Inlining introduces unsized temporaries

[tmiasko]

Inlining introduces unsized temporaries. This is problematic since unsized temporaries fail to uphold alignment requirements as described in #71416. For example:

#![feature(unsized_fn_params)]
use std::any::Any;
use std::hint::black_box;

#[repr(align(1024))]
#[allow(dead_code)]
struct A(u8);

impl A {
    fn f(&self) {
        assert_eq!(0, black_box(self as *const A as usize) % 1024);
    }
}

#[inline(always)]
pub fn f(a: dyn Any) {
    a.downcast_ref::<A>().unwrap().f()
}

pub fn main() {
    let a = Box::new(A(0)) as Box<dyn Any>;
    f(*a);
}

$ rustc d.rs -O -Zinline-mir=off && ./d
$ rustc d.rs -O && ./d
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `992`', d.rs:11:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[tmiasko]

The unsized locals alignment issue itself was fixed in #111374.