Function syntex_syntax2::ext::base::expr_to_spanned_string

pub fn expr_to_spanned_string(
    cx: &mut ExtCtxt, 
    expr: P<Expr>, 
    err_msg: &str
) -> Option<Spanned<(Symbol, StrStyle)>>

Extract a string literal from the macro expanded version of expr, emitting err_msg if expr is not a string literal. This does not stop compilation on error, merely emits a non-fatal error and returns None.