rust-analyzer shows false positive E0308 type mismatch errors with PyO3 macros

[sir-stevie]

Description

rust-analyzer shows false positive E0308 type mismatch errors when using PyO3's #[pyclass] macro in combination with third-party procedural macros (specifically pyo3_stub_gen_derive::gen_stub_pyclass). The errors appear as:

expected &'static [fn() -> TypeInfo], found &[fn() -> TypeInfo; 0]
expected &'static [MemberInfo], found &[MemberInfo; 0]
expected &'static [PyType_Slot], found &[PyType_Slot; 0]
expected &'static [MaybeRuntimePyMethodDef], found &[MaybeRuntimePyMethodDef; 0]

Despite these rust-analyzer errors, the code compiles and runs perfectly.

Steps to Reproduce

1. Create a PyO3 project with the following dependencies:

[dependencies]
pyo3 = { version = "*" }
pyo3-polars = { version = "0.22.0", features = ["derive", "dtype-full", "lazy"] }
pyo3-stub-gen = { version = "0.10.0", default-features = false }

2. Create a struct with both PyO3 and stub generation macros:

use pyo3::prelude::*;
use pyo3_stub_gen::derive::{gen_stub_pyclass, gen_stub_pymethods};

#[gen_stub_pyclass]
#[pyclass]
#[derive(Clone)]
pub struct YourStruct;

#[gen_stub_pymethods]
#[pymethods]
impl YourStruct {
    #[staticmethod]
    fn example_method() -> PyResult<f64> {
        Ok(42.0)
    }
}

3. Open the project in VS Code with rust-analyzer extension
4. Observe the E0308 errors in rust-analyzer, while cargo check passes without issues

Expected Behavior

rust-analyzer should not show type mismatch errors for properly written PyO3 code that compiles successfully.

Actual Behavior

rust-analyzer shows multiple E0308 type mismatch errors related to array-to-slice coercion in macro-generated code.

Environment

• Rust version: 1.89.0 (nightly)
• rust-analyzer version: v0.3.2509
• Operating System: macOS
• Python version: 3.13
• Cargo.toml setup:

  pyo3 = { version = "*" }
  pyo3-polars = { version = "0.22.0", features = ["derive", "dtype-full", "lazy"] }
  pyo3-stub-gen = { version = "0.10.0", default-features = false }

Additional Context

• The issue appears to be related to how rust-analyzer handles procedural macro expansion when multiple proc macros are combined
• The code works perfectly at runtime and passes all cargo commands
• This affects developer experience as it creates noise in the IDE with false error indicators

Workaround

Currently, disabling E0308 diagnostics in rust-analyzer settings as a workaround.

[Distortedlogic]

I believe there is a regression in one of the latest nightly versions .. downgrading to a few days ago resolves .. I seen this 'static type error as well as a slice vs sized slice [u8] vs [u8; 5] in marco expansions with the nightly that has the regression

[ShoyuVanilla]

Strangely enough, I can't reproduce this. Could you please share your Cargo.lock on your reproduction steps for exact dependency versions as well, with exact rustc nightly version(with dates and hash) output that printed with rustc --version?

[sir-stevie]

Strangely enough, I can't reproduce this. Could you please share your Cargo.lock on your reproduction steps for exact dependency versions as well, with exact rustc nightly version(with dates and hash) output that printed with rustc --version?

I shared everything in the description I gave about the setup I was using. I used @Distortedlogic 's trick to get around it, I'm not having the issue anymore. I'll see if a I ca reproduce it again and share the Cargo.lock

[Distortedlogic]

Strangely enough, I can't reproduce this. Could you please share your Cargo.lock on your reproduction steps for exact dependency versions as well, with exact rustc nightly version(with dates and hash) output that printed with rustc --version?

did u use the nightly build of the day the issue was posted? like the june 22nd nightly 1.89? nightly is on 1.90 now.

rust-lang/rust#142794
here is a rollup of 9 PRs which are all in the 1.89 nightly release change log and the timing matches

take this with a grain of salt as I am no rust pro... but I think the issue we saw was related to rust-lang/rust#142532

The macro expansion where I hit this issue were using unsafe and pointer operations .. and the location of the pointer operation was where I was getting the type error of &[u8] vs &[u; 5]

rust-lang/rust#142650 - this one seems related to rust-analyzer but over my head for any kinda guess on its impact

rust-lang/rust#142713 - this one seems related to macros

[ensc]

I am seeing the E0308 when using tracing macros and/or in code like

pub trait Foo: Sync {}
struct Bar;
impl Foo for Bar {}

static BAR: Bar = Bar;
pub static BAR_PTR: &'static dyn Foo = &BAR;    // <<<< expected &'static (dyn Foo + 'static), found &Bar

fn foo() {
    // expected &'static (dyn Callsite + 'static), found &DefaultCallsite
    // expected &'static [&'static str], found &[&'static str; 1]
    tracing::error!("foo");
}

rust-analyzer 1.90.0-nightly (bdaba05 2025-06-27)

UPDATE: ok with rust-analyzer 1.90.0-nightly (71e4c00 2025-07-01)

[Veykril]

I believe this is due to the sized hierachy changes, latest rust-analyzer pre-release should be working again now

[qRoC]

More simple:

struct Test;

impl std::fmt::Debug for Test {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        // expected &(dyn Debug + 'static), found &&'static str
        f.debug_tuple("Test").field(&"hello").finish()
    }
}

rustc 1.90.0-nightly (bdaba05 2025-06-27)
rust-analyzer 0.3.2509-standalone (0100bc7 2025-06-22)

Update
Ok in: rust-analyzer 0.4.2517-standalone (13ffda70eb 2025-06-28)

[ShoyuVanilla]

Update Ok in: rust-analyzer 0.4.2517-standalone (13ffda70eb 2025-06-28)

Good! Is there anyone getting this issue with the latest stable (or nightly) version of rust-analyzer?
If not, I think this is good to be closed and if so, I'll take a look into this.

[ensc]

Still with rust-analyzer 1.90.0-nightly (e43d139 2025-07-09)

use std::any::Any;
use std::sync::Arc;

pub trait Device: Any + Sync + Send {
}

pub struct Gpio;
impl Device for Gpio {}

pub fn ext_dev(dev: Arc<dyn Device>) -> Arc<Gpio> {
    Arc::downcast::<Gpio>(dev).unwrap()
    //                    ^^^
    // expected Arc<dyn Any + Send + Sync, Global>, found Arc<dyn Device, Global>
}

[ChayimFriedman2]

Works now.