Potentially valid code doesn't compile - a silly error message

I tried this code:

use std::cell::RefCell;

thread_local! {
    static REQUESTS_CHECKER: RefCell<()> = RefCell::new(());
}

async fn a() {}

async fn b() {
    REQUESTS_CHECKER.with_borrow_mut(async |c|
        a().await
    ).await
}

fn main() {}

I expected to see this happen: the code to compile.

Instead, this happened:

mismatched types
expected `async` closure body `{async closure body@canister/src/lib.rs:27:9: 35:16}`
   found `async` closure body `{async closure body@canister/src/lib.rs:27:9: 35:16}`
no two async blocks, even if identical, have the same type
consider pinning your async block and casting it to a trait object

This is not two async blocks, it is one.

Meta

rustc --version --verbose:

rustc 1.90.0 (1159e78c4 2025-09-14)

<backtrace>

Real code: this.

Start by compiling the provided async-closure example with rustc 1.90.0 and compare the diagnostic with the reported output. Trace the type-checking path for the call to REQUESTS_CHECKER.with_borrow_mut and the awaited async closure. Done means the valid example compiles and the diagnostic no longer incorrectly reports two distinct async blocks.