1 //! Module for inferring the variance of type and lifetime parameters. See the [rustc dev guide]
2 //! chapter for more info.
4 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/variance.html
7 use rustc_arena::DroplessArena;
9 use rustc_hir::def_id::DefId;
10 use rustc_middle::ty::query::Providers;
11 use rustc_middle::ty::{self, CrateVariancesMap, TyCtxt};
13 /// Defines the `TermsContext` basically houses an arena where we can
17 /// Code to gather up constraints.
20 /// Code to solve constraints and write out the results.
23 /// Code to write unit tests of variance.
26 /// Code for transforming variances.
29 pub fn provide(providers: &mut Providers) {
30 *providers = Providers { variances_of, crate_variances, ..*providers };
33 fn crate_variances(tcx: TyCtxt<'_>, (): ()) -> CrateVariancesMap<'_> {
34 let arena = DroplessArena::default();
35 let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &arena);
36 let constraints_cx = constraints::add_constraints_from_crate(terms_cx);
37 solve::solve_constraints(constraints_cx)
40 fn variances_of(tcx: TyCtxt<'_>, item_def_id: DefId) -> &[ty::Variance] {
41 let id = tcx.hir().local_def_id_to_hir_id(item_def_id.expect_local());
42 let unsupported = || {
43 // Variance not relevant.
44 span_bug!(tcx.hir().span(id), "asked to compute variance for wrong kind of item")
46 match tcx.hir().get(id) {
47 Node::Item(item) => match item.kind {
48 hir::ItemKind::Enum(..)
49 | hir::ItemKind::Struct(..)
50 | hir::ItemKind::Union(..)
51 | hir::ItemKind::Fn(..) => {}
56 Node::TraitItem(item) => match item.kind {
57 hir::TraitItemKind::Fn(..) => {}
62 Node::ImplItem(item) => match item.kind {
63 hir::ImplItemKind::Fn(..) => {}
68 Node::ForeignItem(item) => match item.kind {
69 hir::ForeignItemKind::Fn(..) => {}
74 Node::Variant(_) | Node::Ctor(..) => {}
79 // Everything else must be inferred.
81 let crate_map = tcx.crate_variances(());
82 crate_map.variances.get(&item_def_id).copied().unwrap_or(&[])