Multiple alignments on functions (`#![feature(fn_align)]`)

[jdonszelmann]

This code specifies two alignments, but applies none. A single align(256) does work

#![feature(fn_align)]


#[repr(align(256), align(256))]
pub fn main() {
    let ptr = main as *const u8;
    println!("{ptr:?}");

    assert_eq!(ptr.align_offset(256), 0);
}

See here: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f841ae6318f0f7abdab285ea9ec641ab

CC: #82232

The culprit is this line here matching on slices of length 1:

rust/compiler/rustc_codegen_ssa/src/codegen_attrs.rs

Lines 418 to 420 in 145f9cf

	codegen_fn_attrs.alignment = if let Some(items) = attr.meta_item_list()
	&& let [item] = items.as_slice()
	&& let Some((sym::align, literal)) = item.singleton_lit_list()

It's a one line fix, but honestly this is trivially resolved with rust-lang/compiler-team#796 which I'm working on. I'll make it a separate PR at some point, but I'll assign myself since it makes sure changes conflict a little less :)

@rustbot claim

[jdonszelmann]

@rustbot label +A-repr +A-align