Function syntax::ext::base::expr_to_spanned_string

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

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.