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reduce clutter when reading source

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Tshepang Mbambo 2025-05-10 12:56:03 +02:00
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src/ty-fold.md
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@ -7,9 +7,9 @@ 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? So, how do we implement the `instantiate` methods on the `Early/Binder` types?
The answer is a couple of traits: The answer is a couple of traits:
[`TypeFoldable`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFoldable.html) [`TypeFoldable`]
and and
[`TypeFolder`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFolder.html). [`TypeFolder`].
- `TypeFoldable` is implemented by types that embed type information. It allows you to recursively - `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. process the contents of the `TypeFoldable` and do stuff to them.
@ -17,7 +17,7 @@ and
`TypeFoldable`. `TypeFoldable`.
For example, the `TypeFolder` trait has a method For example, the `TypeFolder` trait has a method
[`fold_ty`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFolder.html#method.fold_ty) [`fold_ty`]
that takes a type as input and returns a new type as a result. `TypeFoldable` invokes the 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 `TypeFolder` `fold_foo` methods on itself, giving the `TypeFolder` access to its contents (the
types, regions, etc that are contained within). types, regions, etc that are contained within).
@ -36,7 +36,7 @@ So to reiterate:
- `TypeFoldable` is a trait that is implemented by things that embed types. - `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`: 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). [`ArgFolder`].
Looking at its implementation, we see where the actual substitutions are happening. 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 However, you might also notice that the implementation calls this `super_fold_with` method. What is
@ -91,17 +91,25 @@ things. We only want to do something when we reach a type. That means there may
implementations. Such implementations of `TypeFoldable` tend to be pretty tedious to write by hand. 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 For this reason, there is a `derive` macro that allows you to `#![derive(TypeFoldable)]`. It is
defined defined
[here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_macros/src/type_foldable.rs). [here].
**`subst`** In the case of substitutions the [actual **`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) folder]
is going to be doing the indexing we’ve already mentioned. There we define a `Folder` and call 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_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) [fold_ty]
the method that process each type it looks for a `ty::Param` and for those it replaces it for 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, something from the list of substitutions, otherwise recursively process the type. To replace it,
calls calls
[ty_for_param](https://github.com/rust-lang/rust/blob/75ff3110ac6d8a0259023b83fd20d7ab295f8dd6/src/librustc_middle/ty/subst.rs#L552-L587) [ty_for_param]
and all that does is index into the list of substitutions with the index of the `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 [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