ObjC ARC `__weak` struct fields are passed by value (`Copy`) instead of the hidden-pointer ABI

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
50/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
c, objective-c, rust
Domain
compilers, tooling

Research direction

Start with input.h and reproduce the bindgen invocation using Objective-C ARC on aarch64-apple-darwin. Compare the generated W definition and takeW declaration with Clang's calling convention for a struct containing __weak id. Done means bindgen no longer emits an incorrect Copy by-value signature, either matching the hidden-pointer ABI or refusing that binding.

Written by the indexing model from the issue text.

Description

Input C/C++ Header
typedef struct objc_object *id;

typedef struct {
    __weak id w;
    int t;
} W;

int takeW(W s, int z);
int takeWp(id p, int t);
id box7(void);
Bindgen Invocation
$ bindgen input.h --no-rustfmt-bindings -- -x objective-c -fobjc-arc
Actual Results

bindgen emits a Copy by-value struct. Layout tests compile (size_of::<W>() == 16).

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct W {
    pub w: id,
    pub t: ::std::os::raw::c_int,
}

unsafe extern "C" {
    pub fn takeW(s: W, z: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
    pub fn takeWp(p: id, t: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
}

An __weak field makes the record nontrivial for calls under ARC. Clang passes takeW indirectly (objc_copyWeak / hidden pointer). rustc passes [2 x i64] in registers.

On Darwin aarch64, s.t = 10, z = 1:

C  takeW(s, 1) = 11    takeWp(o, 10) = 11
Rust takeW(s, 1) = 10    takeWp(o, 10) = 11

The thin __weak id parameter matches. Only the by-value struct disagrees.

This is not #778 (C++ Itanium user-destructor “non-trivial for the purposes of calls”). It is a C struct with an ObjC ARC ownership qualifier. bindgen has no __weak / objc_ownership handling.

Expected Results

A record with an __weak field should not be passed as a Copy by-value argument. Clang’s calling convention for that type is a hidden pointer; the generated signature should match that (or bindgen should refuse to emit a by-value Copy binding).

Environment
bindgen current main: 77cbc72
bindgen:              0.73.1
clang:                Apple clang 21.0.0
rustc:                1.97.1
target:               aarch64-apple-darwin
Dominant language
Rust
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Merged PRs (30d)
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