src/librustc_typeck/variance/terms.rs

   1 // Representing terms
   2 //
   3 // Terms are structured as a straightforward tree. Rather than rely on
   4 // GC, we allocate terms out of a bounded arena (the lifetime of this
   5 // arena is the lifetime 'a that is threaded around).
   6 //
   7 // We assign a unique index to each type/region parameter whose variance
   8 // is to be inferred. We refer to such variables as "inferreds". An
   9 // `InferredIndex` is a newtype'd int representing the index of such
  10 // a variable.
  11
  12 use arena::TypedArena;
  13 use rustc_hir as hir;
  14 use rustc_hir::itemlikevisit::ItemLikeVisitor;
  15 use rustc_hir::HirIdMap;
  16 use rustc_middle::ty::{self, TyCtxt};
  17 use std::fmt;
  18
  19 use self::VarianceTerm::*;
  20
  21 pub type VarianceTermPtr<'a> = &'a VarianceTerm<'a>;
  22
  23 #[derive(Copy, Clone, Debug)]
  24 pub struct InferredIndex(pub usize);
  25
  26 #[derive(Copy, Clone)]
  27 pub enum VarianceTerm<'a> {
  28     ConstantTerm(ty::Variance),
  29     TransformTerm(VarianceTermPtr<'a>, VarianceTermPtr<'a>),
  30     InferredTerm(InferredIndex),
  31 }
  32
  33 impl<'a> fmt::Debug for VarianceTerm<'a> {
  34     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  35         match *self {
  36             ConstantTerm(c1) => write!(f, "{:?}", c1),
  37             TransformTerm(v1, v2) => write!(f, "({:?} \u{00D7} {:?})", v1, v2),
  38             InferredTerm(id) => write!(f, "[{}]", {
  39                 let InferredIndex(i) = id;
  40                 i
  41             }),
  42         }
  43     }
  44 }
  45
  46 // The first pass over the crate simply builds up the set of inferreds.
  47
  48 pub struct TermsContext<'a, 'tcx> {
  49     pub tcx: TyCtxt<'tcx>,
  50     pub arena: &'a TypedArena<VarianceTerm<'a>>,
  51
  52     // For marker types, UnsafeCell, and other lang items where
  53     // variance is hardcoded, records the item-id and the hardcoded
  54     // variance.
  55     pub lang_items: Vec<(hir::HirId, Vec<ty::Variance>)>,
  56
  57     // Maps from the node id of an item to the first inferred index
  58     // used for its type & region parameters.
  59     pub inferred_starts: HirIdMap<InferredIndex>,
  60
  61     // Maps from an InferredIndex to the term for that variable.
  62     pub inferred_terms: Vec<VarianceTermPtr<'a>>,
  63 }
  64
  65 pub fn determine_parameters_to_be_inferred<'a, 'tcx>(
  66     tcx: TyCtxt<'tcx>,
  67     arena: &'a mut TypedArena<VarianceTerm<'a>>,
  68 ) -> TermsContext<'a, 'tcx> {
  69     let mut terms_cx = TermsContext {
  70         tcx,
  71         arena,
  72         inferred_starts: Default::default(),
  73         inferred_terms: vec![],
  74
  75         lang_items: lang_items(tcx),
  76     };
  77
  78     // See the following for a discussion on dep-graph management.
  79     //
  80     // - https://rustc-dev-guide.rust-lang.org/query.html
  81     // - https://rustc-dev-guide.rust-lang.org/variance.html
  82     tcx.hir().krate().visit_all_item_likes(&mut terms_cx);
  83
  84     terms_cx
  85 }
  86
  87 fn lang_items(tcx: TyCtxt<'_>) -> Vec<(hir::HirId, Vec<ty::Variance>)> {
  88     let lang_items = tcx.lang_items();
  89     let all = vec![
  90         (lang_items.phantom_data(), vec![ty::Covariant]),
  91         (lang_items.unsafe_cell_type(), vec![ty::Invariant]),
  92     ];
  93
  94     all.into_iter() // iterating over (Option<DefId>, Variance)
  95         .filter(|&(ref d, _)| d.is_some())
  96         .map(|(d, v)| (d.unwrap(), v)) // (DefId, Variance)
  97         .filter_map(|(d, v)| tcx.hir().as_local_hir_id(d).map(|n| (n, v))) // (HirId, Variance)
  98         .collect()
  99 }
 100
 101 impl<'a, 'tcx> TermsContext<'a, 'tcx> {
 102     fn add_inferreds_for_item(&mut self, id: hir::HirId) {
 103         let tcx = self.tcx;
 104         let def_id = tcx.hir().local_def_id(id);
 105         let count = tcx.generics_of(def_id).count();
 106
 107         if count == 0 {
 108             return;
 109         }
 110
 111         // Record the start of this item's inferreds.
 112         let start = self.inferred_terms.len();
 113         let newly_added = self.inferred_starts.insert(id, InferredIndex(start)).is_none();
 114         assert!(newly_added);
 115
 116         // N.B., in the code below for writing the results back into the
 117         // `CrateVariancesMap`, we rely on the fact that all inferreds
 118         // for a particular item are assigned continuous indices.
 119
 120         let arena = self.arena;
 121         self.inferred_terms.extend(
 122             (start..(start + count)).map(|i| &*arena.alloc(InferredTerm(InferredIndex(i)))),
 123         );
 124     }
 125 }
 126
 127 impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for TermsContext<'a, 'tcx> {
 128     fn visit_item(&mut self, item: &hir::Item<'_>) {
 129         debug!("add_inferreds for item {}", self.tcx.hir().node_to_string(item.hir_id));
 130
 131         match item.kind {
 132             hir::ItemKind::Struct(ref struct_def, _) | hir::ItemKind::Union(ref struct_def, _) => {
 133                 self.add_inferreds_for_item(item.hir_id);
 134
 135                 if let hir::VariantData::Tuple(..) = *struct_def {
 136                     self.add_inferreds_for_item(struct_def.ctor_hir_id().unwrap());
 137                 }
 138             }
 139
 140             hir::ItemKind::Enum(ref enum_def, _) => {
 141                 self.add_inferreds_for_item(item.hir_id);
 142
 143                 for variant in enum_def.variants {
 144                     if let hir::VariantData::Tuple(..) = variant.data {
 145                         self.add_inferreds_for_item(variant.data.ctor_hir_id().unwrap());
 146                     }
 147                 }
 148             }
 149
 150             hir::ItemKind::Fn(..) => {
 151                 self.add_inferreds_for_item(item.hir_id);
 152             }
 153
 154             hir::ItemKind::ForeignMod(ref foreign_mod) => {
 155                 for foreign_item in foreign_mod.items {
 156                     if let hir::ForeignItemKind::Fn(..) = foreign_item.kind {
 157                         self.add_inferreds_for_item(foreign_item.hir_id);
 158                     }
 159                 }
 160             }
 161
 162             _ => {}
 163         }
 164     }
 165
 166     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem<'_>) {
 167         if let hir::TraitItemKind::Fn(..) = trait_item.kind {
 168             self.add_inferreds_for_item(trait_item.hir_id);
 169         }
 170     }
 171
 172     fn visit_impl_item(&mut self, impl_item: &hir::ImplItem<'_>) {
 173         if let hir::ImplItemKind::Fn(..) = impl_item.kind {
 174             self.add_inferreds_for_item(impl_item.hir_id);
 175         }
 176     }
 177 }