release blog post for Rust 1.47

posts/2020-10-08-Rust-1.47.md

@@ -0,0 +1,231 @@
+---
+layout: post
+title: "Announcing Rust 1.47.0"
+author: The Rust Release Team
+release: true
+---
+
+The Rust team is happy to announce a new version of Rust, 1.47.0. 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
+1.47.0 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
+1.47.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/tools/install
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1470-2020-10-08
+
+## What's in 1.47.0 stable
+
+This release contains no new language features, though it does add one
+long-awaited standard library feature. It is mostly quality of life
+improvements, library stabilizations and const-ifications, and toolchain
+improvements. See the [detailed release notes][notes] to learn about other
+changes not covered by this post.
+
+#### Traits on larger arrays
+
+Rust does not currently have a way to be generic over integer values. This
+has long caused problems with arrays, because arrays have an integer as part
+of their type; `[T; N]` is the type of an array of type `T` of `N` length.
+Because there is no way to be generic over `N`, you have to manually implement
+traits for arrays for every `N` you want to support. For the standard library,
+it was decided to support up to `N` of 32.
+
+We have been working on a feature called "const generics" that would allow
+you to be generic over `N`. Fully explaining this feature is out of the scope
+of this post, because we are not stabilizing const generics just yet.
+However, the core of this feature has been implemented in the compiler, and
+it has been decided that the feature is far enough along that we are okay
+with [the standard library using it to implement traits on arrays of any
+length](https://github.com/rust-lang/rust/pull/74060/). What this means in
+practice is that if you try to do something like this on Rust 1.46:
+
+```rust
+fn main() {
+    let xs = [0; 34];
+
+    println!("{:?}", xs);
+}
+```
+
+you'd get this error:
+
+```text
+error[E0277]: arrays only have std trait implementations for lengths 0..=32
+ --> src/main.rs:4:22
+  |
+4 |     println!("{:?}", xs);
+  |                      ^^ the trait `std::array::LengthAtMost32` is not implemented for `[{integer}; 34]`
+  |
+  = note: required because of the requirements on the impl of `std::fmt::Debug` for `[{integer}; 34]`
+  = note: required by `std::fmt::Debug::fmt`
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+```
+
+But with Rust 1.47, it will properly print out the array.
+
+This should make arrays significantly more useful to folks, though it will
+take until the const generics feature stabilizes for libraries to be able to do
+this kind of implementation for their own traits. We do not have a current
+estimated date for the stabilization of const generics.
+
+#### Shorter backtraces
+
+Back in Rust 1.18, we [made some changes to the backtraces `rustc` would
+print on panic](https://github.com/rust-lang/rust/pull/38165). There are a
+number of things in a backtrace that aren't useful the majority of the time.
+However, at some point, [these
+regressed](https://github.com/rust-lang/rust/issues/47429). In Rust 1.47.0,
+the culprit was found, and [this has now been
+fixed](https://github.com/rust-lang/rust/pull/75048). Since the regression,
+this program:
+
+```rust
+fn main() {
+    panic!();
+}
+```
+
+would give you a backtrace that looks like this:
+
+```text
+thread 'main' panicked at 'explicit panic', src/main.rs:2:5
+stack backtrace:
+   0: backtrace::backtrace::libunwind::trace
+             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
+   1: backtrace::backtrace::trace_unsynchronized
+             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
+   2: std::sys_common::backtrace::_print_fmt
+             at src/libstd/sys_common/backtrace.rs:78
+   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
+             at src/libstd/sys_common/backtrace.rs:59
+   4: core::fmt::write
+             at src/libcore/fmt/mod.rs:1076
+   5: std::io::Write::write_fmt
+             at src/libstd/io/mod.rs:1537
+   6: std::sys_common::backtrace::_print
+             at src/libstd/sys_common/backtrace.rs:62
+   7: std::sys_common::backtrace::print
+             at src/libstd/sys_common/backtrace.rs:49
+   8: std::panicking::default_hook::{{closure}}
+             at src/libstd/panicking.rs:198
+   9: std::panicking::default_hook
+             at src/libstd/panicking.rs:217
+  10: std::panicking::rust_panic_with_hook
+             at src/libstd/panicking.rs:526
+  11: std::panicking::begin_panic
+             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/panicking.rs:456
+  12: playground::main
+             at src/main.rs:2
+  13: std::rt::lang_start::{{closure}}
+             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/rt.rs:67
+  14: std::rt::lang_start_internal::{{closure}}
+             at src/libstd/rt.rs:52
+  15: std::panicking::try::do_call
+             at src/libstd/panicking.rs:348
+  16: std::panicking::try
+             at src/libstd/panicking.rs:325
+  17: std::panic::catch_unwind
+             at src/libstd/panic.rs:394
+  18: std::rt::lang_start_internal
+             at src/libstd/rt.rs:51
+  19: std::rt::lang_start
+             at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/rt.rs:67
+  20: main
+  21: __libc_start_main
+  22: _start
+```
+
+Now, in Rust 1.47.0, you'll see this instead:
+
+```text
+thread 'main' panicked at 'explicit panic', src/main.rs:2:5
+stack backtrace:
+   0: std::panicking::begin_panic
+             at /rustc/d6646f64790018719caebeafd352a92adfa1d75a/library/std/src/panicking.rs:497
+   1: playground::main
+             at ./src/main.rs:2
+   2: core::ops::function::FnOnce::call_once
+             at /rustc/d6646f64790018719caebeafd352a92adfa1d75a/library/core/src/ops/function.rs:227
+```
+
+This makes it much easier to see where the panic actually originated, and
+you can still set `RUST_BACKTRACE=full` if you want to see everything.
+
+#### LLVM 11
+
+We have [upgraded to LLVM 11](https://github.com/rust-lang/rust/pull/73526/).
+The compiler still supports being compiled with LLVM versions as old as 8,
+but by default, 11 is what you'll be getting.
+
+#### Control Flow Guard on Windows
+
+`rustc` [now supports](https://github.com/rust-lang/rust/pull/73893/) `-C
+control-flow-guard`, an option that will turn on [Control Flow
+Guard](https://docs.microsoft.com/en-us/windows/win32/secbp/control-flow-guard)
+on Windows. Other platforms ignore this flag.
+
+### Library changes
+
+Additionally, nine new APIs were stabilized this release:
+
+- [`Ident::new_raw`]
+- [`Range::is_empty`]
+- [`RangeInclusive::is_empty`]
+- [`Result::as_deref`]
+- [`Result::as_deref_mut`]
+- [`Vec::leak`]
+- [`pointer::offset_from`]
+- [`f32::TAU`]
+- [`f64::TAU`]
+
+The following previously stable APIs have now been made `const`:
+
+- [The `new` method for all `NonZero` integers.][73858]
+- [The `checked_add`,`checked_sub`,`checked_mul`,`checked_neg`, `checked_shl`,
+  `checked_shr`, `saturating_add`, `saturating_sub`, and `saturating_mul`
+  methods for all integers.][73858]
+- [The `checked_abs`, `saturating_abs`, `saturating_neg`, and `signum`  for all
+  signed integers.][73858]
+- [The `is_ascii_alphabetic`, `is_ascii_uppercase`, `is_ascii_lowercase`,
+  `is_ascii_alphanumeric`, `is_ascii_digit`, `is_ascii_hexdigit`,
+  `is_ascii_punctuation`, `is_ascii_graphic`, `is_ascii_whitespace`, and
+  `is_ascii_control` methods for `char` and `u8`.][73858]
+
+[`Ident::new_raw`]:  https://doc.rust-lang.org/stable/proc_macro/struct.Ident.html#method.new_raw
+[`Range::is_empty`]: https://doc.rust-lang.org/stable/std/ops/struct.Range.html#method.is_empty
+[`RangeInclusive::is_empty`]: https://doc.rust-lang.org/stable/std/ops/struct.RangeInclusive.html#method.is_empty
+[`Result::as_deref_mut`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.as_deref_mut
+[`Result::as_deref`]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.as_deref
+[`TypeId::of`]: https://doc.rust-lang.org/stable/std/any/struct.TypeId.html#method.of
+[`Vec::leak`]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.leak
+[`f32::TAU`]: https://doc.rust-lang.org/stable/std/f32/consts/constant.TAU.html
+[`f64::TAU`]: https://doc.rust-lang.org/stable/std/f64/consts/constant.TAU.html
+[`pointer::offset_from`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.offset_from
+[73858]: https://github.com/rust-lang/rust/pull/73858/
+
+See the [detailed release notes][notes] for more.
+
+### Other changes
+
+[Rustdoc has gained support for the Ayu theme](https://github.com/rust-lang/rust/pull/71237/).
+
+[relnotes-cargo]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-147-2020-10-08
+[relnotes-clippy]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-147
+
+There are other changes in the Rust 1.47.0 release: check out what changed in
+[Rust][notes], [Cargo][relnotes-cargo], and [Clippy][relnotes-clippy].
+
+## Contributors to 1.47.0
+
+Many people came together to create Rust 1.47.0. We couldn't have done it
+without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.47.0/)