When enableExperimental true, rust-analyzer incorrectly reports type mismatch for async-trait implementations: Pin<Box<dyn Future>> vs Pin<Box<impl Future>>

[henryverlin]

I encountered a persistent type mismatch error when using async-trait that only appeared in rust-analyzer but not during compilation. After several hours of troubleshooting, I discovered that the issue was caused by having rust-analyzer.diagnostics.enableExperimental set to true in my workspace settings.

Setting enableExperimental to false completely resolves the issue, while cargo check and cargo build work correctly regardless of this setting.

Impact:

This appears to be a bug in rust-analyzer's experimental diagnostics when processing async-trait macro expansions. The experimental feature incorrectly reports type mismatches between Pin<Box> and Pin<Box> for async trait implementations.

Reproduction:

The issue can be reproduced by:

Setting "rust-analyzer.diagnostics.enableExperimental": true in VS Code settings
Using #[async_trait] with async methods in trait definitions and implementations
Observing the false positive type mismatch errors in the IDE
Workaround:

Setting "rust-analyzer.diagnostics.enableExperimental": false eliminates the issue entirely.

I'm reporting this to help improve the experimental diagnostics feature and assist other developers who may encounter the same issue.

Description:
rust-analyzer shows a type mismatch error for async trait implementations using the async-trait crate when vscode has "rust-analyzer.diagnostics.enableExperimental": true:

error[E0308]: mismatched types
expected Pin<Box<dyn Future<Output = Result<SimpleResult, Box<dyn Error + Send + Sync + 'static, Global>>> + Send + 'async_trait, Global>>
found    Pin<Box<impl Future<Output = Result<SimpleResult, Box<dyn Error + Send + Sync + 'static, Global>>>, Global>>

code snippet to reproduce:

use async_trait::async_trait;
use std::error::Error;

#[derive(Debug, PartialEq)]
pub enum SimpleResult {
    Ok,
    Error,
}

pub struct ExampleData {
    pub id: i32,
    pub name: String,
}

#[async_trait]
pub trait SimpleModel {
    async fn save(&self) -> Result<SimpleResult, Box<dyn Error + Send + Sync + 'static>>;
}

#[async_trait]
impl SimpleModel for ExampleData {
    async fn save(&self) -> Result<SimpleResult, Box<dyn Error + Send + Sync + 'static>> {
        if self.id > 0 {
            Ok(SimpleResult::Ok)
        } else {
            Ok(SimpleResult::Error)
        }
    }
}

vscode workspace file settings:

{
  "folders": [
    {
      "path": "."
    }
  ],
  "settings": {
    // Rust Configuration
    "rust-analyzer.showUnlinkedFileNotification": false,
    "rust-analyzer.check.command": "clippy",
    "rust-analyzer.check.allTargets": false,
    "rust-analyzer.diagnostics.enable": true,
    "rust-analyzer.diagnostics.enableExperimental": true,
    "rust-analyzer.completion.autoimport.enable": true,
    "rust-analyzer.imports.granularity.group": "module",
    "rust-analyzer.imports.prefix": "crate",
    "rust-analyzer.rustfmt.extraArgs": ["+stable"],
  },
}

Expected behavior:
No error should be shown, as the #[async_trait] macro should transform the async fn into the correct Pin<Box> type.

Actual behavior:
rust-analyzer shows a type mismatch between Pin<Box> and Pin<Box>, suggesting it's not correctly processing the async-trait macro expansion.

Additional context:

• cargo check and cargo build complete successfully with no errors
• The error only appears in the IDE, not during compilation
• The error occurs specifically when combining #[async_trait] with async fn methods
• Non-async trait methods work fine
• Issue persists after cache clearing, rust-analyzer restarts, and ensuring single async-trait version

Dependencies:
[dependencies]
async-trait = "0.1.88"

This appears to be a rust-analyzer issue with macro expansion for the async-trait crate, where the IDE incorrectly analyzes the return type of async trait methods.

rust-analyzer version: 0.4.2488-standalone (6acff6c 2025-06-06) [/Users/henryverlin/.vscode/extensions/rust-lang.rust-analyzer-0.4.2488-darwin-arm64/server/rust-analyzer]

rustc version: rustc 1.87.0 (17067e9ac 2025-05-09)

editor or extension: VSCode with rust-analyzer extension version 0.4.2488

relevant settings: Default settings, workspace with Cargo.toml at root defining fetcher as member

repository link (if public, optional): N/A

[ChayimFriedman2]

This is a known issue, that will be fixed when we switch to the new trait solver.

[henryverlin]

This is a known issue, that will be fixed when we switch to the new trait solver.

Thanks for the info! Thats what I figured, but wanted to share here so maybe someone else who is struggling with the same error finds the "solution". Have a good day.

(not sure if closed issues remain visible for the above reason - or if want to keep this open until truly fixed, so wont close with this comment but feel free to do so if you find it appropriate)