False String/str mismatch in diesel-async query with four joins

rust-analyzer version: 0.3.3049-standalone (682a84e95b 2026-09-13)

rustc version: 1.95.0 (59807616e 2026-04-14)

editor or extension: CLI (rust-analyzer diagnostics .)

relevant settings: Default

cargo check --locked succeeds. rust-analyzer diagnostics . reports E0282 on the query and E0308 (expected String, found str) at Ok(value). The value should be String, as specified by .first::<String>(conn).

code snippet to reproduce:

Cargo.toml:

[package]
name = "diesel-ra-inference-repro"
version = "0.1.0"
edition = "2024"

[dependencies]
diesel = { version = "=2.3.13", default-features = false, features = ["mysql_backend"] }
diesel-async = { version = "=0.9.2", default-features = false, features = ["mysql"] }

src/lib.rs:

use diesel::prelude::*;
use diesel_async::{AsyncMysqlConnection, RunQueryDsl};

diesel::table! { a (id) { id -> Integer, name -> Text, } }
diesel::table! { b (id) { id -> Integer, } }
diesel::table! { c (id) { id -> Integer, } }
diesel::table! { d (id) { id -> Integer, } }
diesel::table! { e (id) { id -> Integer, } }
diesel::allow_tables_to_appear_in_same_query!(a, b, c, d, e);

pub async fn query(conn: &mut AsyncMysqlConnection) -> QueryResult<String> {
    let value = a::table
        .inner_join(b::table.on(a::id.eq(b::id)))
        .inner_join(c::table.on(a::id.eq(c::id)))
        .inner_join(d::table.on(a::id.eq(d::id)))
        .inner_join(e::table.on(a::id.eq(e::id)))
        .select(a::name)
        .first::<String>(conn)
        .await?;
    let _: &str = &value;
    Ok(value)
}

Removing the last join eliminates both diagnostics. Adding : String to let value removes E0308 but leaves E0282.

Related: #22402.

Start with the Cargo.toml and src/lib.rs reproduction, then run cargo check --locked and rust-analyzer diagnostics . to compare the results. Investigate the diagnostics around the four-join query and the first::(conn) call; done means the valid query no longer produces E0282 or the false String/str mismatch.