Merge pull request rust-lang#2379 from rust-lang/tshepang-which-chapter

clean TypeFold* chapter

Author: tshepang

src/doc/rustc-dev-guide/src/ty-fold.md

@@ -1,26 +1,28 @@
+<!-- date-check: may 2024 -->
 # `TypeFoldable` and `TypeFolder`
 
-In the previous chapter we discussed instantiating binders. This must involves looking at everything inside of a `Early/Binder`
-to find any usages of the bound vars in order to replace them. Binders can wrap an arbitrary rust type `T` not just a `Ty` so
-how do we implement the `instantiate` methods on the `Early/Binder` types.
+In [a previous chapter], we discussed instantiating binders.
+This involves looking at everything inside of a `Early(Binder)`
+to find any usages of the bound vars in order to replace them.
+Binders can wrap an arbitrary Rust type `T`, not just a `Ty`.
+So, how do we implement the `instantiate` methods on the `Early/Binder` types?
 
 The answer is a couple of traits:
-[`TypeFoldable`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/fold/trait.TypeFoldable.html)
+[`TypeFoldable`]
 and
-[`TypeFolder`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/fold/trait.TypeFolder.html).
+[`TypeFolder`].
 
 - `TypeFoldable` is implemented by types that embed type information. It allows you to recursively
   process the contents of the `TypeFoldable` and do stuff to them.
 - `TypeFolder` defines what you want to do with the types you encounter while processing the
   `TypeFoldable`.
 
-For example, the `TypeFolder` trait has a method
-[`fold_ty`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/fold/trait.TypeFolder.html#method.fold_ty)
-that takes a type as input and returns a new type as a result. `TypeFoldable` invokes the
-`TypeFolder` `fold_foo` methods on itself, giving the `TypeFolder` access to its contents (the
-types, regions, etc that are contained within).
+For example, the `TypeFolder` trait has a method [`fold_ty`]
+that takes a type as input and returns a new type as a result.
+`TypeFoldable` invokes the `TypeFolder` `fold_foo` methods on itself,
+giving the `TypeFolder` access to its contents (the types, regions, etc that are contained within).
 
-You can think of it with this analogy to the iterator combinators we have come to love in rust:
+You can think of it with this analogy to the iterator combinators we have come to love in Rust:
 
 ```rust,ignore
 vec.iter().map(|e1| foo(e2)).collect()
@@ -33,8 +35,7 @@ So to reiterate:
 - `TypeFolder`  is a trait that defines a “map” operation.
 - `TypeFoldable`  is a trait that is implemented by things that embed types.
 
-In the case of `subst`, we can see that it is implemented as a `TypeFolder`:
-[`ArgFolder`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/binder/struct.ArgFolder.html).
+In the case of `subst`, we can see that it is implemented as a `TypeFolder`: [`ArgFolder`].
 Looking at its implementation, we see where the actual substitutions are happening.
 
 However, you might also notice that the implementation calls this `super_fold_with` method. What is
@@ -88,17 +89,22 @@ things. We only want to do something when we reach a type. That means there may
 `TypeFoldable` types whose implementations basically just forward to their fields’ `TypeFoldable`
 implementations. Such implementations of `TypeFoldable` tend to be pretty tedious to write by hand.
 For this reason, there is a `derive` macro that allows you to `#![derive(TypeFoldable)]`. It is
-defined
-[here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_macros/src/type_foldable.rs).
-
-**`subst`** In the case of substitutions the [actual
-folder](https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L440-L451)
-is going to be doing the indexing we’ve already mentioned. There we define a `Folder` and call
-`fold_with` on the `TypeFoldable` to process yourself.  Then
-[fold_ty](https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L512-L536)
-the method that process each type it looks for a `ty::Param` and for those it replaces it for
-something from the list of substitutions, otherwise recursively process the type.  To replace it,
-calls
-[ty_for_param](https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L552-L587)
+defined [here].
+
+**`subst`** In the case of substitutions the [actual folder]
+is going to be doing the indexing we’ve already mentioned.
+There we define a `Folder` and call `fold_with` on the `TypeFoldable` to process yourself.
+Then [fold_ty] the method that process each type it looks for a `ty::Param` and for those
+it replaces it for something from the list of substitutions, otherwise recursively process the type.
+To replace it, calls [ty_for_param]
 and all that does is index into the list of substitutions with the index of the `Param`.
 
+[a previous chapter]: ty_module/instantiating_binders.md
+[`TypeFoldable`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFoldable.html
+[`TypeFolder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFolder.html
+[`fold_ty`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFolder.html#method.fold_ty
+[`ArgFolder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/binder/struct.ArgFolder.html
+[here]: https://github.com/rust-lang/rust/blob/master/compiler/rustc_macros/src/type_foldable.rs
+[actual folder]: https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L440-L451
+[fold_ty]: https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L512-L536
+[ty_for_param]: https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L552-L587