Document Ambig vs Unambig Type/Consts #2474
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nnethercote
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Jun 17, 2025
| parse. | ||
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| ```rust | ||
| fn func<T, const N: usize,>(arg: T) { |
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| fn func<T, const N: usize,>(arg: T) { | |
| fn func<T, const N: usize>(arg: T) { |
nnethercote
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Jun 17, 2025
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| 1. We have different types in the compiler for when a type or const is in an unambig or ambig position, `hir::Ty<AmbigArg>` 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. | ||
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| 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. |
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The visit_infer link doesn't match the visit_ty text.
nnethercote
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Jun 17, 2025
| - 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] |
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Maybe clarify here that points 3 and 4 are the cases where there was enough other information around to disambiguate into either a type or a const?
lcnr
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Jun 18, 2025
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| ``` | ||
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| Most types/consts in ambig positions are able to be disambiguated as either a type or const during either parsing or ast-lowering. |
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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 in AST to HIR lowering.
The only generic arguments which remain ambiguous after lowering are inferred generic arguments -_- in path segments.
I think it's worth it to explicitly call out ambiguous single segment paths here.
lcnr
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Jun 18, 2025
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| 1. We have different types in the compiler for when a type or const is in an unambig or ambig position, `hir::Ty<AmbigArg>` 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. | ||
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| 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. |
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Suggested change
| 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. |
lcnr
reviewed
Jun 18, 2025
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