Rollup merge of #131549 - compiler-errors:try-in-sync, r=spastorino

Add a note for `?` on a `impl Future<Output = Result<..>>` in sync function

It's confusing to `?` a future of a result in a sync function. We have a suggestion to `.await` it if we're in an async function, but not a sync function. Note that this is the case for sync functions, at least.

Let's be a bit more vague about a fix, since it's somewhat context dependent. For example, you could block on it, or you could make your function asynchronous. 🤷
This commit is contained in:
Matthias Krüger 2024-10-22 15:28:41 +02:00 committed by GitHub
commit 457087ed29
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7 changed files with 137 additions and 54 deletions

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@ -10,7 +10,8 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_data_structures::unord::UnordMap;
use rustc_errors::codes::*;
use rustc_errors::{
Applicability, Diag, ErrorGuaranteed, StashKey, Subdiagnostic, pluralize, struct_span_code_err,
Applicability, Diag, ErrorGuaranteed, MultiSpan, StashKey, Subdiagnostic, pluralize,
struct_span_code_err,
};
use rustc_hir::def::{CtorKind, DefKind, Res};
use rustc_hir::def_id::DefId;
@ -2763,12 +2764,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
field_ident.span,
"field not available in `impl Future`, but it is available in its `Output`",
);
err.span_suggestion_verbose(
base.span.shrink_to_hi(),
"consider `await`ing on the `Future` and access the field of its `Output`",
".await",
Applicability::MaybeIncorrect,
);
match self.tcx.coroutine_kind(self.body_id) {
Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
err.span_suggestion_verbose(
base.span.shrink_to_hi(),
"consider `await`ing on the `Future` to access the field",
".await",
Applicability::MaybeIncorrect,
);
}
_ => {
let mut span: MultiSpan = base.span.into();
span.push_span_label(self.tcx.def_span(self.body_id), "this is not `async`");
err.span_note(
span,
"this implements `Future` and its output type has the field, \
but the future cannot be awaited in a synchronous function",
);
}
}
}
fn ban_nonexisting_field(

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@ -3594,52 +3594,64 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
trait_pred: ty::PolyTraitPredicate<'tcx>,
span: Span,
) {
if let Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) =
self.tcx.coroutine_kind(obligation.cause.body_id)
let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
let impls_future = self.type_implements_trait(
future_trait,
[self.tcx.instantiate_bound_regions_with_erased(self_ty)],
obligation.param_env,
);
if !impls_future.must_apply_modulo_regions() {
return;
}
let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
// `<T as Future>::Output`
let projection_ty = trait_pred.map_bound(|trait_pred| {
Ty::new_projection(
self.tcx,
item_def_id,
// Future::Output has no args
[trait_pred.self_ty()],
)
});
let InferOk { value: projection_ty, .. } =
self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
debug!(
normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
);
let try_obligation = self.mk_trait_obligation_with_new_self_ty(
obligation.param_env,
trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
);
debug!(try_trait_obligation = ?try_obligation);
if self.predicate_may_hold(&try_obligation)
&& let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
&& snippet.ends_with('?')
{
let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
let impls_future = self.type_implements_trait(
future_trait,
[self.tcx.instantiate_bound_regions_with_erased(self_ty)],
obligation.param_env,
);
if !impls_future.must_apply_modulo_regions() {
return;
}
let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
// `<T as Future>::Output`
let projection_ty = trait_pred.map_bound(|trait_pred| {
Ty::new_projection(
self.tcx,
item_def_id,
// Future::Output has no args
[trait_pred.self_ty()],
)
});
let InferOk { value: projection_ty, .. } =
self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
debug!(
normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
);
let try_obligation = self.mk_trait_obligation_with_new_self_ty(
obligation.param_env,
trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
);
debug!(try_trait_obligation = ?try_obligation);
if self.predicate_may_hold(&try_obligation)
&& let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
&& snippet.ends_with('?')
{
err.span_suggestion_verbose(
span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
"consider `await`ing on the `Future`",
".await",
Applicability::MaybeIncorrect,
);
match self.tcx.coroutine_kind(obligation.cause.body_id) {
Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _)) => {
err.span_suggestion_verbose(
span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
"consider `await`ing on the `Future`",
".await",
Applicability::MaybeIncorrect,
);
}
_ => {
let mut span: MultiSpan = span.with_lo(span.hi() - BytePos(1)).into();
span.push_span_label(
self.tcx.def_span(obligation.cause.body_id),
"this is not `async`",
);
err.span_note(
span,
"this implements `Future` and its output type supports \
`?`, but the future cannot be awaited in a synchronous function",
);
}
}
}
}

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@ -0,0 +1,13 @@
//@ edition: 2021
struct S {
field: (),
}
async fn foo() -> S { todo!() }
fn main() -> Result<(), ()> {
foo().field;
//~^ ERROR no field `field` on type `impl Future<Output = S>`
Ok(())
}

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@ -0,0 +1,17 @@
error[E0609]: no field `field` on type `impl Future<Output = S>`
--> $DIR/field-in-sync.rs:10:11
|
LL | foo().field;
| ^^^^^ field not available in `impl Future`, but it is available in its `Output`
|
note: this implements `Future` and its output type has the field, but the future cannot be awaited in a synchronous function
--> $DIR/field-in-sync.rs:10:5
|
LL | fn main() -> Result<(), ()> {
| --------------------------- this is not `async`
LL | foo().field;
| ^^^^^
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0609`.

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@ -28,7 +28,7 @@ error[E0609]: no field `0` on type `impl Future<Output = Tuple>`
LL | let _: i32 = tuple().0;
| ^ field not available in `impl Future`, but it is available in its `Output`
|
help: consider `await`ing on the `Future` and access the field of its `Output`
help: consider `await`ing on the `Future` to access the field
|
LL | let _: i32 = tuple().await.0;
| ++++++
@ -39,7 +39,7 @@ error[E0609]: no field `a` on type `impl Future<Output = Struct>`
LL | let _: i32 = struct_().a;
| ^ field not available in `impl Future`, but it is available in its `Output`
|
help: consider `await`ing on the `Future` and access the field of its `Output`
help: consider `await`ing on the `Future` to access the field
|
LL | let _: i32 = struct_().await.a;
| ++++++

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@ -0,0 +1,9 @@
//@ edition: 2021
async fn foo() -> Result<(), ()> { todo!() }
fn main() -> Result<(), ()> {
foo()?;
//~^ ERROR the `?` operator can only be applied to values that implement `Try`
Ok(())
}

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@ -0,0 +1,18 @@
error[E0277]: the `?` operator can only be applied to values that implement `Try`
--> $DIR/try-in-sync.rs:6:5
|
LL | foo()?;
| ^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = Result<(), ()>>`
|
= help: the trait `Try` is not implemented for `impl Future<Output = Result<(), ()>>`
note: this implements `Future` and its output type supports `?`, but the future cannot be awaited in a synchronous function
--> $DIR/try-in-sync.rs:6:10
|
LL | fn main() -> Result<(), ()> {
| --------------------------- this is not `async`
LL | foo()?;
| ^
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0277`.