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1. Clarify that there is a single concrete type for an opaque type
2. Clarify that defining a type alias to an opaque type is an unstable feature
3. Add complete example
Copy file name to clipboardExpand all lines: src/opaque-types-type-alias-impl-trait.md
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@@ -12,22 +12,44 @@ type Foo = impl Bar;
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This declares an opaque type named `Foo`, of which the only information is that
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it implements `Bar`. Therefore, any of `Bar`'s interface can be used on a `Foo`,
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but nothing else (regardless of whether it implements any other traits).
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but nothing else (regardless of whether the concrete type implements any other traits).
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Since there needs to be a concrete background type,
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you can (as of <!-- date-check -->January 2021) express that type
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you can (as of <!-- date-check -->May 2025) express that type
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by using the opaque type in a "defining use site".
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```rust,ignore
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struct Struct;
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impl Bar for Struct { /* stuff */ }
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#[define_opaque(Foo)]
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fn foo() -> Foo {
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Struct
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}
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```
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Any other "defining use site" needs to produce the exact same type.
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Note that defining a type alias to an opaque type is an unstable feature.
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To use it, you need `nightly` and the annotations `#![feature(type_alias_impl_trait)]` on the file and `#[define_opaque(Foo)]` on the method that links the opaque type to the concrete type.
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Complete example:
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```rust
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#![feature(type_alias_impl_trait)]
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traitBar { /* stuff */ }
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typeFoo=implBar;
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structStruct;
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implBarforStruct { /* stuff */ }
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#[define_opaque(Foo)]
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fnfoo() ->Foo {
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Struct
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}
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```
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## Defining use site(s)
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Currently only the return value of a function can be a defining use site
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