Function parse_concat 

fn parse_concat<'psess>(
    iter: &mut TokenStreamIter<'_>,
    psess: &'psess ParseSess,
    outer_span: Span,
    expr_ident_span: Span,
) -> PResult<'psess, MetaVarExpr>

Parse a meta-variable concat expression: concat($metavar, ident, ...).