diff --git a/src/hir/ambig-unambig-ty-and-consts.md b/src/hir/ambig-unambig-ty-and-consts.md index a43ad8b4..70902788 100644 --- a/src/hir/ambig-unambig-ty-and-consts.md +++ b/src/hir/ambig-unambig-ty-and-consts.md @@ -1,11 +1,11 @@ # Ambig/Unambig Types and Consts -Types and Consts args in the HIR can be in two kinds of positions "ambig" or "unambig". Ambig positions are where +Types and Consts args in the HIR can be in two kinds of positions ambiguous (ambig) or unambiguous (unambig). Ambig positions are where it would be valid to parse either a type or a const, unambig positions are where only one kind would be valid to parse. ```rust -fn func(arg: T) { +fn func(arg: T) { // ^ Unambig type position let a: _ = arg; // ^ Unambig type position @@ -21,19 +21,19 @@ fn func(arg: T) { ``` -Most types/consts in ambig positions are able to be disambiguated as either a type or const during either parsing or ast-lowering. -Currently the only exception to this is inferred generic arguments in path segments. In `Foo<_>` it is not clear whether the `_` argument is an -inferred type argument, or an inferred const argument. +Most types/consts in ambig positions are able to be disambiguated as either a type or const during parsing. Single segment paths are always represented as types in the AST but may get resolved to a const parameter during name resolution, then lowered to a const argument during ast-lowering. The only generic arguments which remain ambiguous after lowering are inferred generic arguments (`_`) in path segments. For example, in `Foo<_>` it is not clear whether the `_` argument is an inferred type argument, or an inferred const argument. In unambig positions, inferred arguments are represented with [`hir::TyKind::Infer`][ty_infer] or [`hir::ConstArgKind::Infer`][const_infer] depending on whether it is a type or const position respectively. In ambig positions, inferred arguments are represented with `hir::GenericArg::Infer`. -A naive implementation of this structure would result in there being potentially 5 places where an inferred type/const could be found in the HIR if you just looked at the types: -- In unambig type position as a `hir::TyKind::Infer` -- In unambig const arg position as a `hir::ConstArgKind::Infer` -- In an ambig position as a [`GenericArg::Type(TyKind::Infer)`][generic_arg_ty] -- In an ambig position as a [`GenericArg::Const(ConstArgKind::Infer)`][generic_arg_const] -- In an ambig position as a [`GenericArg::Infer`][generic_arg_infer] +A naive implementation of this would result in there being potentially 5 places where you might think an inferred type/const could be found in the HIR from looking at the structure of the HIR: +1. In unambig type position as a `hir::TyKind::Infer` +2. In unambig const arg position as a `hir::ConstArgKind::Infer` +3. In an ambig position as a [`GenericArg::Type(TyKind::Infer)`][generic_arg_ty] +4. In an ambig position as a [`GenericArg::Const(ConstArgKind::Infer)`][generic_arg_const] +5. In an ambig position as a [`GenericArg::Infer`][generic_arg_infer] + +Note that places 3 and 4 would never actually be possible to encounter as we always lower to `GenericArg::Infer` in generic arg position. This has a few failure modes: - People may write visitors which check for `GenericArg::Infer` but forget to check for `hir::TyKind/ConstArgKind::Infer`, only handling infers in ambig positions by accident. @@ -45,7 +45,7 @@ To make writing HIR visitors less error prone when caring about inferred types/c 1. We have different types in the compiler for when a type or const is in an unambig or ambig position, `hir::Ty` and `hir::Ty<()>`. [`AmbigArg`][ambig_arg] is an uninhabited type which we use in the `Infer` variant of `TyKind` and `ConstArgKind` to selectively "disable" it if we are in an ambig position. -2. The [`visit_ty`][visit_infer] and [`visit_const_arg`][visit_const_arg] methods on HIR visitors only accept the ambig position versions of types/consts. Unambig types/consts are implicitly converted to ambig types/consts during the visiting process, with the `Infer` variant handled by a dedicated [`visit_infer`][visit_infer] method. +2. The [`visit_ty`][visit_ty] and [`visit_const_arg`][visit_const_arg] methods on HIR visitors only accept the ambig position versions of types/consts. Unambig types/consts are implicitly converted to ambig types/consts during the visiting process, with the `Infer` variant handled by a dedicated [`visit_infer`][visit_infer] method. This has a number of benefits: - It's clear that `GenericArg::Type/Const` cannot represent inferred type/const arguments