Add Id::writeback

Cargo.toml

@@ -17,3 +17,5 @@ resolver = "2"
 inherits = "release"
 # Enable LTO to allow testing the `unstable-static-sel-inlined` feature
 lto = true
+# Disable generation of code to handle unwinds.
+panic = "abort"

objc2/src/rc/id.rs

@@ -1,9 +1,9 @@
 use core::fmt;
 use core::marker::PhantomData;
-use core::mem::ManuallyDrop;
+use core::mem::{self, ManuallyDrop};
 use core::ops::{Deref, DerefMut};
 use core::panic::{RefUnwindSafe, UnwindSafe};
-use core::ptr::NonNull;
+use core::ptr::{self, NonNull};
 
 use super::AutoreleasePool;
 use super::{Owned, Ownership, Shared};
@@ -522,6 +522,23 @@ impl<T: Message, O: Ownership> Id<T, O> {
         );
         res
     }
+
+    /// TODO
+    // https://clang.llvm.org/docs/AutomaticReferenceCounting.html#passing-to-an-out-parameter-by-writeback
+    #[inline]
+    pub unsafe fn writeback<R, F>(this: &mut Option<Self>, f: F) -> R
+    where
+        F: FnOnce(&mut *mut T) -> R,
+    {
+        let mut ptr = match this {
+            Some(this) => this.ptr.as_ptr(),
+            None => ptr::null_mut(),
+        };
+        let res = f(&mut ptr);
+        let old = mem::replace(this, unsafe { Self::retain(ptr) });
+        drop(old);
+        res
+    }
 }
 
 // TODO: Consider something like this

test-assembly/crates/test_out_parameters/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "test_out_parameters"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+objc2 = { path = "../../../objc2", default-features = false }
+
+[features]
+default = ["apple", "std"]
+std = ["objc2/std"]
+# Runtime
+apple = ["objc2/apple"]
+gnustep-1-7 = ["objc2/gnustep-1-7"]
+gnustep-1-8 = ["gnustep-1-7", "objc2/gnustep-1-8"]
+gnustep-1-9 = ["gnustep-1-8", "objc2/gnustep-1-9"]
+gnustep-2-0 = ["gnustep-1-9", "objc2/gnustep-2-0"]
+gnustep-2-1 = ["gnustep-2-0", "objc2/gnustep-2-1"]
+
+# Hack
+assembly-features = []

test-assembly/crates/test_out_parameters/expected/apple-x86_64.s

@@ -0,0 +1,82 @@
+	.section	__TEXT,__text,regular,pure_instructions
+	.intel_syntax noprefix
+	.globl	_null
+	.p2align	4, 0x90
+_null:
+	push	rbp
+	mov	rbp, rsp
+	push	rbx
+	push	rax
+	mov	qword ptr [rbp - 16], 0
+	lea	rdx, [rbp - 16]
+	call	_objc_msgSend
+	mov	rbx, rax
+	mov	rdi, qword ptr [rbp - 16]
+	call	_objc_retain
+	mov	rdx, rax
+	mov	rax, rbx
+	add	rsp, 8
+	pop	rbx
+	pop	rbp
+	ret
+
+	.globl	_nonnull
+	.p2align	4, 0x90
+_nonnull:
+	push	rbp
+	mov	rbp, rsp
+	push	r15
+	push	r14
+	push	rbx
+	push	rax
+	mov	rbx, rdx
+	mov	qword ptr [rbp - 32], rdx
+	lea	rdx, [rbp - 32]
+	call	_objc_msgSend
+	mov	r14, rax
+	mov	rdi, qword ptr [rbp - 32]
+	call	_objc_retain
+	mov	r15, rax
+	mov	rdi, rbx
+	call	_objc_release
+	mov	rax, r14
+	mov	rdx, r15
+	add	rsp, 8
+	pop	rbx
+	pop	r14
+	pop	r15
+	pop	rbp
+	ret
+
+	.globl	_generic
+	.p2align	4, 0x90
+_generic:
+	push	rbp
+	mov	rbp, rsp
+	push	r15
+	push	r14
+	push	rbx
+	push	rax
+	mov	rbx, rdx
+	mov	qword ptr [rbp - 32], rdx
+	lea	rdx, [rbp - 32]
+	call	_objc_msgSend
+	mov	r14, rax
+	mov	rdi, qword ptr [rbp - 32]
+	call	_objc_retain
+	mov	r15, rax
+	test	rbx, rbx
+	je	LBB2_2
+	mov	rdi, rbx
+	call	_objc_release
+LBB2_2:
+	mov	rax, r14
+	mov	rdx, r15
+	add	rsp, 8
+	pop	rbx
+	pop	r14
+	pop	r15
+	pop	rbp
+	ret
+
+.subsections_via_symbols

test-assembly/crates/test_out_parameters/lib.rs

@@ -0,0 +1,35 @@
+//! Test that Id::writeback does the correct thing.
+//!
+//! Note that without `panic = "abort"`, this generates a lot more code to
+//! ensure that if the message send panics, the `param` (whether it has been
+//! overwritten yet or not) is still released.
+use objc2::ffi::NSInteger;
+use objc2::rc::{Id, Owned};
+use objc2::runtime::{Object, Sel};
+use objc2::MessageReceiver;
+
+unsafe fn handle(obj: &Object, sel: Sel, param: &mut *mut Object) -> NSInteger {
+    MessageReceiver::send_message(obj, sel, (param,))
+}
+
+type Res = (NSInteger, Option<Id<Object, Owned>>);
+
+#[no_mangle]
+unsafe fn null(obj: &Object, sel: Sel) -> Res {
+    let mut param = None;
+    let res = Id::writeback(&mut param, |param| handle(obj, sel, param));
+    (res, param)
+}
+
+#[no_mangle]
+unsafe fn nonnull(obj: &Object, sel: Sel, param: Id<Object, Owned>) -> Res {
+    let mut param = Some(param);
+    let res = Id::writeback(&mut param, |param| handle(obj, sel, param));
+    (res, param)
+}
+
+#[no_mangle]
+unsafe fn generic(obj: &Object, sel: Sel, mut param: Option<Id<Object, Owned>>) -> Res {
+    let res = Id::writeback(&mut param, |param| handle(obj, sel, param));
+    (res, param)
+}

test-assembly/src/lib.rs

@@ -150,7 +150,7 @@ fn demangle_assembly(assembly: &str) -> String {
         //
         // Example: test_msg_send_static_sel[f7bb0e08e35403f3]::handle_with_sel::NAME_DATA
         // Becomes: test_msg_send_static_sel[CRATE_ID]::handle_with_sel::NAME_DATA
-        static ref RE_CRATE_ID: Regex = Regex::new(r"\[.*\]").unwrap();
+        static ref RE_CRATE_ID: Regex = Regex::new(r"\[.*?\]").unwrap();
 
         // Replace last part of symbol if it looks to be autogenerated
         //
@@ -347,5 +347,10 @@ l_anon.[ID].0:
         "#;
         let output = demangle_assembly(before);
         assert_eq!(output, after, "Got {}", output);
+
+        let before = r#"__RINvNtCshB9ITk7tvJd_4core3ptr13drop_in_placeINtNtB4_6option6OptionINtNtNtCsaWDm3USgSkM_5objc22rc2id2IdNtNtB19_7runtime6ObjectNtNtB17_9ownership6SharedEEECs8tAMaYSsbuV_19test_out_parameters:"#;
+        let after = r#"SYM(core[CRATE_ID]::ptr::drop_in_place::<core[CRATE_ID]::option::Option<objc2[CRATE_ID]::rc::id::Id<objc2[CRATE_ID]::runtime::Object, objc2[CRATE_ID]::rc::ownership::Shared>>>, 0):"#;
+        let output = demangle_assembly(before);
+        assert_eq!(output, after, "Got {}", output);
     }
 }