Make BytesMut::new a const fn

[coolreader18]

This might depend on rust-lang/rust#65801 for the guarantee that Vec::new() is a dangling pointer, but in practice this works and std probably wouldn't change.

[coolreader18]

I also inlined functions as I was originally going to fix BytesMut::new() being compiled as:

foo::foo:
 push    r14
 push    rbx
 sub     rsp, 24
 mov     rbx, rdi
 mov     qword, ptr, [rsp], 1
 xorps   xmm0, xmm0
 movups  xmmword, ptr, [rsp, +, 8], xmm0
 mov     edi, 1
 call    qword, ptr, [rip, +, _ZN5bytes9bytes_mut4vptr17ha5d83dd4c2961c0aE@GOTPCREL]
 mov     r14, rax
 xor     edi, edi
 call    qword, ptr, [rip, +, _ZN5bytes9bytes_mut25original_capacity_to_repr17hedb301e0cc79d76aE@GOTPCREL]
 lea     rax, [4*rax, +, 1]
 mov     qword, ptr, [rbx], r14
 xorps   xmm0, xmm0
 movups  xmmword, ptr, [rbx, +, 8], xmm0
 mov     qword, ptr, [rbx, +, 24], rax
 mov     rax, rbx
 add     rsp, 24
 pop     rbx
 pop     r14
 ret
.LBB1_2:
 mov     rbx, rax
 mov     rdi, rsp
 call    core::ptr::drop_in_place
 mov     rdi, rbx
 call    _Unwind_Resume
 ud2

I'm not sure if making it const fixes that as well, but I'll leave the inlines since I think they make sense anyway.

[coolreader18]

Actually, the inlines do matter lol:

diff --git a/src/bytes_mut.rs b/src/bytes_mut.rs
index 4e638c5..591b092 100644
--- a/src/bytes_mut.rs
+++ b/src/bytes_mut.rs
@@ -1260,7 +1260,7 @@ impl Shared {
     }
 }
 
-#[inline]
+// #[inline]
 const fn original_capacity_to_repr(cap: usize) -> usize {
     let width = PTR_WIDTH - ((cap >> MIN_ORIGINAL_CAPACITY_WIDTH).leading_zeros() as usize);
     let max_capacity = MAX_ORIGINAL_CAPACITY_WIDTH - MIN_ORIGINAL_CAPACITY_WIDTH;

foo::foo:
 push    rbx
 mov     rbx, rdi
 xor     edi, edi
 call    qword, ptr, [rip, +, _ZN5bytes9bytes_mut25original_capacity_to_repr17hedb301e0cc79d76aE@GOTPCREL]
 lea     rax, [4*rax, +, 1]
 mov     qword, ptr, [rbx], 1
 xorps   xmm0, xmm0
 movups  xmmword, ptr, [rbx, +, 8], xmm0
 mov     qword, ptr, [rbx, +, 24], rax
 mov     rax, rbx
 pop     rbx
 ret

[coolreader18]

Oh, maybe LTO would fix the inlining issue, since it was working when I was testing it as foo as a function in bytes, but not as a separate crate. In any case, this works too.

[taiki-e]

Thanks for the PR! The internal representation of BytesMut is not stable and after #435 it may be difficult to keep const so I would prefer to postpone this for now.

[coolreader18]

That makes sense. Would you accept a PR to add some #[inline]s so that the cost of BytesMut::new is as if it was const?

[taiki-e]

Would you accept a PR to add some #[inline]s so that the cost of BytesMut::new is as if it was const?

I would accept a PR to do this.