Document Ambig vs Unambig Type/Consts

src/SUMMARY.md

@@ -121,8 +121,9 @@
     - [Feature gate checking](./feature-gate-ck.md)
     - [Lang Items](./lang-items.md)
 - [The HIR (High-level IR)](./hir.md)
-    - [Lowering AST to HIR](./ast-lowering.md)
-    - [Debugging](./hir-debugging.md)
+    - [Lowering AST to HIR](./hir/lowering.md)
+    - [Ambig/Unambig Types and Consts](./hir/ambig-unambig-ty-and-consts.md)
+    - [Debugging](./hir/debugging.md)
 - [The THIR (Typed High-level IR)](./thir.md)
 - [The MIR (Mid-level IR)](./mir/index.md)
     - [MIR construction](./mir/construction.md)

src/diagnostics.md

@@ -553,7 +553,7 @@ compiler](#linting-early-in-the-compiler).
 
 
 [AST nodes]: the-parser.md
-[AST lowering]: ast-lowering.md
+[AST lowering]: ./hir/lowering.md
 [HIR nodes]: hir.md
 [MIR nodes]: mir/index.md
 [macro expansion]: macro-expansion.md

src/hir.md

@@ -5,7 +5,7 @@
 The HIR – "High-Level Intermediate Representation" – is the primary IR used
 in most of rustc. It is a compiler-friendly representation of the abstract
 syntax tree (AST) that is generated after parsing, macro expansion, and name
-resolution (see [Lowering](./ast-lowering.html) for how the HIR is created).
+resolution (see [Lowering](./hir/lowering.md) for how the HIR is created).
 Many parts of HIR resemble Rust surface syntax quite closely, with
 the exception that some of Rust's expression forms have been desugared away.
 For example, `for` loops are converted into a `loop` and do not appear in

src/hir/ambig-unambig-ty-and-consts.md

@@ -0,0 +1,63 @@
+# 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
+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<T, const N: usize,>(arg: T) {
+    //                           ^ Unambig type position
+    let a: _ = arg; 
+    //     ^ Unambig type position
+
+    func::<T, N>(arg);
+    //     ^  ^
+    //     ^^^^ Ambig position 
+
+    let _: [u8; 10];
+    //      ^^  ^^ Unambig const position
+    //      ^^ Unambig type position
+}
+
+```
+
+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.
+
+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]
+
+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.
+- People may write visitors which check for `hir::TyKind/ConstArgKind::Infer` but forget to check for `GenericArg::Infer`, only handling infers in unambig positions by accident.
+- People may write visitors which check for `GenerArg::Type/Const(TyKind/ConstArgKind::Infer)` and `GenerigArg::Infer`, not realising that we never represent inferred types/consts in ambig positions as a `GenericArg::Type/Const`.
+- People may write visitors which check for *only* `TyKind::Infer` and not `ConstArgKind::Infer` forgetting that there are also inferred const arguments (and vice versa).
+
+To make writing HIR visitors less error prone when caring about inferred types/consts we have a relatively complex system:
+
+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.
+
+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.
+
+This has a number of benefits:
+- It's clear that `GenericArg::Type/Const` cannot represent inferred type/const arguments
+- Implementors of `visit_ty` and `visit_const_arg` will never encounter inferred types/consts making it impossible to write a visitor that seems to work right but handles edge cases wrong 
+- The `visit_infer` method handles *all* cases of inferred type/consts in the HIR making it easy for visitors to handle inferred type/consts in one dedicated place and not forget cases
+
+[ty_infer]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.TyKind.html#variant.Infer
+[const_infer]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.ConstArgKind.html#variant.Infer
+[generic_arg_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.GenericArg.html#variant.Type
+[generic_arg_const]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.GenericArg.html#variant.Const
+[generic_arg_infer]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.GenericArg.html#variant.Infer
+[ambig_arg]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.AmbigArg.html
+[visit_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/intravisit/trait.Visitor.html#method.visit_ty
+[visit_const_arg]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/intravisit/trait.Visitor.html#method.visit_const_arg
+[visit_infer]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/intravisit/trait.Visitor.html#method.visit_infer

src/ast-lowering.md → src/hir/lowering.md

@@ -1,6 +1,6 @@
 # AST lowering
 
-The AST lowering step converts AST to [HIR](hir.html).
+The AST lowering step converts AST to [HIR](../hir.md).
 This means many structures are removed if they are irrelevant
 for type analysis or similar syntax agnostic analyses. Examples
 of such structures include but are not limited to

src/overview.md

@@ -410,7 +410,7 @@ For more details on bootstrapping, see
   - Guide: [The HIR](hir.md)
   - Guide: [Identifiers in the HIR](hir.md#identifiers-in-the-hir)
   - Guide: [The `HIR` Map](hir.md#the-hir-map)
-  - Guide: [Lowering `AST` to `HIR`](ast-lowering.md)
+  - Guide: [Lowering `AST` to `HIR`](./hir/lowering.md)
   - How to view `HIR` representation for your code `cargo rustc -- -Z unpretty=hir-tree`
   - Rustc `HIR` definition: [`rustc_hir`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html)
   - Main entry point: **TODO**