E.g.
#![feature(macro_rules)]
// minimal junk
#![no_std]
macro_rules! foo {
($x: ident) => { y + $x }
}
fn bar() {
let x = 1;
foo!(x)
}
becomes
#![feature(macro_rules)]
// minimal junk
#![no_std]
fn bar /* 61#0 */() { let x /* 58#2 */ = 1; y /* 60#4 */ + x /* 58#5 */ }
Specifically, the syntax contexts of x /* 58#2 */ and x /* 58#5 */ are linked (the xs refer to the same identifier, even with hygiene) but this is not represented in the output at all, so one might interpret them as being distinct identifiers.