Tracking issue: Support for pointers with `asm_const`

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This is a tracking issue for asm_const_ptr.

The feature gate for the issue is #![feature(asm_const_ptr)].

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Approve as lang experiment.
  • https://github.com/rust-lang/rust/issues/128464#issuecomment-3055413949
• Implement in nightly.
  • https://github.com/rust-lang/rust/pull/138618
• Accept an RFC.
  • https://github.com/rust-lang/rfcs/pull/3848
• Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • See the instructions.
• Add formatting for new syntax to the style guide.
  • See the nightly style procedure.
• Stabilize.
  • See the instructions.

Motivation / description

Inline assembly supports an unstable feature called asm_const with tracking issue #93332 that lets you pass constants to inline assembly. However, it only supports integer types. This issue is a feature request to support raw pointers with asm_const.

This would enable code such as the following:

#![feature(const_refs_to_static)]
#![feature(asm_const)]
use core::arch::asm;
use core::ptr::addr_of;
use core::ffi::c_void;

trait Helper {
    const MY_PTR: *const c_void;
}

fn my_asm_wrapper<T: Helper>() {
    unsafe { asm!("mov {},eax", const T::MY_PTR) };
}


extern "C" {
    static FOO: usize;
}

struct HelperForPassingPointerAsConstGeneric {}
impl Helper for HelperForPassingPointerAsConstGeneric {
    const MY_PTR: *const c_void = addr_of!(FOO).cast();
}

fn main() {
    my_asm_wrapper::<HelperForPassingPointerAsConstGeneric>();
}

Currently, the above code would require you to convert my_asm_wrapper into a macro_rules! so that you can write out the path to the global using the sym operand. Supporting this would be useful for the Rust for Linux project, as implementing support for static keys requires a long list of workarounds at the moment.

cc @Amanieu @oli-obk