Hangs on derive macro invoked by format string macro

[dtolnay]

The compiler hangs when compiling the following code.

macro_rules! hang {
    () => {
        {
            #[derive(Clone)]
            struct S;

            ""
        }
    }
}

fn main() {
    format_args!(hang!());
}

This is a regression in 1.20.0. The same code terminates with an error "format argument must be a string literal" in 1.19.0, which is the correct behavior.

[dtolnay]

According to @steffengy in #47188 (comment) -- this is hitting an infinite loop in libsyntax_pos/lib.rs#macro_backtrace. See that comment for details.

[dtolnay]

Transferring nomination by @jonas-schievink in #62167.

[pnkfelix]

triage: P-high. Removing nomination. Assigning to @petrochenkov for primary investigation, and to myself to follow up on it.