implement shim_sig macro

[geetanshjuneja]

Implements shim_sig macro.
More details here - #3842 (comment)

[geetanshjuneja]

@rustbot ready

[CraftSpider]

Outsider question: How many places is check_shim_abi used so far? Could abi, args, and return get packaged into one argument so you could just use the macro directly in the parameters?

[geetanshjuneja]

How many places is check_shim_abi used so far?

It is used in shims/unix/foreign_items.rs.

Could abi, args, and return get packaged into one argument so you could just use the macro directly in the parameters?

Yes, I'll package them into one argument .

[RalfJung]

Yes that is what I suggested in the first place:

                let [name, value, overwrite] = this.check_shim_abi(
                    shim_sig!(extern "C" fn(*const _, *const _, i32) -> i32),
                    link_name,
                    abi,
                    args,
                )?;

[RalfJung]

@rustbot author

[RalfJung]

Please give this macro a more clear name, such as layout_for_ty.

[RalfJung]

Having to use :tt instead of :ty here is unfortunate. I guess that's because *const _ is not a valid type? Maybe we can use *const T instead?

[geetanshjuneja]

Then I won't be able to match libc types.

[RalfJung]

Why not? Please explain the problem.

[geetanshjuneja]

shim_sig!(this, extern "C" fn() -> "pid_t")

"pid_t" won't be able to match ty

[RalfJung]

Why not? That's a valid type, syntactically.

[geetanshjuneja]

macro_rules! layout_and_ty {
    ($this:ident, *const T) => {
        $this.machine.layouts.const_raw_ptr.ty
    };
    ($this:ident, *mut T) => {
        $this.machine.layouts.mut_raw_ptr.ty
    };
    ($this:ident, i32) => {
        $this.tcx.types.i32
    };
    ($this:ident, usize) => {
        $this.tcx.types.usize
    };
    ($this:ident, isize) => {
        $this.tcx.types.isize
    };
    ($this:ident, $x:ty) => {
        $this.libc_ty_layout(stringify!($x)).ty
    };
}

When I call this macro layout_and_ty($this, i32), it matches with ($this:ident, $x:ty) not ($this:ident, i32)

[geetanshjuneja]

In the macro arm of layout_and_ty, is it taking i32 as a keyword not a rust type?

[RalfJung]

i32 is not a keyword though...

But I would anyway say the fallback arm should be

    ($this:ident, libc::$x:ident) => {
        $this.libc_ty_layout(stringify!($x)).ty
    };

Does that help?

Also your macro should probably return the entire layout, not just the type.

[geetanshjuneja]

i32 is not a keyword though...

I think the issue is that when I pass already matched ty token in shim_sig to layout_and_ty, then I can only match $x:ty arm. But if I would have directly called layout_and_ty macro then I would have been able to match i32 arm. See this.
Am I correct here?
So I think I have do if-else inside x:ty arm and return the correct layout.

[RalfJung]

Hm, I don't know if there is a trick to "degenerate" a ty token back to a series of tt. Maybe ask on Zulip? I'm not a macro expert unfortunately.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[geetanshjuneja]

@RalfJung I have created the topic #general > Convert ty token to series of tt token on zulip. Can you tag someone who can help with this?

[RalfJung]

If I knew who could help I'd have already done that.^^