ExplicitBug crash on `m!(a. X::Y)`

Repro against current master (c19b14539b79d44f5f433ac189d70e07fb152354):

$ echo 'm!(a. X::Y);' | RUST_BACKTRACE=1 cargo run --bin rustfmt
m!(a. X::Y);
thread 'main' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1425:13
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   3: core::ptr::drop_in_place<rustc_errors::HandlerInner>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   4: core::ptr::drop_in_place<core::cell::UnsafeCell<rustc_errors::HandlerInner>>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   5: core::ptr::drop_in_place<core::cell::RefCell<rustc_errors::HandlerInner>>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   6: core::ptr::drop_in_place<rustc_data_structures::sync::Lock<rustc_errors::HandlerInner>>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   7: core::ptr::drop_in_place<rustc_errors::Handler>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   8: core::ptr::drop_in_place<rustc_session::parse::ParseSess>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
   9: core::ptr::drop_in_place<rustfmt_nightly::parse::session::ParseSess>
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/library/core/src/ptr/mod.rs:487:1
  10: rustfmt_nightly::formatting::format_project
             at ./src/formatting.rs:175:1
  11: rustfmt_nightly::formatting::<impl rustfmt_nightly::Session<T>>::format_input_inner::{{closure}}
             at ./src/formatting.rs:48:33
  12: scoped_tls::ScopedKey<T>::with
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:171:13
  13: rustc_span::create_session_if_not_set_then::{{closure}}
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/compiler/rustc_span/src/lib.rs:141:50
  14: scoped_tls::ScopedKey<T>::set
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9
  15: rustc_span::create_session_if_not_set_then
             at /rustc/5750a6aa2777382bf421b726f234da23f990a953/compiler/rustc_span/src/lib.rs:141:9
  16: rustfmt_nightly::formatting::<impl rustfmt_nightly::Session<T>>::format_input_inner
             at ./src/formatting.rs:38:9
  17: rustfmt_nightly::Session<T>::format
             at ./src/lib.rs:453:9
  18: rustfmt::format_and_emit_report
             at ./src/bin/main.rs:359:11
  19: rustfmt::format_string
             at ./src/bin/main.rs:281:5
  20: rustfmt::execute
             at ./src/bin/main.rs:241:39
  21: rustfmt::main
             at ./src/bin/main.rs:26:27

Reproduce the crash with the shown cargo run --bin rustfmt command, then trace the formatting path through src/formatting.rs and src/bin/main.rs toward the parser and compiler/rustc_errors/src/lib.rs. Determine why this input reaches ExplicitBug; done means the same input no longer panics and instead produces the expected formatter behavior or diagnostic.