Function rustc_expand::base::expr_to_string

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

Extracts 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.