Improve error messages for #[pyfunction] defined inside #[pymethods] (#…

…4349)

* Improve error messages for #[pyfunction] defined inside #[pymethods]

Make error message more specific when `#[pyfunction]` is used in
`#[pymethods]`.

Effectively, this replaces the error message:

```
error: static method needs #[staticmethod] attribute
```

To:
```
functions inside #[pymethods] do not need to be annotated with #[pyfunction]
```

...and also removes the other misleading error messages to the function in
question.

Fixes #4340

Co-authored-by: László Vaskó <1771332+vlaci@users.noreply.github.com>

* review fixes

---------

Co-authored-by: László Vaskó <1771332+vlaci@users.noreply.github.com>

newsfragments/4349.fixed.md

@@ -0,0 +1 @@
+Improve error messages for `#[pyfunction]` defined inside `#[pymethods]`

pyo3-macros-backend/src/module.rs

@@ -8,7 +8,7 @@ use crate::{
     get_doc,
     pyclass::PyClassPyO3Option,
     pyfunction::{impl_wrap_pyfunction, PyFunctionOptions},
-    utils::{Ctx, LitCStr, PyO3CratePath},
+    utils::{has_attribute, has_attribute_with_namespace, Ctx, IdentOrStr, LitCStr},
 };
 use proc_macro2::{Span, TokenStream};
 use quote::quote;
@@ -584,11 +584,6 @@ fn find_and_remove_attribute(attrs: &mut Vec<syn::Attribute>, ident: &str) -> bo
     found
 }
 
-enum IdentOrStr<'a> {
-    Str(&'a str),
-    Ident(syn::Ident),
-}
-
 impl<'a> PartialEq<syn::Ident> for IdentOrStr<'a> {
     fn eq(&self, other: &syn::Ident) -> bool {
         match self {
@@ -597,36 +592,6 @@ impl<'a> PartialEq<syn::Ident> for IdentOrStr<'a> {
         }
     }
 }
-fn has_attribute(attrs: &[syn::Attribute], ident: &str) -> bool {
-    has_attribute_with_namespace(attrs, None, &[ident])
-}
-
-fn has_attribute_with_namespace(
-    attrs: &[syn::Attribute],
-    crate_path: Option<&PyO3CratePath>,
-    idents: &[&str],
-) -> bool {
-    let mut segments = vec![];
-    if let Some(c) = crate_path {
-        match c {
-            PyO3CratePath::Given(paths) => {
-                for p in &paths.segments {
-                    segments.push(IdentOrStr::Ident(p.ident.clone()));
-                }
-            }
-            PyO3CratePath::Default => segments.push(IdentOrStr::Str("pyo3")),
-        }
-    };
-    for i in idents {
-        segments.push(IdentOrStr::Str(i));
-    }
-
-    attrs.iter().any(|attr| {
-        segments
-            .iter()
-            .eq(attr.path().segments.iter().map(|v| &v.ident))
-    })
-}
 
 fn set_module_attribute(attrs: &mut Vec<syn::Attribute>, module_name: &str) {
     attrs.push(parse_quote!(#[pyo3(module = #module_name)]));

pyo3-macros-backend/src/pyimpl.rs

@@ -1,6 +1,6 @@
 use std::collections::HashSet;
 
-use crate::utils::Ctx;
+use crate::utils::{has_attribute, has_attribute_with_namespace, Ctx, PyO3CratePath};
 use crate::{
     attributes::{take_pyo3_options, CrateAttribute},
     konst::{ConstAttributes, ConstSpec},
@@ -10,6 +10,7 @@ use crate::{
 use proc_macro2::TokenStream;
 use pymethod::GeneratedPyMethod;
 use quote::{format_ident, quote};
+use syn::ImplItemFn;
 use syn::{
     parse::{Parse, ParseStream},
     spanned::Spanned,
@@ -84,6 +85,25 @@ pub fn build_py_methods(
     }
 }
 
+fn check_pyfunction(pyo3_path: &PyO3CratePath, meth: &mut ImplItemFn) -> syn::Result<()> {
+    let mut error = None;
+
+    meth.attrs.retain(|attr| {
+        let attrs = [attr.clone()];
+
+        if has_attribute(&attrs, "pyfunction")
+            || has_attribute_with_namespace(&attrs, Some(pyo3_path),  &["pyfunction"])
+            || has_attribute_with_namespace(&attrs, Some(pyo3_path),  &["prelude", "pyfunction"]) {
+                error = Some(err_spanned!(meth.sig.span() => "functions inside #[pymethods] do not need to be annotated with #[pyfunction]"));
+                false
+        } else {
+            true
+        }
+    });
+
+    error.map_or(Ok(()), Err)
+}
+
 pub fn impl_methods(
     ty: &syn::Type,
     impls: &mut [syn::ImplItem],
@@ -103,6 +123,9 @@ pub fn impl_methods(
                 let ctx = &Ctx::new(&options.krate, Some(&meth.sig));
                 let mut fun_options = PyFunctionOptions::from_attrs(&mut meth.attrs)?;
                 fun_options.krate = fun_options.krate.or_else(|| options.krate.clone());
+
+                check_pyfunction(&ctx.pyo3_path, meth)?;
+
                 match pymethod::gen_py_method(ty, &mut meth.sig, &mut meth.attrs, fun_options, ctx)?
                 {
                     GeneratedPyMethod::Method(MethodAndMethodDef {

pyo3-macros-backend/src/utils.rs

@@ -291,3 +291,39 @@ pub fn apply_renaming_rule(rule: RenamingRule, name: &str) -> String {
 pub(crate) fn is_abi3() -> bool {
     pyo3_build_config::get().abi3
 }
+
+pub(crate) enum IdentOrStr<'a> {
+    Str(&'a str),
+    Ident(syn::Ident),
+}
+
+pub(crate) fn has_attribute(attrs: &[syn::Attribute], ident: &str) -> bool {
+    has_attribute_with_namespace(attrs, None, &[ident])
+}
+
+pub(crate) fn has_attribute_with_namespace(
+    attrs: &[syn::Attribute],
+    crate_path: Option<&PyO3CratePath>,
+    idents: &[&str],
+) -> bool {
+    let mut segments = vec![];
+    if let Some(c) = crate_path {
+        match c {
+            PyO3CratePath::Given(paths) => {
+                for p in &paths.segments {
+                    segments.push(IdentOrStr::Ident(p.ident.clone()));
+                }
+            }
+            PyO3CratePath::Default => segments.push(IdentOrStr::Str("pyo3")),
+        }
+    };
+    for i in idents {
+        segments.push(IdentOrStr::Str(i));
+    }
+
+    attrs.iter().any(|attr| {
+        segments
+            .iter()
+            .eq(attr.path().segments.iter().map(|v| &v.ident))
+    })
+}

tests/test_compile_error.rs

@@ -11,6 +11,7 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pyclass_enum.rs");
     t.compile_fail("tests/ui/invalid_pyclass_item.rs");
     t.compile_fail("tests/ui/invalid_pyfunction_signatures.rs");
+    t.compile_fail("tests/ui/invalid_pyfunction_definition.rs");
     #[cfg(any(not(Py_LIMITED_API), Py_3_11))]
     t.compile_fail("tests/ui/invalid_pymethods_buffer.rs");
     // The output is not stable across abi3 / not abi3 and features

tests/ui/invalid_pyfunction_definition.rs

@@ -0,0 +1,15 @@
+#[pyo3::pymodule]
+mod pyo3_scratch {
+    use pyo3::prelude::*;
+
+    #[pyclass]
+    struct Foo {}
+
+    #[pymethods]
+    impl Foo {
+        #[pyfunction]
+        fn bug() {}
+    }
+}
+
+fn main() {}

tests/ui/invalid_pyfunction_definition.stderr

@@ -0,0 +1,5 @@
+error: functions inside #[pymethods] do not need to be annotated with #[pyfunction]
+  --> tests/ui/invalid_pyfunction_definition.rs:11:9
+   |
+11 |         fn bug() {}
+   |         ^^