1 //! Module for inferring the variance of type and lifetime parameters. See the [rustc guide]
2 //! chapter for more info.
4 //! [rustc guide]: https://rust-lang.github.io/rustc-guide/variance.html
9 use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
10 use rustc::ty::{self, CrateVariancesMap, TyCtxt};
11 use rustc::ty::query::Providers;
12 use rustc_data_structures::sync::Lrc;
14 /// Defines the `TermsContext` basically houses an arena where we can
18 /// Code to gather up constraints.
21 /// Code to solve constraints and write out the results.
24 /// Code to write unit tests of variance.
27 /// Code for transforming variances.
30 pub fn provide(providers: &mut Providers<'_>) {
31 *providers = Providers {
38 fn crate_variances<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, crate_num: CrateNum)
39 -> Lrc<CrateVariancesMap> {
40 assert_eq!(crate_num, LOCAL_CRATE);
41 let mut arena = arena::TypedArena::default();
42 let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &mut arena);
43 let constraints_cx = constraints::add_constraints_from_crate(terms_cx);
44 Lrc::new(solve::solve_constraints(constraints_cx))
47 fn variances_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_def_id: DefId)
48 -> Lrc<Vec<ty::Variance>> {
49 let id = tcx.hir().as_local_hir_id(item_def_id).expect("expected local def-id");
50 let unsupported = || {
51 // Variance not relevant.
52 span_bug!(tcx.hir().span_by_hir_id(id), "asked to compute variance for wrong kind of item")
54 match tcx.hir().get_by_hir_id(id) {
55 Node::Item(item) => match item.node {
56 hir::ItemKind::Enum(..) |
57 hir::ItemKind::Struct(..) |
58 hir::ItemKind::Union(..) |
59 hir::ItemKind::Fn(..) => {}
64 Node::TraitItem(item) => match item.node {
65 hir::TraitItemKind::Method(..) => {}
70 Node::ImplItem(item) => match item.node {
71 hir::ImplItemKind::Method(..) => {}
76 Node::ForeignItem(item) => match item.node {
77 hir::ForeignItemKind::Fn(..) => {}
82 Node::Variant(_) | Node::Ctor(..) => {}
87 // Everything else must be inferred.
89 let crate_map = tcx.crate_variances(LOCAL_CRATE);
90 crate_map.variances.get(&item_def_id)
91 .unwrap_or(&crate_map.empty_variance)