diff options
Diffstat (limited to 'compiler/rustc_trait_selection/src/solve/alias_relate.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/solve/alias_relate.rs | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/alias_relate.rs b/compiler/rustc_trait_selection/src/solve/alias_relate.rs index 73362d82306..c46b4df1125 100644 --- a/compiler/rustc_trait_selection/src/solve/alias_relate.rs +++ b/compiler/rustc_trait_selection/src/solve/alias_relate.rs @@ -1,3 +1,15 @@ +//! Implements the `AliasRelate` goal, which is used to unify two aliases in the +//! new solver, which uses "lazy normalization". +//! +//! This goal, e.g. `A alias-relate B`, may be satisfied by one of three branches: +//! * normalizes-to: If `A` is a projection, we can prove the equivalent +//! projection predicate with B as the right-hand side of the projection. +//! This goal is computed in both directions, if both are aliases. +//! * subst-relate: Equate `A` and `B` by their substs, if they're both +//! aliases with the same def-id. +//! * bidirectional-normalizes-to: If `A` and `B` are both projections, and both +//! may apply, then we can compute the "intersection" of both normalizes-to by +//! performing them together. This is used specifically to resolve ambiguities. use super::{EvalCtxt, SolverMode}; use rustc_infer::traits::query::NoSolution; use rustc_middle::traits::solve::{Certainty, Goal, QueryResult}; @@ -115,6 +127,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> { }) } + // Computes the normalizes-to branch, with side-effects. This must be performed + // in a probe in order to not taint the evaluation context. fn normalizes_to_inner( &mut self, param_env: ty::ParamEnv<'tcx>, @@ -124,9 +138,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> { invert: Invert, ) -> Result<(), NoSolution> { let other = match direction { - // This is purely an optimization. + // This is purely an optimization. No need to instantiate a new + // infer var and equate the RHS to it. ty::AliasRelationDirection::Equate => other, + // Instantiate an infer var and subtype our RHS to it, so that we + // properly represent a subtype relation between the LHS and RHS + // of the goal. ty::AliasRelationDirection::Subtype => { let fresh = self.next_term_infer_of_kind(other); let (sub, sup) = match invert { |