rust/compiler/rustc_builtin_macros/src/cfg_select.rs

use rustc_ast::tokenstream::TokenStream;
use rustc_attr_parsing as attr;
use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
use rustc_parse::parser::cfg_select::{CfgSelectBranches, CfgSelectPredicate, parse_cfg_select};
use rustc_span::{Ident, Span, sym};

use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};

/// Selects the first arm whose predicate evaluates to true.
fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
    for (cfg, tt, arm_span) in branches.reachable {
        if attr::cfg_matches(
            &cfg,
            &ecx.sess,
            ecx.current_expansion.lint_node_id,
            Some(ecx.ecfg.features),
        ) {
            return Some((tt, arm_span));
        }
    }

    branches.wildcard.map(|(_, tt, span)| (tt, span))
}

pub(super) fn expand_cfg_select<'cx>(
    ecx: &'cx mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream,
) -> MacroExpanderResult<'cx> {
    ExpandResult::Ready(match parse_cfg_select(&mut ecx.new_parser_from_tts(tts)) {
        Ok(branches) => {
            if let Some((underscore, _, _)) = branches.wildcard {
                // Warn for every unreachable predicate. We store the fully parsed branch for rustfmt.
                for (predicate, _, _) in &branches.unreachable {
                    let span = match predicate {
                        CfgSelectPredicate::Wildcard(underscore) => underscore.span,
                        CfgSelectPredicate::Cfg(cfg) => cfg.span(),
                    };
                    let err = CfgSelectUnreachable { span, wildcard_span: underscore.span };
                    ecx.dcx().emit_warn(err);
                }
            }

            if let Some((tts, arm_span)) = select_arm(ecx, branches) {
                return ExpandResult::from_tts(
                    ecx,
                    tts,
                    sp,
                    arm_span,
                    Ident::with_dummy_span(sym::cfg_select),
                );
            } else {
                // Emit a compiler error when none of the predicates matched.
                let guar = ecx.dcx().emit_err(CfgSelectNoMatches { span: sp });
                DummyResult::any(sp, guar)
            }
        }
        Err(err) => {
            let guar = err.emit();
            DummyResult::any(sp, guar)
        }
    })
}