`redundant_async_block` not catching uses with nontrivial futures

Summary

The redundant_async_block lint fails to trigger on uses of async { EXPRESSION.await } for seemingly any EXPRESSION other than a single variable.

Lint Name

redundant_async_block

Reproducer

I tried this code:

use std::future::ready;

pub fn foo() {
    tokio::spawn(async { ready(42).await });
}

I expected to see this happen: clippy should have warned about the redundant async block and recommended just writing tokio::spawn(ready(42)) instead.

Instead, this happened: no warnings

Version

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: x86_64-apple-darwin
release: 1.77.2
LLVM version: 17.0.6

Start with the redundant_async_block lint and reproduce the issue using the ready(42).await example. Trace how the lint handles async blocks whose awaited expression is not a single variable. Done means Clippy warns on this case and recommends tokio::spawn(ready(42)) without regressing existing behavior.