rust/src/libsyntax_ext/trace_macros.rs

use syntax::ext::base::ExtCtxt;
use syntax::ext::base;
use syntax::feature_gate;
use syntax::symbol::keywords;
use syntax_pos::Span;
use syntax::tokenstream::TokenTree;

pub fn expand_trace_macros(cx: &mut ExtCtxt,
                           sp: Span,
                           tt: &[TokenTree])
                           -> Box<dyn base::MacResult + 'static> {
    if !cx.ecfg.enable_trace_macros() {
        feature_gate::emit_feature_err(&cx.parse_sess,
                                       "trace_macros",
                                       sp,
                                       feature_gate::GateIssue::Language,
                                       feature_gate::EXPLAIN_TRACE_MACROS);
        return base::DummyResult::any(sp);
    }

    match (tt.len(), tt.first()) {
        (1, Some(&TokenTree::Token(_, ref tok))) if tok.is_keyword(keywords::True) => {
            cx.set_trace_macros(true);
        }
        (1, Some(&TokenTree::Token(_, ref tok))) if tok.is_keyword(keywords::False) => {
            cx.set_trace_macros(false);
        }
        _ => cx.span_err(sp, "trace_macros! accepts only `true` or `false`"),
    }

    base::DummyResult::any(sp)
}