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__str__ with slot wrapper, generate slot with #[pymethod] #6485

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__str__ with slot wrapper, generate slot with #[pymethod] #6485

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96 changes: 96 additions & 0 deletions crates/derive-impl/src/pyclass.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -70,6 +70,7 @@ struct ImplContext {
member_items: MemberNursery,
extend_slots_items: ItemNursery,
class_extensions: Vec<TokenStream>,
extra_impl_items: Vec<syn::ImplItem>,
errors: Vec<syn::Error>,
}

Expand Down Expand Up @@ -196,6 +197,10 @@ pub(crate) fn impl_pyclass_impl(attr: PunctuatedNestedMeta, item: Item) -> Resul
},
];
imp.items.extend(extra_methods);
// Add extra impl items (like __slot_str__ for __str__)
for item in context.extra_impl_items {
imp.items.push(item);
}
let is_main_impl = impl_ty == payload_ty;
if is_main_impl {
let method_defs = if with_method_defs.is_empty() {
Expand Down Expand Up @@ -294,6 +299,8 @@ pub(crate) fn impl_pyclass_impl(attr: PunctuatedNestedMeta, item: Item) -> Resul
},
];
trai.items.extend(extra_methods);
// Note: extra_impl_items (like __slot_str__ for __str__) are not added to traits,
// because traits define the method signature, not the slot wrapper implementation.

trai.into_token_stream()
}
Expand Down Expand Up @@ -925,6 +932,94 @@ where
args.attrs.push(allow_attr);
}

// Special handling for __str__: generate slot wrapper instead of pymethod
if py_name == "__str__" {
// Validate __str__ signature
let sig = func.sig();
let params: Vec<_> = sig.inputs.iter().collect();

// Check parameter count (should be 2: zelf and vm)
if params.len() != 2 {
return Err(syn::Error::new(
sig.inputs.span(),
format!(
"#[pymethod] __str__ must have exactly 2 parameters (zelf, vm), found {}.\n\
Expected signature: fn __str__(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyStrRef>",
params.len()
),
));
}

// Check first parameter is a reference (should be &Py<...> or &self for impl Py<T>)
if let Some(syn::FnArg::Typed(pat_type)) = params.first() {
let ty = &pat_type.ty;
let is_reference = matches!(ty.as_ref(), syn::Type::Reference(_));
if !is_reference {
return Err(syn::Error::new_spanned(
ty,
"#[pymethod] __str__ first parameter must be a reference type.\n\
Expected: &Py<Self> or &self (for impl Py<T>)\n\
Hint: Use `zelf: &Py<Self>` instead of `PyRef<Self>`",
));
}
} else if let Some(syn::FnArg::Receiver(recv)) = params.first() {
// &self is allowed for impl Py<T> blocks (where &self == &Py<T>)
// self by value is not allowed
if recv.reference.is_none() {
return Err(syn::Error::new_spanned(
recv,
"#[pymethod] __str__ cannot take `self` by value.\n\
Expected: fn __str__(zelf: &Py<T>, vm: &VirtualMachine) -> PyResult<PyStrRef>",
));
}
}

// Check return type (should be PyResult<PyStrRef> or PyResult<PyRef<PyStr>>)
let valid_return_type = match &sig.output {
syn::ReturnType::Type(_, ty) => {
let ty_str = quote!(#ty).to_string().replace(' ', "");
ty_str.contains("PyResult")
&& (ty_str.contains("PyStrRef") || ty_str.contains("PyRef<PyStr>"))
}
syn::ReturnType::Default => false,
};
if !valid_return_type {
return Err(syn::Error::new_spanned(
&sig.output,
"#[pymethod] __str__ must return PyResult<PyStrRef>.\n\
Hint: Use `-> PyResult<PyStrRef>` instead of `-> String` or other types",
));
}

// 1. Generate wrapper function as impl item
let wrapper_fn: syn::ImplItem = parse_quote! {
fn slot_str(
zelf: &::rustpython_vm::PyObject,
vm: &::rustpython_vm::VirtualMachine,
) -> ::rustpython_vm::PyResult<::rustpython_vm::builtins::PyStrRef> {
let zelf: &::rustpython_vm::Py<_> = zelf.downcast_ref()
.ok_or_else(|| vm.new_type_error("unexpected payload for __str__"))?;
Self::#ident(zelf, vm)
}
};
args.context.extra_impl_items.push(wrapper_fn);

// 2. Add slot assignment to extend_slots_items
let slot_tokens = quote_spanned! { ident.span() =>
slots.str.store(Some(Self::slot_str as _));
};
args.context.extend_slots_items.add_item(
ident.clone(),
vec!["(slot str)".to_string()],
args.cfgs.to_vec(),
slot_tokens,
2,
)?;

// 3. Don't add to method_items - PySlotWrapper handles dict entry
return Ok(());
}

let doc = args.attrs.doc().map(|doc| format_doc(&sig_doc, &doc));
args.context.method_items.add_item(MethodNurseryItem {
py_name,
Expand All @@ -934,6 +1029,7 @@ where
raw,
attr_name: self.inner.attr_name,
});

Ok(())
}
}
Expand Down
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