Description
We would like to propose showing a warning message when a developer uses an assign attribute to an Objective-C pointer/block when ARC is enabled with -fobjc-arc as clang treats the assign property as __unsafe_unretained which a developer may not expect.
Even though the fact that assign corresponds to __unsafe_unretained is clearly documented in property declarations, this can be surprising in a code base written in Objective-C++ (as Chromium) as this is the only qualifier that is available for pointer to non reference counted object, and thus their use for reference counted may be accidental, especially if the code is modified by a developer less familiar with Objective-C.
Finally, this warning would not prevent declaring properties that keep a non-zeroing weak pointer since it is possible to use unsafe_unretained to declare properties that use __unsafe_unretained which is clearer than the overloaded assign attribute.
Consider the following code:
@interface Bar : NSObject
@property(nonatomic, assign) NSObject* foo1;
@property(nonatomic, strong) NSObject* foo2;
@end
@implementation Bar
@end
It generates the following assembly with the command $ clang -Wall -Wextra -fobjc-arc -o foo.s -S foo.m:
"-[Bar setFoo1:]": ## @"\01-[Bar setFoo1:]"
.cfi_startproc
## %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
movq %rdi, -8(%rbp)
movq %rsi, -16(%rbp)
movq %rdx, -24(%rbp)
movq -24(%rbp), %rcx
movq -8(%rbp), %rax
movq %rcx, 8(%rax)
popq %rbp
retq
.cfi_endproc
"-[Bar setFoo2:]": ## @"\01-[Bar setFoo2:]"
.cfi_startproc
## %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
subq $32, %rsp
movq %rdi, -8(%rbp)
movq %rsi, -16(%rbp)
movq %rdx, -24(%rbp)
movq -24(%rbp), %rsi
movq -8(%rbp), %rdi
addq $16, %rdi
callq _objc_storeStrong
addq $32, %rsp
popq %rbp
retq
.cfi_endproc