//! Used by `rustc` when compiling a plugin crate.

use rustc_hir as hir;
use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
use rustc_hir::itemlikevisit::ItemLikeVisitor;
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::TyCtxt;
use rustc_span::symbol::sym;
use rustc_span::Span;

struct RegistrarFinder<'tcx> {
    tcx: TyCtxt<'tcx>,
    registrars: Vec<(LocalDefId, Span)>,
}

impl<'v, 'tcx> ItemLikeVisitor<'v> for RegistrarFinder<'tcx> {
    fn visit_item(&mut self, item: &hir::Item<'_>) {
        if let hir::ItemKind::Fn(..) = item.kind {
            let attrs = self.tcx.hir().attrs(item.hir_id());
            if self.tcx.sess.contains_name(attrs, sym::plugin_registrar) {
                self.registrars.push((item.def_id, item.span));
            }
        }
    }

    fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem<'_>) {}

    fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem<'_>) {}

    fn visit_foreign_item(&mut self, _foreign_item: &hir::ForeignItem<'_>) {}
}

/// Finds the function marked with `#[plugin_registrar]`, if any.
pub fn find_plugin_registrar(tcx: TyCtxt<'_>) -> Option<DefId> {
    tcx.plugin_registrar_fn(LOCAL_CRATE)
}

fn plugin_registrar_fn(tcx: TyCtxt<'_>, cnum: CrateNum) -> Option<DefId> {
    assert_eq!(cnum, LOCAL_CRATE);

    let mut finder = RegistrarFinder { tcx, registrars: Vec::new() };
    tcx.hir().krate().visit_all_item_likes(&mut finder);

    match finder.registrars.len() {
        0 => None,
        1 => {
            let (def_id, _) = finder.registrars.pop().unwrap();
            Some(def_id.to_def_id())
        }
        _ => {
            let diagnostic = tcx.sess.diagnostic();
            let mut e = diagnostic.struct_err("multiple plugin registration functions found");
            for &(_, span) in &finder.registrars {
                e.span_note(span, "one is here");
            }
            e.emit();
            diagnostic.abort_if_errors();
            unreachable!();
        }
    }
}

pub fn provide(providers: &mut Providers) {
    *providers = Providers { plugin_registrar_fn, ..*providers };
}