clean up error messages for Send / Sync requirements on #[pyclass] (#5750)

* clean up error messages for Send / Sync requirements on `#[pyclass]`

* fix doctest

Author: davidhewitt

guide/src/class.md

@@ -1528,7 +1528,7 @@ impl pyo3::impl_::pyclass::PyClassImpl for MyClass {
     const IS_SEQUENCE: bool = false;
     type Layout = <Self::BaseNativeType as pyo3::impl_::pyclass::PyClassBaseType>::Layout<Self>;
     type BaseType = PyAny;
-    type ThreadChecker = pyo3::impl_::pyclass::SendablePyClass<MyClass>;
+    type ThreadChecker = pyo3::impl_::pyclass::NoopThreadChecker;
     type PyClassMutability = <<pyo3::PyAny as pyo3::impl_::pyclass::PyClassBaseType>::PyClassMutability as pyo3::impl_::pycell::PyClassMutability>::MutableChild;
     type Dict = pyo3::impl_::pyclass::PyClassDummySlot;
     type WeakRef = pyo3::impl_::pyclass::PyClassDummySlot;

pyo3-macros-backend/src/pyclass.rs

@@ -2612,7 +2612,7 @@ impl<'a> PyClassImplsBuilder<'a> {
         let thread_checker = if self.attr.options.unsendable.is_some() {
             quote! { #pyo3_path::impl_::pyclass::ThreadCheckerImpl }
         } else {
-            quote! { #pyo3_path::impl_::pyclass::SendablePyClass<#cls> }
+            quote! { #pyo3_path::impl_::pyclass::NoopThreadChecker }
         };
 
         let (pymethods_items, inventory, inventory_class) = match self.methods_type {
@@ -2697,11 +2697,9 @@ impl<'a> PyClassImplsBuilder<'a> {
         let assertions = if attr.options.unsendable.is_some() {
             TokenStream::new()
         } else {
-            let assert = quote_spanned! { cls.span() => #pyo3_path::impl_::pyclass::assert_pyclass_sync::<#cls>(); };
+            let assert = quote_spanned! { cls.span() => #pyo3_path::impl_::pyclass::assert_pyclass_send_sync::<#cls>() };
             quote! {
-                const _: () = {
-                    #assert
-                };
+                const _: () = #assert;
             }
         };
 

src/impl_/pyclass.rs

@@ -51,7 +51,7 @@ mod sealed {
     impl Sealed for super::PyClassDictSlot {}
     impl Sealed for super::PyClassWeakRefSlot {}
     impl Sealed for super::ThreadCheckerImpl {}
-    impl<T: Send> Sealed for super::SendablePyClass<T> {}
+    impl Sealed for super::NoopThreadChecker {}
 }
 
 /// Represents the `__dict__` field for `#[pyclass]`.
@@ -1028,14 +1028,10 @@ pub trait PyClassThreadChecker<T>: Sized + sealed::Sealed {
 }
 
 /// Default thread checker for `#[pyclass]`.
-///
-/// Keeping the T: Send bound here slightly improves the compile
-/// error message to hint to users to figure out what's wrong
-/// when `#[pyclass]` types do not implement `Send`.
 #[doc(hidden)]
-pub struct SendablePyClass<T: Send>(PhantomData<T>);
+pub struct NoopThreadChecker;
 
-impl<T: Send> PyClassThreadChecker<T> for SendablePyClass<T> {
+impl<T> PyClassThreadChecker<T> for NoopThreadChecker {
     fn ensure(&self) {}
     fn check(&self) -> bool {
         true
@@ -1045,7 +1041,7 @@ impl<T: Send> PyClassThreadChecker<T> for SendablePyClass<T> {
     }
     #[inline]
     fn new() -> Self {
-        SendablePyClass(PhantomData)
+        NoopThreadChecker
     }
 }
 

src/impl_/pyclass/assertions.rs

@@ -1,37 +1,19 @@
 /// Helper function that can be used at compile time to emit a diagnostic if
-/// the type does not implement `Sync` when it should.
-///
-/// The mere act of invoking this function will cause the diagnostic to be
-/// emitted if `T` does not implement `Sync` when it should.
-///
-/// The additional `const IS_SYNC: bool` parameter is used to allow the custom
-/// diagnostic to be emitted; if `PyClassSync`
-#[allow(unused)]
-pub const fn assert_pyclass_sync<T>()
+/// the type does not implement `Send` or `Sync` when it should; the mere act
+/// of invoking this function will cause the diagnostic to be emitted if needed.
+pub const fn assert_pyclass_send_sync<T>()
 where
-    T: PyClassSync + Sync,
+    T: Send + Sync,
 {
 }
 
-#[diagnostic::on_unimplemented(
-    message = "the trait `Sync` is not implemented for `{Self}`",
-    label = "required by `#[pyclass]`",
-    note = "replace thread-unsafe fields with thread-safe alternatives",
-    note = "see <TODO INSERT PYO3 GUIDE> for more information"
-)]
-pub trait PyClassSync<T: Sync = Self> {}
-
-impl<T> PyClassSync for T where T: Sync {}
-
 mod tests {
     #[cfg(feature = "macros")]
     #[test]
-    fn test_assert_pyclass_sync() {
-        use super::assert_pyclass_sync;
-
+    fn test_assert_pyclass_send_sync() {
         #[crate::pyclass(crate = "crate")]
         struct MyClass {}
 
-        assert_pyclass_sync::<MyClass>();
+        super::assert_pyclass_send_sync::<MyClass>();
     }
 }

tests/ui/pyclass_send.stderr

@@ -1,90 +1,23 @@
 error[E0277]: `*mut c_void` cannot be sent between threads safely
- --> tests/ui/pyclass_send.rs:4:1
-  |
-4 | #[pyclass]
-  | ^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
-  |
-  = help: within `NotSyncNotSend`, the trait `Send` is not implemented for `*mut c_void`
-help: the trait `pyo3::impl_::pyclass::PyClassThreadChecker<T>` is implemented for `SendablePyClass<T>`
- --> src/impl_/pyclass.rs
-  |
-  | impl<T: Send> PyClassThreadChecker<T> for SendablePyClass<T> {
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: required because it appears within the type `NotSyncNotSend`
  --> tests/ui/pyclass_send.rs:5:8
   |
 5 | struct NotSyncNotSend(*mut c_void);
-  |        ^^^^^^^^^^^^^^
-  = note: required for `SendablePyClass<NotSyncNotSend>` to implement `pyo3::impl_::pyclass::PyClassThreadChecker<NotSyncNotSend>`
-note: required by a bound in `PyClassImpl::ThreadChecker`
- --> src/impl_/pyclass.rs
-  |
-  |     type ThreadChecker: PyClassThreadChecker<Self>;
-  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `PyClassImpl::ThreadChecker`
-  = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0277]: `*mut c_void` cannot be sent between threads safely
-  --> tests/ui/pyclass_send.rs:11:1
-   |
-11 | #[pyclass]
-   | ^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
-   |
-   = help: within `SyncNotSend`, the trait `Send` is not implemented for `*mut c_void`
-help: the trait `pyo3::impl_::pyclass::PyClassThreadChecker<T>` is implemented for `SendablePyClass<T>`
-  --> src/impl_/pyclass.rs
-   |
-   | impl<T: Send> PyClassThreadChecker<T> for SendablePyClass<T> {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: required because it appears within the type `SyncNotSend`
-  --> tests/ui/pyclass_send.rs:12:8
-   |
-12 | struct SyncNotSend(*mut c_void);
-   |        ^^^^^^^^^^^
-   = note: required for `SendablePyClass<SyncNotSend>` to implement `pyo3::impl_::pyclass::PyClassThreadChecker<SyncNotSend>`
-note: required by a bound in `PyClassImpl::ThreadChecker`
-  --> src/impl_/pyclass.rs
-   |
-   |     type ThreadChecker: PyClassThreadChecker<Self>;
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `PyClassImpl::ThreadChecker`
-   = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0277]: `*mut c_void` cannot be sent between threads safely
- --> tests/ui/pyclass_send.rs:4:1
-  |
-4 | #[pyclass]
-  | ^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
+  |        ^^^^^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
   |
   = help: within `NotSyncNotSend`, the trait `Send` is not implemented for `*mut c_void`
 note: required because it appears within the type `NotSyncNotSend`
  --> tests/ui/pyclass_send.rs:5:8
   |
 5 | struct NotSyncNotSend(*mut c_void);
   |        ^^^^^^^^^^^^^^
-note: required by a bound in `SendablePyClass`
- --> src/impl_/pyclass.rs
+note: required by a bound in `assert_pyclass_send_sync`
+ --> src/impl_/pyclass/assertions.rs
   |
-  | pub struct SendablePyClass<T: Send>(PhantomData<T>);
-  |                               ^^^^ required by this bound in `SendablePyClass`
-  = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0277]: `*mut c_void` cannot be sent between threads safely
-  --> tests/ui/pyclass_send.rs:11:1
-   |
-11 | #[pyclass]
-   | ^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
-   |
-   = help: within `SyncNotSend`, the trait `Send` is not implemented for `*mut c_void`
-note: required because it appears within the type `SyncNotSend`
-  --> tests/ui/pyclass_send.rs:12:8
-   |
-12 | struct SyncNotSend(*mut c_void);
-   |        ^^^^^^^^^^^
-note: required by a bound in `SendablePyClass`
-  --> src/impl_/pyclass.rs
-   |
-   | pub struct SendablePyClass<T: Send>(PhantomData<T>);
-   |                               ^^^^ required by this bound in `SendablePyClass`
-   = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)
+  | pub const fn assert_pyclass_send_sync<T>()
+  |              ------------------------ required by a bound in this function
+  | where
+  |     T: Send + Sync,
+  |        ^^^^ required by this bound in `assert_pyclass_send_sync`
 
 error[E0277]: `*mut c_void` cannot be shared between threads safely
  --> tests/ui/pyclass_send.rs:5:8
@@ -98,14 +31,14 @@ note: required because it appears within the type `NotSyncNotSend`
   |
 5 | struct NotSyncNotSend(*mut c_void);
   |        ^^^^^^^^^^^^^^
-note: required by a bound in `assert_pyclass_sync`
+note: required by a bound in `assert_pyclass_send_sync`
  --> src/impl_/pyclass/assertions.rs
   |
-  | pub const fn assert_pyclass_sync<T>()
-  |              ------------------- required by a bound in this function
+  | pub const fn assert_pyclass_send_sync<T>()
+  |              ------------------------ required by a bound in this function
   | where
-  |     T: PyClassSync + Sync,
-  |                      ^^^^ required by this bound in `assert_pyclass_sync`
+  |     T: Send + Sync,
+  |               ^^^^ required by this bound in `assert_pyclass_send_sync`
 
 error[E0277]: `*mut c_void` cannot be shared between threads safely
  --> tests/ui/pyclass_send.rs:8:8
@@ -119,11 +52,32 @@ note: required because it appears within the type `SendNotSync`
   |
 8 | struct SendNotSync(*mut c_void);
   |        ^^^^^^^^^^^
-note: required by a bound in `assert_pyclass_sync`
+note: required by a bound in `assert_pyclass_send_sync`
  --> src/impl_/pyclass/assertions.rs
   |
-  | pub const fn assert_pyclass_sync<T>()
-  |              ------------------- required by a bound in this function
+  | pub const fn assert_pyclass_send_sync<T>()
+  |              ------------------------ required by a bound in this function
   | where
-  |     T: PyClassSync + Sync,
-  |                      ^^^^ required by this bound in `assert_pyclass_sync`
+  |     T: Send + Sync,
+  |               ^^^^ required by this bound in `assert_pyclass_send_sync`
+
+error[E0277]: `*mut c_void` cannot be sent between threads safely
+  --> tests/ui/pyclass_send.rs:12:8
+   |
+12 | struct SyncNotSend(*mut c_void);
+   |        ^^^^^^^^^^^ `*mut c_void` cannot be sent between threads safely
+   |
+   = help: within `SyncNotSend`, the trait `Send` is not implemented for `*mut c_void`
+note: required because it appears within the type `SyncNotSend`
+  --> tests/ui/pyclass_send.rs:12:8
+   |
+12 | struct SyncNotSend(*mut c_void);
+   |        ^^^^^^^^^^^
+note: required by a bound in `assert_pyclass_send_sync`
+  --> src/impl_/pyclass/assertions.rs
+   |
+   | pub const fn assert_pyclass_send_sync<T>()
+   |              ------------------------ required by a bound in this function
+   | where
+   |     T: Send + Sync,
+   |        ^^^^ required by this bound in `assert_pyclass_send_sync`