The special case breaks several useful invariants (`ExpnId`s are globally unique, and never change). This special case was added back in 2016 in https://github.com/rust-lang/rust/pull/34355
The special case breaks several useful invariants (`ExpnId`s are globally unique, and never change). This special case was added back in 2016 in https://github.com/rust-lang/rust/pull/34355