Rust generic function monomorphization optimization

main.rs

#![allow(dead_code, unused_variables, clippy::from_over_into)]

struct Inner {
    raw: *mut [u8; 0],
}

struct Object {
    inner: Inner,
}

impl AsRef<Inner> for Object {
    #[inline(never)]
    fn as_ref(&self) -> &Inner {
        &self.inner
    }
}

struct SameObject {
    inner: Inner,
}

impl AsRef<Inner> for SameObject {
    #[inline(never)]
    fn as_ref(&self) -> &Inner {
        &self.inner
    }
}

struct SameObjectWithPadding {
    inner: Inner,
    _padding: u8,
}

impl AsRef<Inner> for SameObjectWithPadding {
    #[inline(never)]
    fn as_ref(&self) -> &Inner {
        &self.inner
    }
}

struct SameObjectWithAnotherImpl {
    c: Inner,
}

impl AsRef<Inner> for SameObjectWithAnotherImpl {
    #[inline(never)]
    fn as_ref(&self) -> &Inner {
        println!("c");
        &self.c
    }
}

#[inline(never)]
fn take_any_struct(any_struct: impl AsRef<Inner>) {
    let inner = any_struct.as_ref();
    std::hint::black_box(inner);
}

fn main() {
    let object = Object {
        inner: Inner {
            raw: std::ptr::null_mut(),
        },
    };
    let same_object = SameObject {
        inner: Inner {
            raw: std::ptr::null_mut(),
        },
    };
    let same_object_with_padding = SameObjectWithPadding {
        inner: Inner {
            raw: std::ptr::null_mut(),
        },
        _padding: 42,
    };
    let same_object_with_another_impl = SameObjectWithAnotherImpl {
        c: Inner {
            raw: std::ptr::null_mut(),
        },
    };

    take_any_struct(&object);
    take_any_struct(object);
    take_any_struct(&same_object);
    take_any_struct(same_object);
    take_any_struct(&same_object_with_padding);
    take_any_struct(same_object_with_padding);
    take_any_struct(&same_object_with_another_impl);
    take_any_struct(same_object_with_another_impl);
}

$ rustc --version
rustc 1.69.0-nightly (07c993eba 2023-02-23)

$ rustc main.ll --emit=llvm-ir -C opt-level=s

; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h0855572da314e5b8(ptr noalias noundef nonnull readonly align 8 dereferenceable(8) %object)
  %1 = load ptr, ptr %object, align 8, !noundef !3
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17heaa95f842be98e30E(ptr noundef %1)
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h0855572da314e5b8(ptr noalias noundef nonnull readonly align 8 dereferenceable(8) %same_object)
  %2 = load ptr, ptr %same_object, align 8, !noundef !3
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17heaa95f842be98e30E(ptr noundef %2)
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h95134dd63a3f8201E(ptr noalias noundef nonnull readonly align 8 dereferenceable(16) %same_object_with_padding)
  %3 = load ptr, ptr %same_object_with_padding, align 8, !noundef !3
  %4 = load i8, ptr %0, align 8, !noundef !3
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h62ed7918170547caE(ptr noundef %3, i8 noundef %4)
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h15138211037d7e07E(ptr noalias noundef nonnull readonly align 8 dereferenceable(8) %same_object_with_another_impl)
  %5 = load ptr, ptr %same_object_with_another_impl, align 8, !noundef !3
; call main::take_any_struct
  call fastcc void @_ZN4main15take_any_struct17h4f18d7d3a390031dE(ptr noundef %5)

Unified

• take_any_struct(&object) & take_any_struct(&same_object)
• take_any_struct(object) & take_any_struct(same_object)