Function new_parser_from_source_file 

pub(crate) fn new_parser_from_source_file(
    psess: &ParseSess,
    source_file: Arc<SourceFile>,
    strip_tokens: StripTokens,
) -> Result<Parser<'_>, Vec<Diag<'_>>>

Given a session and a source_file, return a parser. Returns any buffered errors from lexing the initial token stream.