Stabilize conservative_impl_trait

1 files changed, 0 insertions, 66 deletions

diff --git a/src/doc/unstable-book/src/language-features/conservative-impl-trait.md b/src/doc/unstable-book/src/language-features/conservative-impl-trait.md
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-# `conservative_impl_trait`
-
-The tracking issue for this feature is: [#34511]
-
-[#34511]: https://github.com/rust-lang/rust/issues/34511
-
-------------------------
-
-The `conservative_impl_trait` feature allows a conservative form of abstract
-return types.
-
-Abstract return types allow a function to hide a concrete return type behind a
-trait interface similar to trait objects, while still generating the same
-statically dispatched code as with concrete types.
-
-## Examples
-
-```rust
-#![feature(conservative_impl_trait)]
-
-fn even_iter() -> impl Iterator<Item=u32> {
-    (0..).map(|n| n * 2)
-}
-
-fn main() {
-    let first_four_even_numbers = even_iter().take(4).collect::<Vec<_>>();
-    assert_eq!(first_four_even_numbers, vec![0, 2, 4, 6]);
-}
-```
-
-## Background
-
-In today's Rust, you can write function signatures like:
-
-````rust,ignore
-fn consume_iter_static<I: Iterator<Item=u8>>(iter: I) { }
-
-fn consume_iter_dynamic(iter: Box<Iterator<Item=u8>>) { }
-````
-
-In both cases, the function does not depend on the exact type of the argument.
-The type held is "abstract", and is assumed only to satisfy a trait bound.
-
-* In the `_static` version using generics, each use of the function is
-  specialized to a concrete, statically-known type, giving static dispatch,
-  inline layout, and other performance wins.
-* In the `_dynamic` version using trait objects, the concrete argument type is
-  only known at runtime using a vtable.
-
-On the other hand, while you can write:
-
-````rust,ignore
-fn produce_iter_dynamic() -> Box<Iterator<Item=u8>> { }
-````
-
-...but you _cannot_ write something like:
-
-````rust,ignore
-fn produce_iter_static() -> Iterator<Item=u8> { }
-````
-
-That is, in today's Rust, abstract return types can only be written using trait
-objects, which can be a significant performance penalty. This RFC proposes
-"unboxed abstract types" as a way of achieving signatures like
-`produce_iter_static`. Like generics, unboxed abstract types guarantee static
-dispatch and inline data layout.