Rustfmt incorrectly adds whitespace inside multiline string literals within macro calls that have curly braces

Code:

macro_rules! test {
    ($l:literal) => {};
}

fn main() {
    test!{
        "
"};
}

expected output:

macro_rules! test {
    ($l:literal) => {};
}

fn main() {
    test!{
        "
"
    };
}

actual output:

macro_rules! test {
    ($l:literal) => {};
}

fn main() {
    test! {
            "
    "};
}

Rustfmt added an extra indentation to the literal, including inside the string itself. This only occurs when the macro invocation is delimited by curly brackets, it works fine when square or parentheses are used.

Start by running rustfmt on the supplied Rust snippet and compare the curly-braced macro invocation with square- and parenthesized forms. Trace the macro-call formatting path that handles multiline string literals and delimiters. Done means the curly-braced example matches the expected output without changing the string contents, while the other delimiter cases remain correct.