Fails to identify type of vec![] macro

[zeramorphic]

In an empty (latest rust-analyzer, nightly rust) VSCode project, I am getting the following (related) errors to do with the vec! macro.

It seems as if rust-analyzer is maybe unable to resolve the second type parameter related to the new allocator API, and that error stops it from filling in the first parameter. Weirdly, it semi-correctly identifies a char in the following image, but thinks that it is part of an array.

It also identifies vec!["]; to be a Vec<[&str; _]>.

This is probably not a direct repeat of #6668 - I'm using a nightly version of rust-analyzer released after that issue was patched. Also, using the normal Vec constructor, and adding elements in afterwards, works fine thanks to type inference - it's only a problem with the macro, it seems.

[flodiebold]

I can't reproduce this with current rust-analyzer. Can you check whether it's still happening?

[zeramorphic]

Yes, I've just updated rust-analyzer vscode plugin, the language server, and rustup. The issue is still here. If it's relevant, I'm using x86_64-pc-windows-msvc.

[flodiebold]

Ah wait, I missed that this is with nightly rust.

[zeramorphic]

Yeah, I'm pretty sure it's to do with the new allocator API adding extra type parameters to Vec.

[flodiebold]

Ah, vec![1i32] now expands to <[_]>::into_vec(box [1i32]), and we're having problems either with the Box self type or with something about the new allocator parameter.

[Pawel82S]

I've got the same problem but with stable rust v 1.50

[detrumi]

It's the box_syntax feature:

let a = <[_]>::into_vec(Box::new([1i32])); // Vec<i32, Global>
let a = <[_]>::into_vec(box [1i32]); // Vec<{unknown}, {unknown}>

I think RA doesn't figure out the allocator parameter when using box ... syntax:

< [INFO hir_ty::traits] trait_solve_query(Implements(fn new<?0.0>(?0.0) -> Box<?0.0, Global>: Deref))
< [INFO hir_ty::traits] trait_solve_query(Implements(Box<[i32; _], Global>: CoerceUnsized<Box<[?0.0], ?0.1>>))
> [INFO hir_ty::traits] trait_solve_query(Implements(Box<[i32; _], ?0.0>: CoerceUnsized<Box<[?0.1], ?0.2>>))

[flodiebold]

How does box interact with the allocator parameter? Does it always use the default?

[detrumi]

According to rust#58457):

Ideally, inherent methods like Box::new would be applied to
Box<T, A: Alloc + Default>, but as of now, that would be backwards
incompatible because it would require type annotations in places where
they currently aren't required.

So at the moment Box::new always uses std::alloc::Global, but that may change in the future.
box_syntax might already supports custom allocators, and defaults to the Global (like how Vec does for example, but in this case built into the syntax instead of as a default type param). Not sure though, I'll ask around.

[detrumi]

Tried it out, and turns out that box_syntax currently always uses std::alloc::Global (playground link)
So I think rust-analyzer has to set that default somewhere, instead of trying to infer it.

[flodiebold]

Ok, that's probably the reason it doesn't work then. The handling for the box syntax is here:
https://github.com/rust-analyzer/rust-analyzer/blob/803ff2e55eeb6bd1d9028fbbffe34721f0e6d459/crates/hir_ty/src/infer/expr.rs#L509-L512
Currently we just fill it with a type variable; we could instead look up the generic defaults (using db.generic_defaults) and put them there.