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Quinn Sinclair 2024-02-13 23:48:14 +01:00
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34
book/src/development/trait_checking.md
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@ -96,11 +96,16 @@ impl LateLintPass<'_> for CheckTokioAsyncReadExtTrait {
## Creating Types Programmatically
Traits are often generic over a type e.g. `Borrow<T>` is generic over `T`, and rust allows us to implement a trait for
a specific type. For example, we can implement `Borrow<str>` for a hypothetical type `Foo`. Let's suppose that we
would like to find whether our type actually implements `Borrow<[u8]>`. To do so, we need to supply a type that
represents `[u8]`, but `[u8]` is also a generic, it's a slice over `u8`. We can create this type using Ty::new_slice
method. The following code demonstrates how to do this:
Traits are often generic over a type parameter, e.g. `Borrow<T>` is generic
over `T`. Rust allows us to implement a trait for a specific type. For example,
we can implement `Borrow<[u8]>` for a hypothetical type `Foo`. Let's suppose
that we would like to find whether our type actually implements `Borrow<[u8]>`.
To do so, we can use the same `implements_trait` function as above, and supply
a parameter type that represents `[u8]`. Since `[u8]` is a specialization of
`[T]`, we can use the [`Ty::new_slice`][new_slice] method to create a type
that represents `[T]` and supply `u8` as a type parameter. The following code
demonstrates how to do this:
```rust
@ -108,17 +113,19 @@ use rustc_middle::ty::Ty;
use clippy_utils::ty::implements_trait;
use rustc_span::symbol::sym;
let ty = todo!();
let ty = todo!("Get the `Foo` type to check for a trait implementation");
let borrow_id = cx.tcx.get_diagnostic_item(sym::Borrow).unwrap(); // avoid unwrap in real code
if implements_trait(cx, ty, borrow_id, &[Ty::new_slice(cx.tcx, cx.tcx.types.u8).into()]) {
todo!()
let slice_bytes = Ty::new_slice(cx.tcx, cx.tcx.types.u8);
let generic_param = slice_bytes.into();
if implements_trait(cx, ty, borrow_id, &[generic_param]) {
todo!("Rest of lint implementation")
}
```
Here, we use `Ty::new_slice` to create a type that represents `[T]` and supply `u8` as a type parameter, and then we go
on normally with `implements_trait` function. The [Ty] struct allows us to create types programmatically, and it's
useful when we need to create types that we can't obtain through the usual means.
In essence, the [`Ty`] struct allows us to create types programmatically in a
representation that can be used by the compiler and the query engine. We then
use the `rustc_middle::Ty` of the type we are interested in, and query the
compiler to see if it indeed implements the trait we are interested in.
[DefId]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html
@ -129,5 +136,6 @@ useful when we need to create types that we can't obtain through the usual means
[symbol]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/symbol/struct.Symbol.html
[symbol_index]: https://doc.rust-lang.org/beta/nightly-rustc/rustc_span/symbol/sym/index.html
[TyCtxt]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html
[Ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html
[`Ty`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html
[rust]: https://github.com/rust-lang/rust
[new_slice]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.new_slice