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Richard Smith
9feff92f22
Change explorer's handling of witnesses to track them wherever possible. Associated constant support needs witnesses to be available much more pervasively. The main changes here are: - `Substitute` now takes a set of bindings covering both `Value`s for `GenericBinding`s and `Witness`es for `ImplBinding`s, and substitutes both. - Witnesses are now tracked within `ConstraintType`s. The `.Self` type has an `ImplBinding` that self-references from parts of the constraint to other parts of the constraint can use to access the witness for the constraint. - The constraint type for an `impl` and for a `GenericBinding` are now stored in pre-substituted form, with references to the actual constrained type rather than symbolic references to `.Self`. This results in minor changes in diagnostic text. Argument deduction still performs substitutions without remapping witnesses.