rustdoc shows __arg0 for mutable parameter to async function

[Hawk777]

I tried this code:

/// Does a thing.
pub async fn do_thing(mut x: u32) {
	x
}

I expected to see this happen: In the Rustdoc output, a parameter named x.

Instead, this happened: In the Rustdoc output, pub async fn do_thing(__arg0: u32).

Meta

rustc --version --verbose:

rustc 1.46.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.46.0
LLVM version: 10.0

[camelid]

Minimized to:

async fn foo(mut x: i32) {}

[camelid]

Applying P-medium since I think this is important to fix.

[jyn514]

Duplicate of #76517