Implement builtin # syntax and use it for offset_of!(...)

[est31]

Add builtin # syntax to the parser, as well as a generic infrastructure to support both item and expression position builtin syntaxes. The PR also uses this infrastructure for the implementation of the offset_of! macro, added by #106934.

cc @petrochenkov @drmeepster

cc #110680 builtin # tracking issue
cc #106655 offset_of! tracking issue

[rustbot]

r? @WaffleLapkin

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[clubby789]

Once builtin# syntax is merged, would it make sense to implement builtin#ptr_write, as suggested in #108516 (comment) ?

[est31]

@clubby789 personally I'd like that, yes. It is imo better to have such a primitive than #[rustc_box], as its main use is inside the vec![] macro which immediately converts the boxed slice into a Vec. Outside of rust, builtin#write (and its macro wrapper) can support users who don't have alloc or std but do have core, say rust-on-linux folks. The offset_of macro btw helps people with these use cases as well because it makes it easier for them to build macros that get you 95% there... but all of this obviously doesn't change the fact that one can't remove builtin#write/#[rustc_box] from vec![]'s implementation and IMO it's simpler to have builtin#write than to have #[rustc_box].

[beepster4096]

It might be better to have a builtin#shallow_init_box instead to more closely match the mir that #[rustc_box] generates.

[est31]

Oh I was unaware of rust-lang/compiler-team#460 and #89030 . It seems to be a special case just to support box syntax / #[rustc_box]. Maybe it would be possible to use something that builds on MaybeUninit instead, say for example Box::new_uninit_slice? IIRC the magic that avoids the copies onto the stack is inside the expr_into_dest stuff to write into the Box pointer, not the stuff before it.

Unwinding seems to be implemented weirdly, order depending on whether there is a panic call or not:

struct LoudDrop(u8);

impl Drop for LoudDrop {
    fn drop(&mut self) {
        println!("dropping {}", self.0);
    }
}
fn main() {
    {
        let _v = vec!(LoudDrop(0), LoudDrop(1));
    }
    println!("Hi");
    {
        let _v = vec!(LoudDrop(2), LoudDrop(3), LoudDrop(4), panic!());
    }
}

prints

dropping 0
dropping 1
Hi
dropping 4
dropping 3
dropping 2

anyways we should better discuss this in a separate thread, I want to keep this one dedicated to my PR. Edit: filed #110715

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[petrochenkov]

#110970 has landed, r=me after rebase.
@rustbot author

[est31]

@rustbot ready

[petrochenkov]

@bors r+

[bors]

📌 Commit d5669bf7f881d194c9c5ff1536c67fa635d4030c has been approved by petrochenkov

It is now in the queue for this repository.

[petrochenkov]

@bors r+

[bors]

📌 Commit 83b4df4 has been approved by petrochenkov

It is now in the queue for this repository.