Create a better error message for moving self (#2238)

pyo3-macros-backend/src/method.rs

@@ -157,14 +157,14 @@ impl SelfType {
                 quote! {
                     let _cell = #cell;
                     let _ref = _cell.try_borrow()?;
-                    let _slf = &_ref;
+                    let _slf: &#cls = &*_ref;
                 }
             }
             SelfType::Receiver { mutable: true } => {
                 quote! {
                     let _cell = #cell;
                     let mut _ref = _cell.try_borrow_mut()?;
-                    let _slf = &mut _ref;
+                    let _slf: &mut #cls = &mut *_ref;
                 }
             }
             SelfType::TryFromPyCell(span) => {
@@ -235,8 +235,15 @@ pub fn get_return_info(output: &syn::ReturnType) -> syn::Type {
 
 pub fn parse_method_receiver(arg: &syn::FnArg) -> Result<SelfType> {
     match arg {
-        syn::FnArg::Receiver(recv) => Ok(SelfType::Receiver {
+        syn::FnArg::Receiver(
-            mutable: recv.mutability.is_some(),
+            recv @ syn::Receiver {
+                reference: None, ..
+            },
+        ) => {
+            bail_spanned!(recv.span() => RECEIVER_BY_VALUE_ERR);
+        }
+        syn::FnArg::Receiver(syn::Receiver { mutability, .. }) => Ok(SelfType::Receiver {
+            mutable: mutability.is_some(),
         }),
         syn::FnArg::Typed(syn::PatType { ty, .. }) => {
             if let syn::Type::ImplTrait(_) = &**ty {
@@ -729,3 +736,6 @@ fn parse_method_attributes(
 }
 
 const IMPL_TRAIT_ERR: &str = "Python functions cannot have `impl Trait` arguments";
+const RECEIVER_BY_VALUE_ERR: &str =
+    "Python objects are shared, so 'self' cannot be moved out of the Python interpreter.
+Try `&self`, `&mut self, `slf: PyRef<'_, Self>` or `slf: PyRefMut<'_, Self>`.";

tests/ui/invalid_pymethods.rs

@@ -119,6 +119,11 @@ impl MyClass {
     fn default_arg_before_required(&self, has_default: isize, required: isize) {}
 }
 
+#[pymethods]
+impl MyClass {
+    fn method_self_by_value(self){}
+}
+
 struct TwoNew { }
 
 #[pymethods]

tests/ui/invalid_pymethods.stderr

@@ -102,10 +102,17 @@ error: `pass_module` cannot be used on Python methods
 112 |     #[pyo3(pass_module)]
     |            ^^^^^^^^^^^
 
-error[E0592]: duplicate definitions with name `__pymethod__new__`
+error: Python objects are shared, so 'self' cannot be moved out of the Python interpreter.
-   --> tests/ui/invalid_pymethods.rs:124:1
+       Try `&self`, `&mut self, `slf: PyRef<'_, Self>` or `slf: PyRefMut<'_, Self>`.
+   --> tests/ui/invalid_pymethods.rs:124:29
     |
-124 | #[pymethods]
+124 |     fn method_self_by_value(self){}
+    |                             ^^^^
+
+error[E0592]: duplicate definitions with name `__pymethod__new__`
+   --> tests/ui/invalid_pymethods.rs:129:1
+    |
+129 | #[pymethods]
     | ^^^^^^^^^^^^
     | |
     | duplicate definitions for `__pymethod__new__`