Announcing Rust 1.32.0

Author: steveklabnik

posts/2019-01-17-Rust-1.32.0.md

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+---
+layout: post
+title: "Announcing Rust 1.32.0"
+author: The Rust Release Team
+---
+
+The Rust team is happy to announce a new version of Rust, 1.32.0. 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
+1.32.0 is as easy as:
+
+```
+$ 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
+1.32.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/install.html
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1320-2019-01-17
+
+## What's in 1.32.0 stable
+
+Rust 1.32.0 has a few quality of life improvements, switches the default
+allocator, and makes additional functions `const`. Read on for a few
+highlights, or see the [detailed release notes][notes] for additional
+information.
+
+#### The `dbg` macro
+
+First up, a small quality of life improvement. Are you a "printf debugger"? If you are, and
+you've wanted to print out some value while working on some code, you have to do this:
+
+```rust
+let x = 5;
+
+println!("{:?}", x);
+
+// or maybe even this
+println!("{:#?}", x);
+```
+
+This isn't the *largest* speed bump, but it is a lot of stuff to simply show the value of `x`.
+Additionally, there's no context here. If you have several of these `println!`s, it can be hard
+to tell which is which, unless you add your own context to each invocation, causing even more work.
+
+In Rust 1.32.0, [we've added a new macro,
+`dbg!`](https://github.com/rust-lang/rust/pull/56395/), for this purpose:
+
+```rust
+fn main() {
+    let x = 5;
+    
+    dbg!(x);
+}
+```
+
+If you run this program, you'll see:
+
+```text
+[src/main.rs:4] x = 5
+```
+
+You get the file and line number of where this was invoked, as well as the
+name and value. Additionally, `println!` prints to the standard output, so
+you really should be using `eprintln!` to print to standard error. `dbg!`
+does the right thing and goes to `stderr` by default.
+
+It even works in more complex circumstances. Consider this factorial example:
+
+```rust
+fn factorial(n: u32) -> u32 {
+    if n <= 1 {
+        n
+    } else {
+        n * factorial(n - 1)
+    }
+}
+```
+
+If we wanted to debug this, we might write it like this with `eprintln!`:
+
+```rust
+fn factorial(n: u32) -> u32 {
+    eprintln!("n: {}", n);
+
+    if n <= 1 {
+        eprintln!("n <= 1");
+
+        n
+    } else {
+        let n = n * factorial(n - 1);
+
+        eprintln!("n: {}", n);
+
+        n
+    }
+}
+```
+
+We want to log `n` on each iteration, as well as have some kind of context
+for each of the branches. We see this output for `factorial(4)`:
+
+```text
+n: 4
+n: 3
+n: 2
+n: 1
+n <= 1
+n: 2
+n: 6
+n: 24
+```
+
+This is servicable, but not particularly great. Maybe we could work on how we
+print out the context to make it more clear, but now we're not debugging our code,
+we're figuring out how to make our debugging code better.
+
+Consider this version using `dbg!`:
+
+```rust
+fn factorial(n: u32) -> u32 {
+    if dbg!(n <= 1) {
+        dbg!(1)
+    } else {
+        dbg!(n * factorial(n - 1))
+    }
+}
+```
+
+We simply wrap each of the various expressions we want to print with the macro. We
+get this output instead:
+
+```text
+[src/main.rs:3] n <= 1 = false
+[src/main.rs:3] n <= 1 = false
+[src/main.rs:3] n <= 1 = false
+[src/main.rs:3] n <= 1 = true
+[src/main.rs:4] 1 = 1
+[src/main.rs:5] n * factorial(n - 1) = 2
+[src/main.rs:5] n * factorial(n - 1) = 6
+[src/main.rs:5] n * factorial(n - 1) = 24
+[src/main.rs:11] factorial(4) = 24
+```
+
+Because the `dbg!` macro returns the value of what it's debugging, instead of
+`eprintln!` which returns `()`, we need to make *no* changes to the structure
+of our code. Additionally, we have *vastly* more useful output.
+
+That's a lot to say about a little macro, but we hope it improves your
+debugging experience! We are contining to work on support for `gdb` and
+friends as well, of course.
+
+#### `jemalloc` is removed by default
+
+Long, long ago, Rust had a large, Erlang-like runtime. We chose to use
+[jemalloc] instead of the system allocator, because it often improved
+performance over the default system one. Over time, we shed more and more of
+this runtime, and eventually almost all of it was removed, but jemalloc
+didn't. We didn't have a way to choose a custom allocator, and so we couldn't
+really remove it without causing a regression for people who do need
+jemalloc.
+
+Also, saying that `jemalloc` was always the default is a bit UNIX-centric,
+as it was only the default on *some* platforms. Notably, the MSVC target on
+Windows has shipped the system allocator for a long time.
+
+Finally, while jemalloc *usually* has great performance, that's not always
+the case. Additionally, it adds about 300kb to every Rust binary. We've also
+had a host of [other
+issues](https://github.com/rust-lang/rust/issues/36963#issuecomment-252029017)
+with jemalloc in the past. It has also felt a little strange that a systems
+language does not default to the system's allocator.
+
+For all of these reasons, once [Rust 1.28 shipped a way to choose a global
+allocator](https://blog.rust-lang.org/2018/08/02/Rust-1.28.html#whats-in-1.28.0-stable),
+we started making plans to switch the default to the system allocator, and
+allow you to use `jemalloc` via a crate. In Rust 1.32, we've finally finished
+this work, and by default, you will get the system allocator for your
+programs.
+
+If you'd like to continue to use jemalloc, use [the jemallocator crate]. In
+your `Cargo.toml`:
+
+```toml
+jemallocator = "0.1.8"
+```
+
+And in your crate root:
+
+```rust
+#[global_allocator]
+static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
+```
+
+That's it! If you don't need jemalloc, it's not forced upon you, and if
+you do need it, it's a few lines of code away.
+
+[jemalloc]: http://jemalloc.net/
+[the jemallocator crate]: https://crates.io/crates/jemallocator
+
+#### Macro improvements
+
+A few improvements to macros have landed in Rust 1.32.0. First, [a new
+`literal` matcher](https://github.com/rust-lang/rust/pull/56072/) was added:
+
+```rust
+macro_rules! m {
+    ($lt:literal) => {};
+}
+
+fn main() {
+    m!("some string literal");
+}
+```
+
+`literal` mactches against literals of any type; string literals, numeric literals, `char` literals.
+
+In the 2018 edition, `macro_rules` macros can also use `?`, like this:
+
+```rust
+macro_rules! bar {
+    ($(a)?) => {}
+}
+```
+
+The `?` will match zero or one repitions of the pattern, similar to the
+already-existing `*` for "zero or more" and `+` for "one or more."
+
+#### Final module improvements
+
+In Rust 1.30.0, [we announced several improvements to the module
+system](https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#module-system-changes).
+We have one last tweak landing in 1.32.0 and the 2018 edition. Nicknamed
+["uniform paths"](https://github.com/rust-lang/rust/pull/56759/), the simplest way to explain
+it is to show you this code:
+
+```rust
+enum Color { Red, Green, Blue }
+
+use Color::*;
+```
+
+This code did *not* previously compile, as `use` statements had to start with
+`super`, `self`, or `crate`. With this change, this code will work, and do
+what you probably expect: import the variants of the `Color` enum defined
+above the `use` statement.
+
+With this change in place, we've completed our efforts at revising the module
+system. We hope you've been enjoying the simplified system so far!
+
+### Library stabilizations
+
+We talked above about the `dbg!` macro, which is a big library addition.
+Beyond that, 19 functions were made `const fn`s, and all 72 combinations of
+the `to_*_bytes` and `from_*_bytes` methods, where `*` is one of `{be, le,
+ne}`, were added to `{usize,isize,i8,i16,i32,i64,i128,u8,u16,u32,u64,u128}`.
+See the [detailed release notes][notes] for more details.
+
+### Cargo features
+
+Cargo gained [`cargo c` as an alias for `cargo
+check`](https://github.com/rust-lang/cargo/pull/6218/), and now [allows
+usernames in registry URLs](https://github.com/rust-lang/cargo/pull/6242/).
+
+See the [detailed release notes][notes] for more.
+
+## Contributors to 1.32.0
+
+Many people came together to create Rust 1.32.0. We couldn't have done it
+without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.32.0)