Unnamed repository; edit this file 'description' to name the repository.
Diffstat (limited to 'crates/hir-ty/src/next_solver/infer/relate/lattice.rs')
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diff --git a/crates/hir-ty/src/next_solver/infer/relate/lattice.rs b/crates/hir-ty/src/next_solver/infer/relate/lattice.rs new file mode 100644 index 0000000000..c7f771ffe3 --- /dev/null +++ b/crates/hir-ty/src/next_solver/infer/relate/lattice.rs @@ -0,0 +1,269 @@ +//! # Lattice variables +//! +//! Generic code for operating on [lattices] of inference variables +//! that are characterized by an upper- and lower-bound. +//! +//! The code is defined quite generically so that it can be +//! applied both to type variables, which represent types being inferred, +//! and fn variables, which represent function types being inferred. +//! (It may eventually be applied to their types as well.) +//! In some cases, the functions are also generic with respect to the +//! operation on the lattice (GLB vs LUB). +//! +//! ## Note +//! +//! Although all the functions are generic, for simplicity, comments in the source code +//! generally refer to type variables and the LUB operation. +//! +//! [lattices]: https://en.wikipedia.org/wiki/Lattice_(order) + +use rustc_type_ir::{ + AliasRelationDirection, TypeVisitableExt, Upcast, Variance, + inherent::{IntoKind, Span as _}, + relate::{ + Relate, StructurallyRelateAliases, TypeRelation, VarianceDiagInfo, + combine::{PredicateEmittingRelation, super_combine_consts, super_combine_tys}, + }, +}; + +use crate::next_solver::{ + AliasTy, Binder, Const, DbInterner, Goal, ParamEnv, Predicate, PredicateKind, Region, Span, Ty, + TyKind, + infer::{ + DefineOpaqueTypes, InferCtxt, TypeTrace, + relate::RelateResult, + traits::{Obligation, PredicateObligations}, + }, +}; + +#[derive(Clone, Copy)] +pub(crate) enum LatticeOpKind { + Glb, + Lub, +} + +impl LatticeOpKind { + fn invert(self) -> Self { + match self { + LatticeOpKind::Glb => LatticeOpKind::Lub, + LatticeOpKind::Lub => LatticeOpKind::Glb, + } + } +} + +/// A greatest lower bound" (common subtype) or least upper bound (common supertype). +pub(crate) struct LatticeOp<'infcx, 'db> { + infcx: &'infcx InferCtxt<'db>, + // Immutable fields + trace: TypeTrace<'db>, + param_env: ParamEnv<'db>, + // Mutable fields + kind: LatticeOpKind, + obligations: PredicateObligations<'db>, +} + +impl<'infcx, 'db> LatticeOp<'infcx, 'db> { + pub(crate) fn new( + infcx: &'infcx InferCtxt<'db>, + trace: TypeTrace<'db>, + param_env: ParamEnv<'db>, + kind: LatticeOpKind, + ) -> LatticeOp<'infcx, 'db> { + LatticeOp { infcx, trace, param_env, kind, obligations: PredicateObligations::new() } + } + + pub(crate) fn into_obligations(self) -> PredicateObligations<'db> { + self.obligations + } +} + +impl<'db> TypeRelation<DbInterner<'db>> for LatticeOp<'_, 'db> { + fn cx(&self) -> DbInterner<'db> { + self.infcx.interner + } + + fn relate_with_variance<T: Relate<DbInterner<'db>>>( + &mut self, + variance: Variance, + _info: VarianceDiagInfo<DbInterner<'db>>, + a: T, + b: T, + ) -> RelateResult<'db, T> { + match variance { + Variance::Invariant => { + self.obligations.extend( + self.infcx + .at(&self.trace.cause, self.param_env) + .eq_trace(DefineOpaqueTypes::Yes, self.trace.clone(), a, b)? + .into_obligations(), + ); + Ok(a) + } + Variance::Covariant => self.relate(a, b), + // FIXME(#41044) -- not correct, need test + Variance::Bivariant => Ok(a), + Variance::Contravariant => { + self.kind = self.kind.invert(); + let res = self.relate(a, b); + self.kind = self.kind.invert(); + res + } + } + } + + /// Relates two types using a given lattice. + fn tys(&mut self, a: Ty<'db>, b: Ty<'db>) -> RelateResult<'db, Ty<'db>> { + if a == b { + return Ok(a); + } + + let infcx = self.infcx; + + let a = infcx.shallow_resolve(a); + let b = infcx.shallow_resolve(b); + + match (a.kind(), b.kind()) { + // If one side is known to be a variable and one is not, + // create a variable (`v`) to represent the LUB. Make sure to + // relate `v` to the non-type-variable first (by passing it + // first to `relate_bound`). Otherwise, we would produce a + // subtype obligation that must then be processed. + // + // Example: if the LHS is a type variable, and RHS is + // `Box<i32>`, then we current compare `v` to the RHS first, + // which will instantiate `v` with `Box<i32>`. Then when `v` + // is compared to the LHS, we instantiate LHS with `Box<i32>`. + // But if we did in reverse order, we would create a `v <: + // LHS` (or vice versa) constraint and then instantiate + // `v`. This would require further processing to achieve same + // end-result; in particular, this screws up some of the logic + // in coercion, which expects LUB to figure out that the LHS + // is (e.g.) `Box<i32>`. A more obvious solution might be to + // iterate on the subtype obligations that are returned, but I + // think this suffices. -nmatsakis + (TyKind::Infer(rustc_type_ir::TyVar(..)), _) => { + let v = infcx.next_ty_var(); + self.relate_bound(v, b, a)?; + Ok(v) + } + (_, TyKind::Infer(rustc_type_ir::TyVar(..))) => { + let v = infcx.next_ty_var(); + self.relate_bound(v, a, b)?; + Ok(v) + } + + ( + TyKind::Alias(rustc_type_ir::Opaque, AliasTy { def_id: a_def_id, .. }), + TyKind::Alias(rustc_type_ir::Opaque, AliasTy { def_id: b_def_id, .. }), + ) if a_def_id == b_def_id => super_combine_tys(infcx, self, a, b), + + _ => super_combine_tys(infcx, self, a, b), + } + } + + fn regions(&mut self, a: Region<'db>, b: Region<'db>) -> RelateResult<'db, Region<'db>> { + let mut inner = self.infcx.inner.borrow_mut(); + let mut constraints = inner.unwrap_region_constraints(); + Ok(match self.kind { + // GLB(&'static u8, &'a u8) == &RegionLUB('static, 'a) u8 == &'static u8 + LatticeOpKind::Glb => constraints.lub_regions(self.cx(), a, b), + + // LUB(&'static u8, &'a u8) == &RegionGLB('static, 'a) u8 == &'a u8 + LatticeOpKind::Lub => constraints.glb_regions(self.cx(), a, b), + }) + } + + fn consts(&mut self, a: Const<'db>, b: Const<'db>) -> RelateResult<'db, Const<'db>> { + super_combine_consts(self.infcx, self, a, b) + } + + fn binders<T>( + &mut self, + a: Binder<'db, T>, + b: Binder<'db, T>, + ) -> RelateResult<'db, Binder<'db, T>> + where + T: Relate<DbInterner<'db>>, + { + // GLB/LUB of a binder and itself is just itself + if a == b { + return Ok(a); + } + + if a.skip_binder().has_escaping_bound_vars() || b.skip_binder().has_escaping_bound_vars() { + // When higher-ranked types are involved, computing the GLB/LUB is + // very challenging, switch to invariance. This is obviously + // overly conservative but works ok in practice. + self.relate_with_variance(Variance::Invariant, VarianceDiagInfo::default(), a, b)?; + Ok(a) + } else { + Ok(Binder::dummy(self.relate(a.skip_binder(), b.skip_binder())?)) + } + } +} + +impl<'infcx, 'db> LatticeOp<'infcx, 'db> { + // Relates the type `v` to `a` and `b` such that `v` represents + // the LUB/GLB of `a` and `b` as appropriate. + // + // Subtle hack: ordering *may* be significant here. This method + // relates `v` to `a` first, which may help us to avoid unnecessary + // type variable obligations. See caller for details. + fn relate_bound(&mut self, v: Ty<'db>, a: Ty<'db>, b: Ty<'db>) -> RelateResult<'db, ()> { + let at = self.infcx.at(&self.trace.cause, self.param_env); + match self.kind { + LatticeOpKind::Glb => { + self.obligations.extend(at.sub(DefineOpaqueTypes::Yes, v, a)?.into_obligations()); + self.obligations.extend(at.sub(DefineOpaqueTypes::Yes, v, b)?.into_obligations()); + } + LatticeOpKind::Lub => { + self.obligations.extend(at.sub(DefineOpaqueTypes::Yes, a, v)?.into_obligations()); + self.obligations.extend(at.sub(DefineOpaqueTypes::Yes, b, v)?.into_obligations()); + } + } + Ok(()) + } +} + +impl<'db> PredicateEmittingRelation<InferCtxt<'db>> for LatticeOp<'_, 'db> { + fn span(&self) -> Span { + Span::dummy() + } + + fn structurally_relate_aliases(&self) -> StructurallyRelateAliases { + StructurallyRelateAliases::No + } + + fn param_env(&self) -> ParamEnv<'db> { + self.param_env + } + + fn register_predicates( + &mut self, + preds: impl IntoIterator<Item: Upcast<DbInterner<'db>, Predicate<'db>>>, + ) { + self.obligations.extend(preds.into_iter().map(|pred| { + Obligation::new(self.infcx.interner, self.trace.cause.clone(), self.param_env, pred) + })) + } + + fn register_goals(&mut self, goals: impl IntoIterator<Item = Goal<'db, Predicate<'db>>>) { + self.obligations.extend(goals.into_iter().map(|goal| { + Obligation::new( + self.infcx.interner, + self.trace.cause.clone(), + goal.param_env, + goal.predicate, + ) + })) + } + + fn register_alias_relate_predicate(&mut self, a: Ty<'db>, b: Ty<'db>) { + self.register_predicates([Binder::dummy(PredicateKind::AliasRelate( + a.into(), + b.into(), + // FIXME(deferred_projection_equality): This isn't right, I think? + AliasRelationDirection::Equate, + ))]); + } +} |