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Rollup merge of #111087 - ibraheemdev:patch-15, r=dtolnay

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Implement `Sync` for `mpsc::Sender`

`mpsc::Sender` is currently `!Sync` because the previous implementation contained an optimization where the channel started out as single-producer and was dynamically upgraded on the first clone, which relied on a unique reference to the sender. This optimization is one of the main reasons the old implementation was so complex and was removed in #93563. `mpsc::Sender` can now soundly implement `Sync`.

Note for any potential confusion, this chance does *not* add MPMC behavior. This only affects the already `Send + Clone` *sender*, not *receiver*.

It's technically possible to rely on the `!Sync` behavior in the same way as a `PhantomData<*mut T>`, but that seems very unlikely in practice. Either way, this change is insta-stable and needs an FCP.

`@rustbot` label +T-libs-api -T-libs
This commit is contained in:
Michael Goulet 2023-06-23 19:47:19 -07:00 committed by GitHub
commit 4a01a38466
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9 changed files with 61 additions and 81 deletions
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4
library/std/src/sync/mpsc/mod.rs
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@ -347,8 +347,8 @@ pub struct Sender<T> {
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for Sender<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> !Sync for Sender<T> {}
#[stable(feature = "mpsc_sender_sync", since = "CURRENT_RUSTC_VERSION")]
unsafe impl<T: Send> Sync for Sender<T> {}
/// The sending-half of Rust's synchronous [`sync_channel`] type.
///

31
tests/ui/async-await/issue-70935-complex-spans.drop_tracking.stderr
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@ -1,18 +1,25 @@
error[E0277]: `Sender<i32>` cannot be shared between threads safely
--> $DIR/issue-70935-complex-spans.rs:13:45
error[E0277]: `*mut ()` cannot be shared between threads safely
--> $DIR/issue-70935-complex-spans.rs:18:23
|
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ `Sender<i32>` cannot be shared between threads safely
LL | fn foo(x: NotSync) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ `*mut ()` cannot be shared between threads safely
|
= help: the trait `Sync` is not implemented for `Sender<i32>`
= note: required for `&Sender<i32>` to implement `Send`
= help: within `NotSync`, the trait `Sync` is not implemented for `*mut ()`
note: required because it appears within the type `PhantomData<*mut ()>`
--> $SRC_DIR/core/src/marker.rs:LL:COL
note: required because it appears within the type `NotSync`
--> $DIR/issue-70935-complex-spans.rs:12:8
|
LL | struct NotSync(PhantomData<*mut ()>);
| ^^^^^^^
= note: required for `&NotSync` to implement `Send`
note: required because it's used within this closure
--> $DIR/issue-70935-complex-spans.rs:17:13
--> $DIR/issue-70935-complex-spans.rs:22:13
|
LL | baz(|| async{
LL | baz(|| async {
| ^^
note: required because it's used within this `async fn` body
--> $DIR/issue-70935-complex-spans.rs:10:67
--> $DIR/issue-70935-complex-spans.rs:15:67
|
LL | async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {
| ___________________________________________________________________^
@ -20,11 +27,11 @@ LL | | }
| |_^
= note: required because it captures the following types: `ResumeTy`, `impl Future<Output = ()>`, `()`
note: required because it's used within this `async` block
--> $DIR/issue-70935-complex-spans.rs:16:5
--> $DIR/issue-70935-complex-spans.rs:21:5
|
LL | / async move {
LL | | baz(|| async{
LL | | foo(tx.clone());
LL | | baz(|| async {
LL | | foo(x.clone());
LL | | }).await;
LL | | }
| |_____^

31
tests/ui/async-await/issue-70935-complex-spans.drop_tracking_mir.stderr
View File

@ -1,18 +1,25 @@
error[E0277]: `Sender<i32>` cannot be shared between threads safely
--> $DIR/issue-70935-complex-spans.rs:13:45
error[E0277]: `*mut ()` cannot be shared between threads safely
--> $DIR/issue-70935-complex-spans.rs:18:23
|
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ `Sender<i32>` cannot be shared between threads safely
LL | fn foo(x: NotSync) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ `*mut ()` cannot be shared between threads safely
|
= help: the trait `Sync` is not implemented for `Sender<i32>`
= note: required for `&Sender<i32>` to implement `Send`
= help: within `NotSync`, the trait `Sync` is not implemented for `*mut ()`
note: required because it appears within the type `PhantomData<*mut ()>`
--> $SRC_DIR/core/src/marker.rs:LL:COL
note: required because it appears within the type `NotSync`
--> $DIR/issue-70935-complex-spans.rs:12:8
|
LL | struct NotSync(PhantomData<*mut ()>);
| ^^^^^^^
= note: required for `&NotSync` to implement `Send`
note: required because it's used within this closure
--> $DIR/issue-70935-complex-spans.rs:17:13
--> $DIR/issue-70935-complex-spans.rs:22:13
|
LL | baz(|| async{
LL | baz(|| async {
| ^^
note: required because it's used within this `async fn` body
--> $DIR/issue-70935-complex-spans.rs:10:67
--> $DIR/issue-70935-complex-spans.rs:15:67
|
LL | async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {
| ___________________________________________________________________^
@ -20,11 +27,11 @@ LL | | }
| |_^
= note: required because it captures the following types: `impl Future<Output = ()>`
note: required because it's used within this `async` block
--> $DIR/issue-70935-complex-spans.rs:16:5
--> $DIR/issue-70935-complex-spans.rs:21:5
|
LL | / async move {
LL | | baz(|| async{
LL | | foo(tx.clone());
LL | | baz(|| async {
LL | | foo(x.clone());
LL | | }).await;
LL | | }
| |_____^

16
tests/ui/async-await/issue-70935-complex-spans.no_drop_tracking.stderr
View File

@ -1,21 +1,21 @@
error: future cannot be sent between threads safely
--> $DIR/issue-70935-complex-spans.rs:13:45
--> $DIR/issue-70935-complex-spans.rs:18:23
|
LL | fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
LL | fn foo(x: NotSync) -> impl Future + Send {
| ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
|
= help: the trait `Sync` is not implemented for `Sender<i32>`
= help: within `NotSync`, the trait `Sync` is not implemented for `*mut ()`
note: future is not `Send` as this value is used across an await
--> $DIR/issue-70935-complex-spans.rs:19:12
--> $DIR/issue-70935-complex-spans.rs:24:12
|
LL | baz(|| async{
LL | baz(|| async {
| _____________-
LL | | foo(tx.clone());
LL | | foo(x.clone());
LL | | }).await;
| | - ^^^^^- the value is later dropped here
| | | |
| |_________| await occurs here, with the value maybe used later
| has type `[closure@$DIR/issue-70935-complex-spans.rs:17:13: 17:15]` which is not `Send`
| has type `[closure@$DIR/issue-70935-complex-spans.rs:22:13: 22:15]` which is not `Send`
error: aborting due to previous error

17
tests/ui/async-await/issue-70935-complex-spans.rs
View File

@ -6,16 +6,21 @@
// with newlines which lead complex diagnostics.
use std::future::Future;
use std::marker::PhantomData;
#[derive(Clone)]
struct NotSync(PhantomData<*mut ()>);
unsafe impl Send for NotSync {}
async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {
}
fn foo(tx: std::sync::mpsc::Sender<i32>) -> impl Future + Send {
fn foo(x: NotSync) -> impl Future + Send {
//[no_drop_tracking]~^ ERROR future cannot be sent between threads safely
//[drop_tracking,drop_tracking_mir]~^^ ERROR `Sender<i32>` cannot be shared between threads
//[drop_tracking,drop_tracking_mir]~^^ ERROR `*mut ()` cannot be shared between threads
async move {
baz(|| async{
foo(tx.clone());
baz(|| async {
foo(x.clone());
}).await;
}
}
@ -24,6 +29,6 @@ fn bar(_s: impl Future + Send) {
}
fn main() {
let (tx, _rx) = std::sync::mpsc::channel();
bar(foo(tx));
let x = NotSync(PhantomData);
bar(foo(x));
}

6
tests/ui/closures/closure-move-sync.rs
View File

@ -13,10 +13,4 @@ fn bar() {
t.join().unwrap();
}
fn foo() {
let (tx, _rx) = channel();
thread::spawn(|| tx.send(()).unwrap());
//~^ ERROR `Sender<()>` cannot be shared between threads safely
}
fn main() {}

20
tests/ui/closures/closure-move-sync.stderr
View File

@ -20,24 +20,6 @@ LL | let t = thread::spawn(|| {
note: required by a bound in `spawn`
--> $SRC_DIR/std/src/thread/mod.rs:LL:COL
error[E0277]: `Sender<()>` cannot be shared between threads safely
--> $DIR/closure-move-sync.rs:18:19
|
LL | thread::spawn(|| tx.send(()).unwrap());
| ------------- ^^^^^^^^^^^^^^^^^^^^^^^ `Sender<()>` cannot be shared between threads safely
| |
| required by a bound introduced by this call
|
= help: the trait `Sync` is not implemented for `Sender<()>`
= note: required for `&Sender<()>` to implement `Send`
note: required because it's used within this closure
--> $DIR/closure-move-sync.rs:18:19
|
LL | thread::spawn(|| tx.send(()).unwrap());
| ^^
note: required by a bound in `spawn`
--> $SRC_DIR/std/src/thread/mod.rs:LL:COL
error: aborting due to 2 previous errors
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.

2
tests/ui/stdlib-unit-tests/not-sync.rs
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@ -17,6 +17,4 @@ fn main() {
test::<Receiver<i32>>();
//~^ ERROR `std::sync::mpsc::Receiver<i32>` cannot be shared between threads safely [E0277]
test::<Sender<i32>>();
//~^ ERROR `Sender<i32>` cannot be shared between threads safely [E0277]
}

15
tests/ui/stdlib-unit-tests/not-sync.stderr
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@ -65,19 +65,6 @@ note: required by a bound in `test`
LL | fn test<T: Sync>() {}
| ^^^^ required by this bound in `test`
error[E0277]: `Sender<i32>` cannot be shared between threads safely
--> $DIR/not-sync.rs:20:12
|
LL | test::<Sender<i32>>();
| ^^^^^^^^^^^ `Sender<i32>` cannot be shared between threads safely
|
= help: the trait `Sync` is not implemented for `Sender<i32>`
note: required by a bound in `test`
--> $DIR/not-sync.rs:5:12
|
LL | fn test<T: Sync>() {}
| ^^^^ required by this bound in `test`
error: aborting due to 6 previous errors
error: aborting due to 5 previous errors
For more information about this error, try `rustc --explain E0277`.