single-match lint expands println! macro in suggestion

[kimsnj]

The example in the README file:

    let x = Some(1u8);
    match x {
        Some(y) => println!("{:?}", y),
        _ => ()
    }

produces this suggestions:

help: try this: `if let Some(y) = x { $ crate :: io :: _print ( format_args ! ( $ ( $ arg ) * ) ) }`

Using current master (commit: 6b3487a)
And rustc 1.25.0-nightly (b8398d9 2018-02-11)

Note putting the statement inside a block { println!("{:?}", y) } works properly (and is the current case covered by ui tests).

[kimsnj]

The issue seems to be more general than just the single arm lint. for example:

4 | /     match x {
5 | |         true => {println!("yay")},
6 | |         false => println!("nay")
7 | |     }
  | |_____^ help: consider using an if/else expression: `if x {println!("yay")} else { $ crate :: io :: _print ( format_args ! ( $ ( $ arg ) * ) ) }`

The difference in behaviour between the two arms can be observed by directly calling utils::snippet with either arms[0].body.span or arms[1].body.span.
This calls a method on LintContext session…

Can this be fixed on clippy side or should it be on rustc ?
I am not quite sure where to go from here.

[oli-obk]

We should be checking the span for macro expansions and not produce a snippet in that case (or change it to approximate_suggestion and insert a placeholder