Nested `macro_rules` utilising inner metavariable are infinitely indented with repeated formatting

Input

macro_rules! outer {
    ($d:tt) => {
        macro_rules! inner {
            ($d s:expr) => {
                println!("{}", $d s);
            }
        }
    };
}

outer!($);

fn main() {
    inner!("hi");
}

Output
Note that the body and trailing closing braces of the nested macro_rules block is indented. With further invocations of rustfmt, this block is continuously indented to the next level.

Removing the usage of the inner macro_rules's $s in the println! usage causes the formatting to behave as expected.

macro_rules! outer {
    ($d:tt) => {
        macro_rules! inner {
                    ($d s:expr) => {
                        println!("{}", $d s);
                    }
                }
    };
}

outer!($);

fn main() {
    inner!("hi");
}

Expected output
The input should likely remain unchanged.

Meta

• rustfmt version: 1.4.27-nightly (2020-11-16 580d826), as used on Rust Playground
• From where did you install rustfmt?: Rust Playground

Reproduce the provided nested macro_rules input with rustfmt and run formatting repeatedly to confirm the indentation grows. Start by tracing rustfmt's macro_rules formatting path, then add a regression test showing that inner metavariable usage leaves the input stable across repeated formatting.