Function source_str_to_stream 

pub fn source_str_to_stream(
    psess: &ParseSess,
    name: FileName,
    source: String,
    override_span: Option<Span>,
) -> Result<TokenStream, Vec<Diag<'_>>>

Given a source string, produces a sequence of token trees.

NOTE: This only strips shebangs, not frontmatter!