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|
//! **Canonicalization** is the key to constructing a query in the
//! middle of type inference. Ordinarily, it is not possible to store
//! types from type inference in query keys, because they contain
//! references to inference variables whose lifetimes are too short
//! and so forth. Canonicalizing a value T1 using `canonicalize_query`
//! produces two things:
//!
//! - a value T2 where each unbound inference variable has been
//! replaced with a **canonical variable**;
//! - a map M (of type `CanonicalVarValues`) from those canonical
//! variables back to the original.
//!
//! We can then do queries using T2. These will give back constraints
//! on the canonical variables which can be translated, using the map
//! M, into constraints in our source context. This process of
//! translating the results back is done by the
//! `instantiate_query_result` method.
//!
//! For a more detailed look at what is happening here, check
//! out the [chapter in the rustc dev guide][c].
//!
//! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html
use crate::infer::{ConstVariableOrigin, ConstVariableOriginKind};
use crate::infer::{InferCtxt, RegionVariableOrigin, TypeVariableOrigin, TypeVariableOriginKind};
use rustc_index::vec::IndexVec;
use rustc_middle::ty::fold::TypeFoldable;
use rustc_middle::ty::subst::GenericArg;
use rustc_middle::ty::{self, BoundVar, List};
use rustc_span::source_map::Span;
pub use rustc_middle::infer::canonical::*;
use substitute::CanonicalExt;
mod canonicalizer;
pub mod query_response;
mod substitute;
impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
/// Creates a substitution S for the canonical value with fresh
/// inference variables and applies it to the canonical value.
/// Returns both the instantiated result *and* the substitution S.
///
/// This is only meant to be invoked as part of constructing an
/// inference context at the start of a query (see
/// `InferCtxtBuilder::enter_with_canonical`). It basically
/// brings the canonical value "into scope" within your new infcx.
///
/// At the end of processing, the substitution S (once
/// canonicalized) then represents the values that you computed
/// for each of the canonical inputs to your query.
pub fn instantiate_canonical_with_fresh_inference_vars<T>(
&self,
span: Span,
canonical: &Canonical<'tcx, T>,
) -> (T, CanonicalVarValues<'tcx>)
where
T: TypeFoldable<'tcx>,
{
// For each universe that is referred to in the incoming
// query, create a universe in our local inference context. In
// practice, as of this writing, all queries have no universes
// in them, so this code has no effect, but it is looking
// forward to the day when we *do* want to carry universes
// through into queries.
let universes: IndexVec<ty::UniverseIndex, _> = std::iter::once(ty::UniverseIndex::ROOT)
.chain((0..canonical.max_universe.as_u32()).map(|_| self.create_next_universe()))
.collect();
let canonical_inference_vars =
self.instantiate_canonical_vars(span, canonical.variables, |ui| universes[ui]);
let result = canonical.substitute(self.tcx, &canonical_inference_vars);
(result, canonical_inference_vars)
}
/// Given the "infos" about the canonical variables from some
/// canonical, creates fresh variables with the same
/// characteristics (see `instantiate_canonical_var` for
/// details). You can then use `substitute` to instantiate the
/// canonical variable with these inference variables.
fn instantiate_canonical_vars(
&self,
span: Span,
variables: &List<CanonicalVarInfo<'tcx>>,
universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
) -> CanonicalVarValues<'tcx> {
let var_values: IndexVec<BoundVar, GenericArg<'tcx>> = variables
.iter()
.map(|info| self.instantiate_canonical_var(span, info, &universe_map))
.collect();
CanonicalVarValues { var_values }
}
/// Given the "info" about a canonical variable, creates a fresh
/// variable for it. If this is an existentially quantified
/// variable, then you'll get a new inference variable; if it is a
/// universally quantified variable, you get a placeholder.
fn instantiate_canonical_var(
&self,
span: Span,
cv_info: CanonicalVarInfo<'tcx>,
universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
) -> GenericArg<'tcx> {
match cv_info.kind {
CanonicalVarKind::Ty(ty_kind) => {
let ty = match ty_kind {
CanonicalTyVarKind::General(ui) => self.next_ty_var_in_universe(
TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span },
universe_map(ui),
),
CanonicalTyVarKind::Int => self.next_int_var(),
CanonicalTyVarKind::Float => self.next_float_var(),
};
ty.into()
}
CanonicalVarKind::PlaceholderTy(ty::PlaceholderType { universe, name }) => {
let universe_mapped = universe_map(universe);
let placeholder_mapped = ty::PlaceholderType { universe: universe_mapped, name };
self.tcx.mk_ty(ty::Placeholder(placeholder_mapped)).into()
}
CanonicalVarKind::Region(ui) => self
.next_region_var_in_universe(
RegionVariableOrigin::MiscVariable(span),
universe_map(ui),
)
.into(),
CanonicalVarKind::PlaceholderRegion(ty::PlaceholderRegion { universe, name }) => {
let universe_mapped = universe_map(universe);
let placeholder_mapped = ty::PlaceholderRegion { universe: universe_mapped, name };
self.tcx.mk_region(ty::RePlaceholder(placeholder_mapped)).into()
}
CanonicalVarKind::Const(ui, ty) => self
.next_const_var_in_universe(
ty,
ConstVariableOrigin { kind: ConstVariableOriginKind::MiscVariable, span },
universe_map(ui),
)
.into(),
CanonicalVarKind::PlaceholderConst(ty::PlaceholderConst { universe, name }, ty) => {
let universe_mapped = universe_map(universe);
let placeholder_mapped = ty::PlaceholderConst { universe: universe_mapped, name };
self.tcx
.mk_const(ty::ConstS {
kind: ty::ConstKind::Placeholder(placeholder_mapped),
ty,
})
.into()
}
}
}
}
|
