release blog post for Rust 1.46.0

posts/2020-08-27-Rust-1.46.0.md

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+---
+layout: post
+title: "Announcing Rust 1.46.0"
+author: The Rust Release Team
+release: true
+---
+
+The Rust team is happy to announce a new version of Rust, 1.46.0. Rust is a
+programming language that is empowering everyone to build reliable and
+efficient software.
+
+If you have a previous version of Rust installed via rustup, getting Rust
+1.46.0 is as easy as:
+
+```console
+rustup update stable
+```
+
+If you don't have it already, you can [get `rustup`][install] from the
+appropriate page on our website, and check out the [detailed release notes for
+1.46.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/tools/install
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1460-2020-08-27
+
+## What's in 1.46.0 stable
+
+This release enables quite a lot of new things to appear in `const fn`, two
+new standard library APIs, and one feature useful for library authors. See
+the [detailed release notes][notes] to learn about other changes not covered
+by this post.
+
+### `const fn` improvements
+
+There are [several core language features] you can now use in a `const fn`:
+
+* `if`, `if let`, and `match`
+* `while`, `while let`, and `loop`
+* the `&&` and `||` operators
+
+You can also [cast to a slice][cast-to-slice]:
+
+```rust
+const fn foo() {
+  let x = [1, 2, 3, 4, 5];
+
+  // cast the array to a slice
+  let y: &[_] = &x;
+}
+```
+
+While these features may not feel *new*, given that you could use them all
+outside of `const fn`, they add a lot of compile-time computation power! As
+an example, the [`const-sha1` crate][sha1] can let you compute SHA-1 hashes
+at compile time. This led to a [40x performance improvement][const-perf] in
+Microsoft's WinRT bindings for Rust.
+
+[several core language features]: https://github.com/rust-lang/rust/pull/72437/
+[cast-to-slice]: https://github.com/rust-lang/rust/pull/73862/
+[sha1]: https://github.com/rylev/const-sha1
+[const-perf]: https://github.com/microsoft/winrt-rs/pull/279#issuecomment-668436700
+
+
+### `#[track_caller]`
+
+Back in March, the release of Rust 1.42 introduced [better error messages when `unwrap` and related functions would panic][better-errors]. At the time, we mentioned that the way
+this was implemented was not yet stable. Rust 1.46 stabilizes this feature.
+
+[better-errors]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#useful-line-numbers-in-option-and-result-panic-messages
+
+This attribute is called `#[track_caller]`, which was originally proposed in
+[RFC 2091][rfc-2091] way back in July of 2017! If you're writing a function
+like `unwrap` that may panic, you can put this annotation on your functions,
+and the default panic formatter will use its caller as the location in its
+error message. For example, here is `unwrap` previously:
+
+```rust
+pub fn unwrap(self) -> T {
+    match self {
+        Some(val) => val,
+        None => panic!("called `Option::unwrap()` on a `None` value"),
+    }
+}
+```
+
+It now looks like this:
+
+```rust
+#[track_caller]
+pub fn unwrap(self) -> T {
+    match self {
+        Some(val) => val,
+        None => panic!("called `Option::unwrap()` on a `None` value"),
+    }
+}
+```
+
+That's it!
+
+If you are implementing a panic hook yourself, you can use the [caller] method
+on `std::panic::Location` to get access to this information.
+
+[rfc-2091]: https://github.com/rust-lang/rfcs/pull/2091
+[caller]: https://doc.rust-lang.org/stable/std/panic/struct.Location.html#method.caller
+
+### Library changes
+
+Keeping with the theme of `const fn` improvements, [`std::mem::forget` is now
+a `const fn`][forget]. Additionally, two new APIs were stabilized this release:
+
+* [`Option::zip`][zip]
+* [`vec::Drain::as_slice`][as_slice]
+
+[forget]: https://github.com/rust-lang/rust/pull/73887/
+[zip]: https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.zip
+[as_slice]:  https://doc.rust-lang.org/stable/std/vec/struct.Drain.html#method.as_slice
+
+See the [detailed release notes][notes] for more.
+
+### Other changes
+
+[relnotes-cargo]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-146-2020-08-27
+[relnotes-clippy]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-146
+
+There are other changes in the Rust 1.46.0 release: check out what changed in
+[Rust][notes], [Cargo][relnotes-cargo], and [Clippy][relnotes-clippy].
+
+## Contributors to 1.46.0
+
+Many people came together to create Rust 1.46.0. We couldn't have done it
+without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.46.0/)