Function snippet_with_applicability 

pub fn snippet_with_applicability<'a, 'sm>(
    sm: impl HasSourceMap<'sm>,
    span: Span,
    default: &'a str,
    applicability: &mut Applicability,
) -> Cow<'a, str>

Same as snippet, but it adapts the applicability level by following rules:

• Applicability level Unspecified will never be changed.
• If the span is inside a macro, change the applicability level to MaybeIncorrect.
• If the default value is used and the applicability level is MachineApplicable, change it to HasPlaceholders

If the span might realistically contain a macro call (e.g. vec![]), consider using snippet_with_context instead.