mirror
/
rustc-dev-guide
mirror of https://github.com/rust-lang/rustc-dev-guide.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

reduce clutter when reading source

This commit is contained in:
Tshepang Mbambo 2025-05-10 12:56:03 +02:00
parent 6b12439a07
commit 1dff715902
1 changed files with 16 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
src/ty-fold.md
View File

@ -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?
The answer is a couple of traits:
[`TypeFoldable`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/trait.TypeFoldable.html)
[`TypeFoldable`]
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
process the contents of the `TypeFoldable` and do stuff to them.
@ -17,7 +17,7 @@ and
`TypeFoldable`.
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
`TypeFolder` `fold_foo` methods on itself, giving the `TypeFolder` access to its contents (the
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.
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.
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.
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).
[here].
**`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
`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
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)
[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