Rust does not have type-based alias analysis

[narpfel]

Summary

gccrs seems to compile pointers/pointer casts with C’s strict aliasing/type-based alias analysis (TBAA) semantics, i. e. with the assumption that a pointer to type T cannot alias a pointer to type U for T != U. This is not true in Rust.

Reproducer

I tried this code:

#![feature(no_core)]
#![no_core]

extern "C" {
    fn malloc(n: u64) -> *mut u8;
}

pub fn f() -> i32 {
    unsafe {
        let p = malloc(4) as *mut i32;
        *p = 27;
        *(p as *mut i16) = 42;
        *p
    }
}

pub fn main() -> i32 {
    f()
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

https://godbolt.org/z/889vTb4eK

Actual behavior

The current behaviour is that gccrs ignores writes to *p through *mut i16 pointers when compiling with optimisations (-O2 and above): f returns 27 (like the equivalent C code when compiled with GCC 15.2). This is incorrect in Rust.

Expected behavior

f should return 42.

GCC Version

679aad3