rust-analyser doesn't provide suggestions/hover/types for `alloc` types with `Deref` when using a custom target

[matanui159]

rust-analyzer version: rust-analyzer version: 0.4.2355-standalone (3ed13b4 2025-03-24) [~/.vscode/extensions/rust-lang.rust-analyzer-0.4.2355-darwin-arm64/server/rust-analyzer]

I also tested with the non pre-release version

rustc version: rustc 1.87.0-nightly (f8c27dfe1 2025-03-24)

editor or extension: VSCode, release and pre-release

relevant settings: "rust-analyzer.cargo.target": "mipsel-sony-psx.json"

repository link (if public, optional): https://github.com/matanui159/rust-analyzer-bug

code snippet to reproduce:

#![no_std]
extern crate alloc;

use core::cell::Ref;
use alloc::{rc::Rc, vec::Vec};

pub fn vec_end(vec: &Vec<i32>) -> *const i32 {
    let range = vec.as_ptr_range();
    range.end
}

pub fn get_inner(rc: &Rc<i32>) -> i32 {
    let inner = **rc;
    inner + 1
}

pub fn get_inner_borrow(borrow: &Ref<i32>) -> i32 {
    let inner = **borrow;
    inner + 1
}

The range and first inner variable have the type {unknown}. Meanwhile the second inner variable (which uses a type from core) has the correct i32 type. Additionally the as_ptr_range method was not suggested when typing and hovering does not provide any docs.

If the rust-analyser target is set to mipsel-sony-psx (without the .json, e.g. the official rust target), even with all other settings the same (still nightly with build-std) the types work again. The JSON file is a direct copy of the official target generated using:

rustc +nightly -Zunstable-options --target=mipsel-sony-psx --print=target-spec-json > mipsel-sony-psx.json

I didn't originally get this issue with this target specifically (hence why I needed a custom target) but using one of the official targets copied into a JSON ruled out any issues with the target config.

[matanui159]

It doesn't seem to be detecting the traits at all on the alloc types. For example it doesn't even detect Rc.deref or Vec.into_iter

[matanui159]

Found some more oddities. Associated types don't seem to be working either. Doing <alloc::rc::Rc<i32> as Deref>::Target doesn't simplify to i32 but <core::cell::Ref<i32> as Deref>::Target does. Also if I add the following:

trait DerefExt {
    fn foobar(&self) {}
}

impl<T: Deref> DerefExt for T {}

Then foobar does show up in suggestions and hover, which implies rust-analyser does know that Rc/Vec implement Deref, just doesn't know the methods/types they get because of that. Instead limiting the above impl to T: Deref<Target = i32> instead removes foobar from suggestions on Rc<i32>, likely because it doesn't know about the associated type.

Anyway I'm gonna stop playing around with this for tonight. Hopefully the information is useful :)

[TitanHZZ]

Yeah, i seem to be running into the same problem. Also using rust nightly in a no_std environment.

[kkew3]

Running into the same problem too. rust-analyzer v1.88.0 on vscode fails to hover over Vec itself---hovering over Vec::new works fine. However, the difference is that I'm using std environment, and rust stable (v1.88.0).

[lnicola]

Does Go to definition work on the relevant types?

[kkew3]

Go to definition seems to work fine for either Vec or Vec::new in my case. Just that hovering over Vec (probably on some other types too) fails.

[lnicola]

@kkew3 that's probably #20190 and it should be fixed in the pre-release version.

[kkew3]

Thanks for pointing out! Yeah it should be that issue.