fn debug($self: $S::Span) -> String;
fn def_site() -> $S::Span;
fn call_site() -> $S::Span;
+ fn mixed_site() -> $S::Span;
fn source_file($self: $S::Span) -> $S::SourceFile;
fn parent($self: $S::Span) -> Option<$S::Span>;
fn source($self: $S::Span) -> $S::Span;