1 //! **Canonicalization** is the key to constructing a query in the
2 //! middle of type inference. Ordinarily, it is not possible to store
3 //! types from type inference in query keys, because they contain
4 //! references to inference variables whose lifetimes are too short
5 //! and so forth. Canonicalizing a value T1 using `canonicalize_query`
6 //! produces two things:
8 //! - a value T2 where each unbound inference variable has been
9 //! replaced with a **canonical variable**;
10 //! - a map M (of type `CanonicalVarValues`) from those canonical
11 //! variables back to the original.
13 //! We can then do queries using T2. These will give back constraints
14 //! on the canonical variables which can be translated, using the map
15 //! M, into constraints in our source context. This process of
16 //! translating the results back is done by the
17 //! `instantiate_query_result` method.
19 //! For a more detailed look at what is happening here, check
20 //! out the [chapter in the rustc dev guide][c].
22 //! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html
24 use crate::infer::{ConstVariableOrigin, ConstVariableOriginKind};
25 use crate::infer::{InferCtxt, RegionVariableOrigin, TypeVariableOrigin, TypeVariableOriginKind};
26 use rustc_index::vec::IndexVec;
27 use rustc_middle::ty::fold::TypeFoldable;
28 use rustc_middle::ty::subst::GenericArg;
29 use rustc_middle::ty::{self, List};
30 use rustc_span::source_map::Span;
32 pub use rustc_middle::infer::canonical::*;
33 use substitute::CanonicalExt;
36 pub mod query_response;
39 impl<'tcx> InferCtxt<'tcx> {
40 /// Creates a substitution S for the canonical value with fresh
41 /// inference variables and applies it to the canonical value.
42 /// Returns both the instantiated result *and* the substitution S.
44 /// This can be invoked as part of constructing an
45 /// inference context at the start of a query (see
46 /// `InferCtxtBuilder::build_with_canonical`). It basically
47 /// brings the canonical value "into scope" within your new infcx.
49 /// At the end of processing, the substitution S (once
50 /// canonicalized) then represents the values that you computed
51 /// for each of the canonical inputs to your query.
52 pub fn instantiate_canonical_with_fresh_inference_vars<T>(
55 canonical: &Canonical<'tcx, T>,
56 ) -> (T, CanonicalVarValues<'tcx>)
58 T: TypeFoldable<'tcx>,
60 // For each universe that is referred to in the incoming
61 // query, create a universe in our local inference context. In
62 // practice, as of this writing, all queries have no universes
63 // in them, so this code has no effect, but it is looking
64 // forward to the day when we *do* want to carry universes
65 // through into queries.
67 // Instantiate the root-universe content into the current universe,
68 // and create fresh universes for the higher universes.
69 let universes: IndexVec<ty::UniverseIndex, _> = std::iter::once(self.universe())
70 .chain((1..=canonical.max_universe.as_u32()).map(|_| self.create_next_universe()))
73 let canonical_inference_vars =
74 self.instantiate_canonical_vars(span, canonical.variables, |ui| universes[ui]);
75 let result = canonical.substitute(self.tcx, &canonical_inference_vars);
76 (result, canonical_inference_vars)
79 /// Given the "infos" about the canonical variables from some
80 /// canonical, creates fresh variables with the same
81 /// characteristics (see `instantiate_canonical_var` for
82 /// details). You can then use `substitute` to instantiate the
83 /// canonical variable with these inference variables.
84 fn instantiate_canonical_vars(
87 variables: &List<CanonicalVarInfo<'tcx>>,
88 universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
89 ) -> CanonicalVarValues<'tcx> {
91 var_values: self.tcx.mk_substs(
94 .map(|info| self.instantiate_canonical_var(span, info, &universe_map)),
99 /// Given the "info" about a canonical variable, creates a fresh
100 /// variable for it. If this is an existentially quantified
101 /// variable, then you'll get a new inference variable; if it is a
102 /// universally quantified variable, you get a placeholder.
103 fn instantiate_canonical_var(
106 cv_info: CanonicalVarInfo<'tcx>,
107 universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
108 ) -> GenericArg<'tcx> {
110 CanonicalVarKind::Ty(ty_kind) => {
111 let ty = match ty_kind {
112 CanonicalTyVarKind::General(ui) => self.next_ty_var_in_universe(
113 TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span },
117 CanonicalTyVarKind::Int => self.next_int_var(),
119 CanonicalTyVarKind::Float => self.next_float_var(),
124 CanonicalVarKind::PlaceholderTy(ty::PlaceholderType { universe, name }) => {
125 let universe_mapped = universe_map(universe);
126 let placeholder_mapped = ty::PlaceholderType { universe: universe_mapped, name };
127 self.tcx.mk_ty(ty::Placeholder(placeholder_mapped)).into()
130 CanonicalVarKind::Region(ui) => self
131 .next_region_var_in_universe(
132 RegionVariableOrigin::MiscVariable(span),
137 CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion { universe, name }) => {
138 let universe_mapped = universe_map(universe);
139 let placeholder_mapped = ty::PlaceholderRegion { universe: universe_mapped, name };
140 self.tcx.mk_region(ty::RePlaceholder(placeholder_mapped)).into()
143 CanonicalVarKind::Const(ui, ty) => self
144 .next_const_var_in_universe(
146 ConstVariableOrigin { kind: ConstVariableOriginKind::MiscVariable, span },
151 CanonicalVarKind::PlaceholderConst(ty::PlaceholderConst { universe, name }, ty) => {
152 let universe_mapped = universe_map(universe);
153 let placeholder_mapped = ty::PlaceholderConst { universe: universe_mapped, name };
154 self.tcx.mk_const(placeholder_mapped, ty).into()