add field_of! macro and field representing types

[y86-dev]

I implemented the core of rust-lang/rfcs#3318 for personal experimentation and found out that you can get experimental features merged easier than a whole RFC. I do not know the correct process to do this, but I thought a PR would be the way to go.

This PR introduces field info types. These types are generated for every field of every struct. They can be named by using the core::mem::field_of!() macro also introduced by this PR.

I have not had any experience modifying the Rust compiler, so there will be mistakes. I could definitely use some help refining this, so any comments are welcomed. The way I implemented was by first looking at offset_of! then realizing I want something that resolves to a type and not an expression, I then went and took a look at how impl Trait types are implemented and copied a lot from that. At some point I also realized that type equality and trait resolution were incorrect, so I went digging and I think I fixed it.

Here a list of things to complete before even thinking about merging:

• create an issue to use for the unstability attribute on field_of!
• handle TODOs in the code
• bikeshed the name FieldInfo: should the hir version be named FieldOf?
• how to handle enums/unions?
• can this also handle slices/arrays?
• fix type inference, i.e. allow field_of!(_, foo)
• remove some debugging stuff
• solve ICEs when using cargo instead of rustc

Example Code

#![feature(field_project, offset_of)]
use std::mem::{field_of, offset_of, MaybeUninit};

unsafe trait Field {
    type Base;
    type Type;
    const OFFSET: usize;
}

trait Project<F: Field> {
    type Output;
    fn project(self) -> Self::Output;
}

impl<'a, F: Field> Project<F> for &'a mut MaybeUninit<F::Base>
where
    F::Type: 'a,
{
    type Output = &'a mut MaybeUninit<F::Type>;
    fn project(self) -> Self::Output {
        let ptr = self.as_mut_ptr().cast::<u8>().wrapping_add(F::OFFSET);
        unsafe { &mut *ptr.cast() }
    }
}

#[derive(Debug)]
struct Foo {
    value: usize,
    next: Option<Box<Foo>>,
}

unsafe impl Field for field_of!(Foo, value) {
    type Base = Foo;
    type Type = usize;
    const OFFSET: usize = offset_of!(Foo, value);
}

unsafe impl Field for field_of!(Foo, next) {
    type Base = Foo;
    type Type = Option<Box<Foo>>;
    const OFFSET: usize = offset_of!(Foo, next);
}

fn main() {
    let mut foo = MaybeUninit::uninit();
    Project::<field_of!(Foo, value)>::project(&mut foo).write(42);
    Project::<field_of!(Foo, next)>::project(&mut foo).write(Some(Box::new(Foo {
        value: 84,
        next: None,
    })));
    println!("{:?}", unsafe { foo.assume_init_ref() });
}

Output: Foo { value: 42, next: Some(Foo { value: 84, next: None }) }

[rustbot]

r? @b-naber

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[rustbot]

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino, @ouz-a

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[rust-log-analyzer]

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[bors]

☔ The latest upstream changes (presumably #118679) made this pull request unmergeable. Please resolve the merge conflicts.

[b-naber]

Can you convert this to a draft PR?

Unfortunately I currently don't have enough time to help you with any of the outstanding implementation problems. Have you tried asking for help on zulip?

[y86-dev]

Thanks for the info, I will go ask there.

[Dylan-DPC]

@y86-dev any updates on this pr?

[y86-dev]

currently I am very busy. @nbdd0121 created an improved version some time ago: https://github.com/nbdd0121/rust/tree/field-projection

[Dylan-DPC]

Closing this in favour of the improved version

@nbdd0121 can you submit a draft pr here with those changes (even if wip) and mention this pr in the description, makes it easier for future tracking.

[nyabinary]

bump @nbdd0121