Add explanatory comments for GenericArg::Infer and TyKind::Infer

Author: voidc

compiler/rustc_hir/src/hir.rs

@@ -290,7 +290,7 @@ pub enum ConstArgKind<'hir> {
     /// However, in the future, we'll be using it for all of those.
     Path(QPath<'hir>),
     Anon(&'hir AnonConst),
-    /// **Note:** Not all const inference variables are represented as
+    /// **Note:** Not all inferred consts are represented as
     /// `ConstArgKind::Infer`. In cases where it is ambiguous whether
     /// a generic arg is a type or a const, inference variables are
     /// represented as `GenericArg::Infer` instead.
@@ -314,6 +314,10 @@ pub enum GenericArg<'hir> {
     Lifetime(&'hir Lifetime),
     Type(&'hir Ty<'hir>),
     Const(&'hir ConstArg<'hir>),
+    /// **Note:** Inference variables are only represented as
+    /// `GenericArg::Infer` in cases where it is ambiguous whether
+    /// a generic arg is a type or a const. Otherwise, inference variables
+    /// are represented as `TyKind::Infer` or `ConstArgKind::Infer`.
     Infer(InferArg),
 }
 
@@ -2844,6 +2848,11 @@ pub enum TyKind<'hir> {
     Typeof(&'hir AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been
     /// specified. This can appear anywhere in a type.
+    ///
+    /// **Note:** Not all inferred types are represented as
+    /// `TyKind::Infer`. In cases where it is ambiguous whether
+    /// a generic arg is a type or a const, inference variables are
+    /// represented as `GenericArg::Infer` instead.
     Infer,
     /// Placeholder for a type that has failed to be defined.
     Err(rustc_span::ErrorGuaranteed),