missed optimization: coalesce PathBuf reallocation

compare the following disassemblies:

#![allow(dead_code)]

use std::path::{Path, PathBuf};

#[no_mangle]
pub fn boxed_path() -> Box<Path> {
    Path::new("aaa").into()
}

boxed_path:
        lea     rdi, [rip + .L__unnamed_1]
        mov     esi, 3
        jmp     qword ptr [rip + std::path::<impl core::convert::From<&std::path::Path> for alloc::boxed::Box<std::path::Path>>::from::hfd2d6fbd02ea1491@GOTPCREL]

.L__unnamed_1:
        .zero   3,97

#![allow(dead_code)]

use std::path::{Path, PathBuf};

#[no_mangle]
pub fn boxed_path() -> Box<Path> {
    PathBuf::from("aaa").into()
}

boxed_path:
        push    rbx
        sub     rsp, 32
        lea     rsi, [rip + .L__unnamed_1]
        lea     rbx, [rsp + 8]
        mov     edx, 3
        mov     rdi, rbx
        call    qword ptr [rip + std::sys::os_str::bytes::Slice::to_owned::hda0e6bb365f52f1f@GOTPCREL]
        mov     rdi, rbx
        call    qword ptr [rip + std::ffi::os_str::OsString::into_boxed_os_str::haa190229882bb506@GOTPCREL]
        add     rsp, 32
        pop     rbx
        ret

.L__unnamed_1:
        .zero   3,97

Start by reproducing the two boxed_path examples and comparing their disassemblies, focusing on PathBuf::from("aaa").into() versus Path::new("aaa").into() and the Box<Path> conversion. Done means the PathBuf conversion no longer performs the unnecessary reallocation, with the generated assembly confirming the optimization.