src/librustc_builtin_macros/cfg.rs

//! The compiler code necessary to support the cfg! extension, which expands to
//! a literal `true` or `false` based on whether the given cfg matches the
//! current compilation environment.

use rustc_ast::ast;
use rustc_ast::token;
use rustc_ast::tokenstream::TokenStream;
use rustc_attr as attr;
use rustc_errors::DiagnosticBuilder;
use rustc_expand::base::{self, *};
use rustc_span::Span;

pub fn expand_cfg(
    cx: &mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream,
) -> Box<dyn base::MacResult + 'static> {
    let sp = cx.with_def_site_ctxt(sp);

    match parse_cfg(cx, sp, tts) {
        Ok(cfg) => {
            let matches_cfg = attr::cfg_matches(&cfg, cx.parse_sess, cx.ecfg.features);
            MacEager::expr(cx.expr_bool(sp, matches_cfg))
        }
        Err(mut err) => {
            err.emit();
            DummyResult::any(sp)
        }
    }
}

fn parse_cfg<'a>(
    cx: &mut ExtCtxt<'a>,
    sp: Span,
    tts: TokenStream,
) -> Result<ast::MetaItem, DiagnosticBuilder<'a>> {
    let mut p = cx.new_parser_from_tts(tts);

    if p.token == token::Eof {
        let mut err = cx.struct_span_err(sp, "macro requires a cfg-pattern as an argument");
        err.span_label(sp, "cfg-pattern required");
        return Err(err);
    }

    let cfg = p.parse_meta_item()?;

    let _ = p.eat(&token::Comma);

    if !p.eat(&token::Eof) {
        return Err(cx.struct_span_err(sp, "expected 1 cfg-pattern"));
    }

    Ok(cfg)
}