Function new_parser_from_simple_source_str 

pub fn new_parser_from_simple_source_str(
    psess: &ParseSess,
    name: FileName,
    source: String,
) -> Result<Parser<'_>, Vec<Diag<'_>>>

Creates a new parser from a simple (no shebang, no frontmatter) source string.

On failure, the errors must be consumed via unwrap_or_emit_fatal, emit, cancel, etc., otherwise a panic will occur when they are dropped.