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

[the6p4c]

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

[davidBar-On]

The issue is caused because original code snippet is used, since the macro code cannot be parsed properly: a , is missing after the $d in println!("{}", $d s). Submitted PR #4629 with a proposed solution to the issue.

[the6p4c]

The original code does compile and execute as expected - I’m not sure what you mean by “cannot be parsed properly”.

[davidBar-On]

Oops! You are right. It is a while since I evaluated the root case .... Sorry for that.
The real issue is that rusfmt doesn't know to distinguish between a $ parameter and other parameters (see issue #8), so it expects that there will be a , after the $d. This is the reason for using the original code snippet.

Note that the proposed fix is handling the general issue of formatting macro body when original code snippet is used, and not the specific issue of handling the $ items (which seem to require significant refactoring of the code).

[chris-morgan]

Another example:

macro_rules! alpha {
    () => {
        macro_rules! beta {
            () => {
                gamma!(*)
            };
        }
    };
}

Where I’ve written *, you look to be able to put anything that doesn’t parse as an expression.

I found a hint at a likely underlying problem: if you take just the inner macro_rules!, you get a case where it just gives up and leaves indentation as it is for lines 2–5, so that this is a stable wonky indentation:

macro_rules! delta {
 () => {
  epsilon!(*)
   };
    }

Whereas if you remove that * or replace it with something that can be parsed as an expression, it formats to what you’d expect.