I'm building a simple interner with an arena backing it, and would like to take advantage of pointer equality for interned values to speed up equality checking, which I believe is fairly common. However, I am encountering some strange behavior due to optimizations:
#Shared reference pointer equality unexpectedly optimized to false
ruby hearth
?eval ```rust
pub struct Interned<'arena, T: ?Sized>(&'arena T);
impl<T: ?Sized> PartialEq for Interned<', T> {
fn eq(&self, other: &Self) -> bool {
core::ptr::eq(self, other)
}
}
impl<T: ?Sized> Eq for Interned<', T> {}
pub fn main() {
let s1: &'static str = "abcd";
let i1 = Interned(s1);
let i2 = Interned(s1);
assert_eq!(
core::ptr::from_ref(i1.0).addr(),
core::ptr::from_ref(i2.0).addr()
);
assert!(i1 == i2);
}```
amber copperBOT
thread 'main' (11) panicked at src/main.rs:20:5:
assertion failed: i1 == i2
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Even though the first assertion passes, the PartialEq impl is optimized to always return false, which is not what I would expect from reading https://doc.rust-lang.org/stable/core/ptr/fn.eq.html. Am I missing something super obvious?
?godbolt ```rust
pub struct Interned<'arena, T: ?Sized>(&'arena T);
impl<T: ?Sized> PartialEq for Interned<', T> {
fn eq(&self, other: &Self) -> bool {
core::ptr::eq(self, other)
}
}
impl<T: ?Sized> Eq for Interned<', T> {}
pub fn main() {
let s1: &'static str = "abcd";
let i1 = Interned(s1);
let i2 = Interned(s1);
assert_eq!(
core::ptr::from_ref(i1.0).addr(),
core::ptr::from_ref(i2.0).addr()
);
assert!(i1 == i2);
}```
amber copperBOT
main:
push rax
lea rdi, [rip + .Lanon.fad58de7366495db4650cfefac2fcd61.1]
lea rdx, [rip + .Lanon.fad58de7366495db4650cfefac2fcd61.2]
mov esi, 26
call qword ptr [rip + core::panicking::panic@GOTPCREL]
.Lanon.fad58de7366495db4650cfefac2fcd61.0:
.asciz "/app/example.rs"
.Lanon.fad58de7366495db4650cfefac2fcd61.1:
.ascii "assertion failed: i1 == i2"
.Lanon.fad58de7366495db4650cfefac2fcd61.2:
.quad .Lanon.fad58de7366495db4650cfefac2fcd61.0
.asciz "\017\000\000\000\000\000\000\000\024\000\000\000\005\000\000"
Shared reference pointer equality unexpectedly optimized to false
native wigeon
You're comparing the addresses of i1 and i2, aka the struct holding the reference to the string. You need to do
core::ptr::eq(self.0, other.0)