Announcing Rust 1.29

_posts/2018-09-13-Rust-1.29.md

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+---
+layout: post
+title: "Announcing Rust 1.29"
+author: The Rust Core Team
+---
+
+The Rust team is happy to announce a new version of Rust, 1.29.0. Rust is a
+systems programming language focused on safety, speed, and concurrency.
+
+If you have a previous version of Rust installed via rustup, getting Rust
+1.29.0 is as easy as:
+
+```bash
+$ 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
+1.29.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/install.html
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1290-2018-09-13
+
+## What's in 1.29.0 stable
+
+The 1.29 release is fairly small; Rust 1.30 and 1.31 are going to have a lot
+in them, and so much of the 1.29 cycle was spent preparing for those
+releases. The two most significant things in this release aren't even language
+features: they're new abilities that Cargo has grown, and they're both about lints.
+
+* `cargo fix` can automatically fix your code that has warnings
+* `cargo clippy` is a bunch of lints to catch common mistakes and improve your Rust code
+
+### `cargo fix`
+
+With the release of Rust 1.29, Cargo has a new subcommand: `cargo fix`. If you've written
+code in Rust before, you've probably seen a compiler warning before. For example, consider
+this code:
+
+```rust
+fn do_something() {}
+
+fn main() {
+    for i in 0..100 {
+        do_something();
+    }
+}
+```
+
+Here, we're calling `do_something` a hundred times. But we never use the variable `i`.
+And so Rust warns:
+
+```text
+> cargo build
+   Compiling myprogram v0.1.0 (file:///path/to/myprogram)
+warning: unused variable: `i`
+ --> src\main.rs:4:9
+  |
+4 |     for i in 1..100 {
+  |         ^ help: consider using `_i` instead
+  |
+  = note: #[warn(unused_variables)] on by default
+
+    Finished dev [unoptimized + debuginfo] target(s) in 0.50s
+```
+
+See how it suggests that we use `_i` as a name instead? We can automatically
+apply that suggestion with `cargo fix`:
+
+```console
+> cargo fix
+    Checking myprogram v0.1.0 (file:///C:/Users/steve/tmp/fix)
+      Fixing src\main.rs (1 fix)
+    Finished dev [unoptimized + debuginfo] target(s) in 0.59s
+```
+
+If we look at `src\main.rs` again, we'll see that the code has changed:
+
+```rust
+fn do_something() {}
+
+fn main() {
+    for _i in 0..100 {
+        do_something();
+    }
+}
+```
+
+We're now using `_i`, and the warning will no longer appear.
+
+This initial release of `cargo fix` only fixes up a small number of warnings.
+The compiler has an API for this, and it only suggests fixing lints that
+we're confident recommend correct code. Over time, as our suggestions
+improve, we'll be expanding this to automatically fix more warnings.
+
+### `cargo clippy`
+
+Speaking of warnings, you can now check out a preview of `cargo clippy` through Rustup.
+Clippy is a large number of additional warnings that you can run against your Rust code.
+
+For example:
+
+```rust
+let mut lock_guard = mutex.lock();
+
+std::mem::drop(&lock_guard)
+
+operation_that_requires_mutex_to_be_unlocked();
+```
+
+This code is syntactically correct, but may have a deadlock! You see, we
+dropped *a reference to `lock_guard`*, not the guard itself. Dropping
+a reference is a no-op, and so this is almost certainly a bug.
+
+We can get the preview of Clippy from Rustup:
+
+```console
+$ rustup component add clippy-preview
+```
+
+and then run it:
+
+```console
+$ cargo clippy
+error: calls to `std::mem::drop` with a reference instead of an owned value. Dropping a reference does nothing.
+ --> src\main.rs:5:5
+  |
+5 |     std::mem::drop(&lock_guard);
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: #[deny(drop_ref)] on by default
+note: argument has type &std::result::Result<std::sync::MutexGuard<'_, i32>, std::sync::PoisonError<std::sync::MutexGuard<'_, i32>>>
+ --> src\main.rs:5:20
+  |
+5 |     std::mem::drop(&lock_guard);
+  |                    ^^^^^^^^^^^
+  = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#drop_ref
+```
+
+As you can see from that help message, you can view all of the lints that
+clippy offers on the web.
+
+Please note that this is a preview; clippy has not yet reached 1.0. As such,
+its lints may change. We'll release a `clippy` component once it has stabilized;
+please give the preview a try and let us know how it goes.
+
+Oh, and one more thing: [you can't use clippy with `cargo-fix` yet,
+really](https://github.com/rust-lang-nursery/rustfix/issues/130). It's in the works!
+
+See the [detailed release notes][notes] for more.
+
+### Library stabilizations
+
+Three APIs were stabilized this release:
+
+* [`Arc<T>::downcast`](https://doc.rust-lang.org/std/sync/struct.Arc.html#method.downcast)
+* [`Rc<T>::downcast`](https://doc.rust-lang.org/std/rc/struct.Rc.html#method.downcast)
+* [`Iterator::flatten`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flatten)
+
+Additionally, you can [now compare `&str` and
+`OsString`](https://github.com/rust-lang/rust/pull/51178/).
+
+See the [detailed release notes][notes] for more.
+
+### Cargo features
+
+We covered the two new subcommands to Cargo above, but additionally, [Cargo
+will now try to fix up lockfiles that have been corrupted by a `git
+merge`](https://github.com/rust-lang/cargo/pull/5831/). You can pass
+`--locked` to disable this behavior.
+
+`cargo doc` has also grown a new flag:
+[`--document-private-items`](https://github.com/rust-lang/cargo/pull/5543).  By
+default, `cargo doc` only documents public things, as the docs it produces are
+intended for end-users. But if you're working on your own crate, and you have
+internal documentation for yourself to refer to, `--document-private-items`
+will generate docs for all items, not just public ones.
+
+See the [detailed release notes][notes] for more.
+
+## Contributors to 1.29.0
+
+Many people came together to create Rust 1.29. We couldn't have done it
+without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.29.0)