(format!("the type of `{}` is chosen by the caller and may be a type that is different from `{}` ", expected_ty_as_param.name, found.to_string()));

Converting found to string still does not return the type expected.


pub(in super::super) fn suggest_missing_break_or_return_expr(
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1 change: 1 addition & 0 deletions tests/ui/return/return-impl-trait-bad.stderr
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Expand Up @@ -53,6 +53,7 @@ LL | "don't suggest this, because the generic param is used in the bound."
|
= note: expected type parameter `T`
found reference `&'static str`
= note: the type of `T` is chosen by the caller

error: aborting due to 4 previous errors

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2 changes: 2 additions & 0 deletions tests/ui/return/return-impl-trait.stderr
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Expand Up @@ -12,6 +12,7 @@ LL | ()
|
= note: expected type parameter `T`
found unit type `()`
= note: the type of `T` is chosen by the caller

error[E0308]: mismatched types
--> $DIR/return-impl-trait.rs:23:5
Expand All @@ -28,6 +29,7 @@ LL | ()
|
= note: expected type parameter `T`
found unit type `()`
= note: the type of `T` is chosen by the caller

error: aborting due to 2 previous errors

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