Macro expansion moves comments around.

[steffahn]

pub fn bar() {
    println!("Hello, World!");
}


// hello...

pub fn foo() {
    /*
    some
    comment
    */
}

// ...world!

(Playground)

Macro Expansion:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std;
pub fn bar() {


    // hello...

    /*
    some
    comment
    */

    // ...world!
    {
        ::std::io::_print(::core::fmt::Arguments::new_v1(&["Hello, World!\n"],
                                                         &match () {
                                                              () => [],
                                                          }));
    };
}
pub fn foo() { }

[petrochenkov]

I first thought this would be about doc comments, then it would be a duplicate of #67839.

But this is about regular comments that are not parsed into AST and don't participate in expansion at all.
Pretty-printing used by --pretty=expanded then tries its best to put comments into their old places based on spans, but there aren't any guarantees.