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-rw-r--r-- | compiler/rustc_infer/src/infer/lattice.rs | 124 |
1 files changed, 124 insertions, 0 deletions
diff --git a/compiler/rustc_infer/src/infer/lattice.rs b/compiler/rustc_infer/src/infer/lattice.rs new file mode 100644 index 000000000..1e3293efa --- /dev/null +++ b/compiler/rustc_infer/src/infer/lattice.rs @@ -0,0 +1,124 @@ +//! # 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 super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind}; +use super::InferCtxt; + +use crate::traits::{ObligationCause, PredicateObligation}; +use rustc_middle::ty::relate::{RelateResult, TypeRelation}; +use rustc_middle::ty::TyVar; +use rustc_middle::ty::{self, Ty}; + +/// Trait for returning data about a lattice, and for abstracting +/// over the "direction" of the lattice operation (LUB/GLB). +/// +/// GLB moves "down" the lattice (to smaller values); LUB moves +/// "up" the lattice (to bigger values). +pub trait LatticeDir<'f, 'tcx>: TypeRelation<'tcx> { + fn infcx(&self) -> &'f InferCtxt<'f, 'tcx>; + + fn cause(&self) -> &ObligationCause<'tcx>; + + fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>); + + fn define_opaque_types(&self) -> bool; + + // 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<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>; +} + +/// Relates two types using a given lattice. +#[instrument(skip(this), level = "debug")] +pub fn super_lattice_tys<'a, 'tcx: 'a, L>( + this: &mut L, + a: Ty<'tcx>, + b: Ty<'tcx>, +) -> RelateResult<'tcx, Ty<'tcx>> +where + L: LatticeDir<'a, 'tcx>, +{ + debug!("{}", this.tag()); + + if a == b { + return Ok(a); + } + + let infcx = this.infcx(); + + let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a); + let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(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 + (&ty::Infer(TyVar(..)), _) => { + let v = infcx.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::LatticeVariable, + span: this.cause().span, + }); + this.relate_bound(v, b, a)?; + Ok(v) + } + (_, &ty::Infer(TyVar(..))) => { + let v = infcx.next_ty_var(TypeVariableOrigin { + kind: TypeVariableOriginKind::LatticeVariable, + span: this.cause().span, + }); + this.relate_bound(v, a, b)?; + Ok(v) + } + + (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => { + infcx.super_combine_tys(this, a, b) + } + (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..)) + if this.define_opaque_types() && did.is_local() => + { + this.add_obligations( + infcx + .handle_opaque_type(a, b, this.a_is_expected(), this.cause(), this.param_env())? + .obligations, + ); + Ok(a) + } + + _ => infcx.super_combine_tys(this, a, b), + } +} |