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Suggest a T: Send bound for &mut T upvars in Send generators #105839

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Merged
merged 1 commit into from
Dec 19, 2022
Merged

Suggest a T: Send bound for &mut T upvars in Send generators #105839

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39 changes: 22 additions & 17 deletions compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2344,28 +2344,33 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
}
}
GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
// `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
let refers_to_non_sync = match target_ty.kind() {
ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
Ok(eval) if !eval.may_apply() => Some(ref_ty),
// `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
let non_send = match target_ty.kind() {
ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
_ => None,
},
_ => None,
};

let (span_label, span_note) = match refers_to_non_sync {
// if `target_ty` is `&T` and `T` fails to impl `Sync`,
// include suggestions to make `T: Sync` so that `&T: Send`
Some(ref_ty) => (
format!(
"has type `{}` which {}, because `{}` is not `Sync`",
target_ty, trait_explanation, ref_ty
),
format!(
"captured value {} because `&` references cannot be sent unless their referent is `Sync`",
trait_explanation
),
),
let (span_label, span_note) = match non_send {
// if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
// include suggestions to make `T: Sync` so that `&T: Send`,
// or to make `T: Send` so that `&mut T: Send`
Some((ref_ty, is_mut)) => {
let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
let ref_kind = if is_mut { "&mut" } else { "&" };
(
format!(
"has type `{}` which {}, because `{}` is not `{}`",
target_ty, trait_explanation, ref_ty, ref_ty_trait
),
format!(
"captured value {} because `{}` references cannot be sent unless their referent is `{}`",
trait_explanation, ref_kind, ref_ty_trait
),
)
}
None => (
format!("has type `{}` which {}", target_ty, trait_explanation),
format!("captured value {}", trait_explanation),
Expand Down
31 changes: 31 additions & 0 deletions src/test/ui/generator/ref-upvar-not-send.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,31 @@
// For `Send` generators, suggest a `T: Sync` requirement for `&T` upvars,
// and suggest a `T: Send` requirement for `&mut T` upvars.

#![feature(generators)]

fn assert_send<T: Send>(_: T) {}
//~^ NOTE required by a bound in `assert_send`
//~| NOTE required by this bound in `assert_send`
//~| NOTE required by a bound in `assert_send`
//~| NOTE required by this bound in `assert_send`

fn main() {
let x: &*mut () = &std::ptr::null_mut();
let y: &mut *mut () = &mut std::ptr::null_mut();
assert_send(move || {
//~^ ERROR generator cannot be sent between threads safely
//~| NOTE generator is not `Send`
yield;
let _x = x;
});
//~^^ NOTE captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
//~| NOTE has type `&*mut ()` which is not `Send`, because `*mut ()` is not `Sync`
assert_send(move || {
//~^ ERROR generator cannot be sent between threads safely
//~| NOTE generator is not `Send`
yield;
let _y = y;
});
//~^^ NOTE captured value is not `Send` because `&mut` references cannot be sent unless their referent is `Send`
//~| NOTE has type `&mut *mut ()` which is not `Send`, because `*mut ()` is not `Send`
}
50 changes: 50 additions & 0 deletions src/test/ui/generator/ref-upvar-not-send.stderr
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@@ -0,0 +1,50 @@
error: generator cannot be sent between threads safely
--> $DIR/ref-upvar-not-send.rs:15:17
|
LL | assert_send(move || {
| _________________^
LL | |
LL | |
LL | | yield;
LL | | let _x = x;
LL | | });
| |_____^ generator is not `Send`
|
= help: the trait `Sync` is not implemented for `*mut ()`
note: captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync`
--> $DIR/ref-upvar-not-send.rs:19:18
|
LL | let _x = x;
| ^ has type `&*mut ()` which is not `Send`, because `*mut ()` is not `Sync`
note: required by a bound in `assert_send`
--> $DIR/ref-upvar-not-send.rs:6:19
|
LL | fn assert_send<T: Send>(_: T) {}
| ^^^^ required by this bound in `assert_send`

error: generator cannot be sent between threads safely
--> $DIR/ref-upvar-not-send.rs:23:17
|
LL | assert_send(move || {
| _________________^
LL | |
LL | |
LL | | yield;
LL | | let _y = y;
LL | | });
| |_____^ generator is not `Send`
|
= help: within `[generator@$DIR/ref-upvar-not-send.rs:23:17: 23:24]`, the trait `Send` is not implemented for `*mut ()`
note: captured value is not `Send` because `&mut` references cannot be sent unless their referent is `Send`
--> $DIR/ref-upvar-not-send.rs:27:18
|
LL | let _y = y;
| ^ has type `&mut *mut ()` which is not `Send`, because `*mut ()` is not `Send`
note: required by a bound in `assert_send`
--> $DIR/ref-upvar-not-send.rs:6:19
|
LL | fn assert_send<T: Send>(_: T) {}
| ^^^^ required by this bound in `assert_send`

error: aborting due to 2 previous errors