Why access subtyping is not allowed in type extensions?

[RustyYato]

I know the difference, I don't see how that's relevant here though. Traits can't have fields so your second example doesn't make sense (unless you are proposing an extension to Rust). Currently, if you wanted to access a field you would need to use a function like fn field(&self) -> &FieldType. Of course this runs into issues with the borrow checker because it can't see disjoint borrows across function boundaries.

(this issue can be alleviated with some sort of partial borrows)

Pre-RFC: Partial Borrow Syntax Sugar

I woke up with this idea this morning. It is still very much a work in progress, but I want to get the idea out there to get some feedback and let the wider community iterate on it. rendered Edit: updated rendered link to point to feature branch.

Partial borrows have a long history, going back to 2016 on these forums

Partial self borrowing syntax

This is valid D code: struct Foo { uint x; uint[2] v; void bar() { x++; } void spam() { foreach (_; v) bar(); } } void main() {} You can’t translate it to this Rust code: struct Foo { x: u32, v: [u32; 2] } impl Foo { fn new() -> Self { Foo { x: 0, v: [0; 2] } } fn bar(&mut self) { self.x += 1; } fn spam(&mut self) { for _ in self.v.iter() { self.bar(); } } } f…