Generic function produces multiple identical methods in ASM.

[rickvanprim]

Compiled on Stable with Release target.

use std::any::Any;

#[inline(never)]
fn cast<T1: ?Sized, T2>(t: Box<T1>) -> Box<T2>
{
    unsafe { Box::from_raw(Box::into_raw(t) as *mut T2) }
}

fn main()
{
    let t: Box<Any> = Box::new(5);
    let s: Box<u32> = cast(t);
    println!("{}", *s);

    let t: Box<Any> = Box::new(10.0f32);
    let s: Box<f32> = cast(t);
    println!("{}", *s);
}

In the ASM, I see the following produced.

playground::cast:
    mov    rax, rdi
    ret

playground::cast:
    mov    rax, rdi
    ret

I observed this earlier today on Rust Playground. I don't know if #[inline(never)] is causing them not to be merged, or if it's expected that they don't get merged despite being identical. It seems to me like they should be merged, as they are identical aside from the type parameters, which is only a difference at compile time, not runtime.

[sfackler]

I think there has been some investigation of duplicate function deduplication but I don't know how far it's gone.

There are some caveats here - it seems like it would make stack traces more confusing, since you'll have frames which correspond to function calls you don't actually make in the source.

[nox]

Probably helped by #49479.

[rickvanprim]

Sorry for the slow response. Just tested now and issue no longer reproduces!