Emit a better error for bad argument names

pyo3-macros-backend/src/method.rs

@@ -40,14 +40,14 @@ impl<'a> FnArg<'a> {
                 }
 
                 let arg_attrs = PyFunctionArgPyO3Attributes::from_attrs(&mut cap.attrs)?;
-                let (ident, by_ref, mutability) = match *cap.pat {
+                let (ident, by_ref, mutability) = match &*cap.pat {
                     syn::Pat::Ident(syn::PatIdent {
-                        ref ident,
-                        ref by_ref,
-                        ref mutability,
+                        ident,
+                        by_ref,
+                        mutability,
                         ..
                     }) => (ident, by_ref, mutability),
-                    _ => bail_spanned!(cap.pat.span() => "unsupported argument"),
+                    other => return Err(handle_argument_error(other)),
                 };
 
                 Ok(FnArg {
@@ -67,6 +67,19 @@ impl<'a> FnArg<'a> {
     }
 }
 
+fn handle_argument_error(pat: &syn::Pat) -> syn::Error {
+    let span = pat.span();
+    let msg = match pat {
+        syn::Pat::Wild(_) => "wildcard argument names are not supported",
+        syn::Pat::Struct(_)
+        | syn::Pat::Tuple(_)
+        | syn::Pat::TupleStruct(_)
+        | syn::Pat::Slice(_) => "destructuring in arguments is not supported",
+        _ => "unsupported argument",
+    };
+    syn::Error::new(span, msg)
+}
+
 #[derive(Clone, PartialEq, Debug, Copy, Eq)]
 pub enum MethodTypeAttribute {
     /// `#[new]`

tests/ui/invalid_pyfunctions.rs

@@ -9,4 +9,10 @@ fn impl_trait_function(impl_trait: impl AsRef<PyAny>) {}
 #[pyfunction]
 async fn async_function() {}
 
+#[pyfunction]
+fn wildcard_argument(_: i32) {}
+
+#[pyfunction]
+fn destructured_argument((a, b): (i32, i32)) {}
+
 fn main() {}

tests/ui/invalid_pyfunctions.stderr

@@ -17,3 +17,15 @@ error: `async fn` is not yet supported for Python functions.
    |
 10 | async fn async_function() {}
    | ^^^^^
+
+error: wildcard argument names are not supported
+  --> tests/ui/invalid_pyfunctions.rs:13:22
+   |
+13 | fn wildcard_argument(_: i32) {}
+   |                      ^
+
+error: destructuring in arguments is not supported
+  --> tests/ui/invalid_pyfunctions.rs:16:26
+   |
+16 | fn destructured_argument((a, b): (i32, i32)) {}
+   |                          ^^^^^^