C++23 explicit object member functions get an extra implicit this pointer

[lwz23]

Input C/C++ Header

struct S {
  int x;
  int get(this S self, int y);
};

int S::get(this S self, int y) { return self.x + y; }

Bindgen Invocation

$ bindgen input.h --output bindings.rs -- -x c++ -std=c++23

Layout tests are left on (the default).

Actual Results

clang's AST for S::get is already int (S, int) — the explicit object parameter replaces the implicit this pointer:

CXXMethodDecl get 'int (S, int)'
  ParmVarDecl self this 'S'
  ParmVarDecl y 'int'

sizeof(S) is 4 in both clang++ and the generated Rust, so the layout tests compile. S is trivial (Copy/Clone, no user-provided copy ctor or destructor); this is not #778.

clang++ compiles the method as two LLVM arguments (the object by value, then y):

define i32 @_ZNH1S3getES_i(i64 %0, i32 noundef %1)

bindgen still prepends an implicit this pointer, so the generated signature has three arguments:

#[repr(C)]
pub struct S { pub x: c_int }

pub fn S_get(this: *mut S, self_: S, y: c_int) -> c_int;

impl S {
    pub unsafe fn get(&mut self, self_: S, y: c_int) -> c_int {
        S_get(self, self_, y)
    }
}

rustc therefore calls with three arguments:

declare i32 @"\01__ZNH1S3getES_i"(ptr, i64, i32)
call i32 @"\01__ZNH1S3getES_i"(ptr %s, i64 %1, i32 3)

Runtime, S s; s.x = 10; s.get(3) versus the generated S_get:

C++:  13
Rust: 1829318430   // low bits of the extra this pointer interpreted as s.x

bindgen does not panic. rustc accepts the bindings, including the layout tests.

The same extra-this insertion happens for int get(this S& self, int y) (clang: (S*, int); bindgen: (*mut S, *mut S, int)).

Expected Results

The generated signature should match the C++ function type in the AST / the clang ABI: no implicit this pointer when the method already has an explicit object parameter. For the header above that is effectively fn get(self_: S, y: c_int) -> c_int (or whatever equivalent rustc will lower the same way).

On current main, bindgen/ir/function.rs inserts this for every non-static CXCursor_CXXMethod and does not check for an explicit object parameter.

Environment

bindgen:  0.72.1 (same insertion still present on rust-lang/rust-bindgen main)
clang++:  Apple clang 21.0.0
rustc:    1.97.1
target:   aarch64-apple-darwin