Function print 

pub fn print(
    main: FileId,
    tokens: &[Tok],
    lines: &[LineDirective],
    sources: &SourceMap,
    interner: &Interner,
    opts: PrintOptions,
) -> String

Renders tokens the way -E prints them.

main is the file named on the command line, which is what the first line marker says even when the first token comes from a header. lines is the #line directives the run read, in the order it read them, because each one is a marker in the output at the point it was written rather than at the point its effect is first visible.