src/librustc_typeck/variance/mod.rs

   1 //! Module for inferring the variance of type and lifetime parameters. See the [rustc dev guide]
   2 //! chapter for more info.
   3 //!
   4 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/variance.html
   5
   6 use hir::Node;
   7 use rustc_arena::TypedArena;
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
  10 use rustc_middle::ty::query::Providers;
  11 use rustc_middle::ty::{self, CrateVariancesMap, TyCtxt};
  12
  13 /// Defines the `TermsContext` basically houses an arena where we can
  14 /// allocate terms.
  15 mod terms;
  16
  17 /// Code to gather up constraints.
  18 mod constraints;
  19
  20 /// Code to solve constraints and write out the results.
  21 mod solve;
  22
  23 /// Code to write unit tests of variance.
  24 pub mod test;
  25
  26 /// Code for transforming variances.
  27 mod xform;
  28
  29 pub fn provide(providers: &mut Providers) {
  30     *providers = Providers { variances_of, crate_variances, ..*providers };
  31 }
  32
  33 fn crate_variances(tcx: TyCtxt<'_>, crate_num: CrateNum) -> CrateVariancesMap<'_> {
  34     assert_eq!(crate_num, LOCAL_CRATE);
  35     let mut arena = TypedArena::default();
  36     let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &mut arena);
  37     let constraints_cx = constraints::add_constraints_from_crate(terms_cx);
  38     solve::solve_constraints(constraints_cx)
  39 }
  40
  41 fn variances_of(tcx: TyCtxt<'_>, item_def_id: DefId) -> &[ty::Variance] {
  42     let id = tcx.hir().local_def_id_to_hir_id(item_def_id.expect_local());
  43     let unsupported = || {
  44         // Variance not relevant.
  45         span_bug!(tcx.hir().span(id), "asked to compute variance for wrong kind of item")
  46     };
  47     match tcx.hir().get(id) {
  48         Node::Item(item) => match item.kind {
  49             hir::ItemKind::Enum(..)
  50             | hir::ItemKind::Struct(..)
  51             | hir::ItemKind::Union(..)
  52             | hir::ItemKind::Fn(..) => {}
  53
  54             _ => unsupported(),
  55         },
  56
  57         Node::TraitItem(item) => match item.kind {
  58             hir::TraitItemKind::Fn(..) => {}
  59
  60             _ => unsupported(),
  61         },
  62
  63         Node::ImplItem(item) => match item.kind {
  64             hir::ImplItemKind::Fn(..) => {}
  65
  66             _ => unsupported(),
  67         },
  68
  69         Node::ForeignItem(item) => match item.kind {
  70             hir::ForeignItemKind::Fn(..) => {}
  71
  72             _ => unsupported(),
  73         },
  74
  75         Node::Variant(_) | Node::Ctor(..) => {}
  76
  77         _ => unsupported(),
  78     }
  79
  80     // Everything else must be inferred.
  81
  82     let crate_map = tcx.crate_variances(LOCAL_CRATE);
  83     crate_map.variances.get(&item_def_id).copied().unwrap_or(&[])
  84 }