Add compile error for associated_to attribute

crates/swift-bridge-ir/src/errors/parse_error.rs

@@ -56,6 +56,8 @@ pub(crate) enum ParseError {
     ArgCopyAndRefMut { arg: FnArg },
     /// There was an unsupported item in the module, such as a `use` statement.
     InvalidModuleItem { item: Item },
+    /// The associated_to attribute is used for only an associated method.
+    InvalidAssociatedTo { self_: FnArg },
 }
 
 /// An error while parsing a function attribute.
@@ -206,6 +208,12 @@ struct {struct_name};
                 let message = format!(r#"Only `extern` blocks, structs and enums are supported."#);
                 Error::new_spanned(item, message)
             }
+            ParseError::InvalidAssociatedTo { self_ } => {
+                let message = format!(
+                    r#"The associated_to attribute is used for only an associated method."#
+                );
+                Error::new_spanned(self_, message)
+            }
         }
     }
 }

crates/swift-bridge-ir/src/parse/parse_extern_mod.rs

@@ -297,6 +297,11 @@ impl<'a> ForeignModParser<'a> {
     ) -> syn::Result<Option<TypeDeclaration>> {
         let associated_type = match first {
             Some(FnArg::Receiver(recv)) => {
+                if let Some(_) = attributes.associated_to {
+                    self.errors.push(ParseError::InvalidAssociatedTo {
+                        self_: first.unwrap().clone(),
+                    })
+                }
                 if local_type_declarations.len() == 1 {
                     let ty = local_type_declarations.iter_mut().next().unwrap().1;
                     let associated_type = Some(TypeDeclaration::Opaque(ty.clone()));

crates/swift-bridge-ir/src/parse/parse_extern_mod/function_attributes.rs

@@ -80,6 +80,7 @@ impl Parse for FunctionAttr {
         let key: Ident = input.parse()?;
 
         let attrib = match key.to_string().as_str() {
+            // TODO: Replace this with "static_method_of" before we release 0.2.0.
             "associated_to" => {
                 input.parse::<Token![=]>()?;
                 let value: Ident = input.parse()?;

crates/swift-bridge-macro/tests/ui/invalid-associated-to-attribute.rs

@@ -0,0 +1,34 @@
+//! # To Run
+//! cargo test -p swift-bridge-macro -- ui trybuild=invalid-associated-to-attribute.rs
+
+#[swift_bridge::bridge]
+mod ffi {
+    extern "Rust" {
+        type SomeType;
+
+        #[swift_bridge(associated_to = SomeType)]
+        fn immutable_method(&self);
+
+        #[swift_bridge(associated_to = SomeType)]
+        fn mutable_method(&mut self);
+
+        #[swift_bridge(associated_to = SomeType)]
+        fn owned_method(self);
+    }
+}
+
+pub struct SomeType;
+
+impl SomeType {
+    fn immutable_method(&self) {
+
+    }
+    fn mutable_method(&mut self) {
+
+    }
+    fn owned_method() {
+
+    }
+}
+
+fn main() {}

crates/swift-bridge-macro/tests/ui/invalid-associated-to-attribute.stderr

@@ -0,0 +1,17 @@
+error: The associated_to attribute is used for only an associated method.
+  --> tests/ui/invalid-associated-to-attribute.rs:10:29
+   |
+10 |         fn immutable_method(&self);
+   |                             ^^^^^
+
+error: The associated_to attribute is used for only an associated method.
+  --> tests/ui/invalid-associated-to-attribute.rs:13:27
+   |
+13 |         fn mutable_method(&mut self);
+   |                           ^^^^^^^^^
+
+error: The associated_to attribute is used for only an associated method.
+  --> tests/ui/invalid-associated-to-attribute.rs:16:25
+   |
+16 |         fn owned_method(self);
+   |                         ^^^^