Clarify whether inline assembly nomem implies nostack

[lukas-code]

Location: https://doc.rust-lang.org/nightly/reference/inline-assembly.html#options

i.e. is the following UB:

pub fn foo() {
    unsafe { core::arch::asm!("push rax", "pop rax", options(nomem)) }
}

This code obviously does write to memory, but the reference only talks about global variables for nomem, which make me think this probably fine? It would be nice to clarify whether nomem inline assembly is allowed to access to stack at all.

[Freax13]

I think this code would not be UB.

The set of memory locations that assembly code is allowed to read and write are the same as those allowed for an FFI function. [...] If the readonly option is set, then only memory reads are allowed. If the nomem option is set then no reads or writes to memory are allowed. [...] These rules do not apply to memory which is private to the asm code, such as stack space allocated within the asm block.

(source, emphasis mine)

[lukas-code]

@Freax13 Thanks, I must have missed that.