Include ABI information in rustc_layout(debug)

[RalfJung]

It is currently rather hard to figure out which PassMode a type will get. It would be great to have some way to display that information, similar to rustc_layout(debug). The logic for computing PassMode is currently here and we'd have to find some way to invoke it:

rust/compiler/rustc_ty_utils/src/abi.rs

Lines 404 to 416 in 795ade0

	let mut arg = ArgAbi::new(cx, layout, |layout, scalar, offset| {
	let mut attrs = ArgAttributes::new();
	adjust_for_rust_scalar(
	*cx,
	&mut attrs,
	scalar,
	*layout,
	offset,
	is_return,
	drop_target_pointee,
	);
	attrs
	});

@eddyb @bjorn3 any advice on how to implement this? Or is it meaningless to ask about the PassMode of a type, since this has to always be considered in the context of a particular function signature?

[RalfJung]

I should also note that the PassMode type is really confusing. For example, PassMode::Indirect has an on_stack field, but it also carries an ArgAttributes that has an IN_REG flag -- so the argument can be both on the stack and in a register at the same time? That obviously wouldn't make any sense, but the representation allows it...

[RalfJung]

I found this gem

rust/compiler/rustc_ty_utils/src/abi.rs

Line 451 in 795ade0

fn_abi_adjust_for_abi(cx, &mut fn_abi, sig.abi, fn_def_id)?;

so yeah I guess there's no way to say anything about the context of a concrete function signature. Fun times...

[bjorn3]

For example, PassMode::Indirect has an on_stack field, but it also carries an ArgAttributes that has an IN_RET flag -- so the argument can be both on the stack and in a register at the same time?

You mean InReg? I'm not quite sure what it exactly does, but I did assume it would be better served as separate flag for PassMode::Direct and possibly PassMode::Pair.

Maybe we could have a #[rustc_fn_abi(debug)] attribute to be applied to functions and then whenever said function is monomorphized (or in case of an extern {} function whenever it is referenced) we could print the computed FnAbi?