Announcing Rust 1.9

May 26, 2016 · The Rust Core Team

The Rust team is happy to announce the latest version of Rust, 1.9. Rust is a systems programming language focused on safety, speed, and concurrency.

As always, you can install Rust 1.9 from the appropriate page on our website, and check out the detailed release notes for 1.9 on GitHub. About 1000 patches were landed in this release.

What's in 1.9 stable

Controlled unwinding

The biggest shift in Rust 1.9 is the stabilization of the std::panic module, which includes methods for halting the unwinding process started by a panic:

use std::panic;

let result = panic::catch_unwind(|| {

let result = panic::catch_unwind(|| {
    panic!("oh no!");

This new API was defined in RFC 1236.

In general, Rust distinguishes between two ways that an operation can fail:

  • Due to an expected problem, like a file not being found.
  • Due to an unexpected problem, like an index being out of bounds for an array.

Expected problems usually arise from conditions that are outside of your control; robust code should be prepared for anything its environment might throw at it. In Rust, expected problems are handled via the Result type, which allows a function to return information about the problem to its caller, which can then handle the error in a fine-grained way.

Unexpected problems are bugs: they arise due to a contract or assertion being violated. Since they are unexpected, it doesn't make sense to handle them in a fine-grained way. Instead, Rust employs a "fail fast" approach by panicking, which by default unwinds the stack (running destructors but no other code) of the thread which discovered the error. Other threads continue running, but will discover the panic any time they try to communicate with the panicked thread (whether through channels or shared memory). Panics thus abort execution up to some "isolation boundary", with code on the other side of the boundary still able to run, and perhaps to "recover" from the panic in some very coarse-grained way. A server, for example, does not necessarily need to go down just because of an assertion failure in one of its threads.

The new catch_unwind API offers a way to introduce new isolation boundaries within a thread. There are a couple of key motivating examples:

  • Embedding Rust in other languages
  • Abstractions that manage threads

For the first case, unwinding across a language boundary is undefined behavior, and often leads to segfaults in practice. Allowing panics to be caught means that you can safely expose Rust code via a C API, and translate unwinding into an error on the C side.

For the second case, consider a threadpool library. If a thread in the pool panics, you generally don't want to kill the thread itself, but rather catch the panic and communicate it to the client of the pool. The catch_unwind API is paired with resume_unwind, which can then be used to restart the panicking process on the client of the pool, where it belongs.

In both cases, you're introducing a new isolation boundary within a thread, and then translating the panic into some other form of error elsewhere.

A final point: why catch_unwind rather than catch_panic? We are in the process of adding an additional strategy for panics: aborting the entire process (possibly after running a general hook). For some applications, this is the most reasonable way to deal with a programmer error, and avoiding unwinding can have performance and code size wins.

Deprecation warnings

We introduced a new attribute for library authors: #[deprecated]. This attribute allows you to tag an API with a deprecation warning, which users of their crate will receive whenever they use the API, directing them to a replacement API. Deprecation warnings have long been a part of the standard library, but thanks to RFC 1270 they're now usable ecosystem-wide.

New targets

We now publish standard library binaries for several new targets:

  • mips-unknown-linux-musl,
  • mipsel-unknown-linux-musl, and
  • i586-pc-windows-msvc.

The first two targets are particularly interesting from a cross-compilation perspective; see the recent blog post on rustup for more details.

Compile time improvements

The time complexity of comparing variables for equivalence during type unification is reduced from O(n!) to O(n). As a result, some programming patterns compile much, much more quickly.

Rolling out use of specialization

This release sees some of the first use of specialization within the standard library. Specialization, which is currently available only on nightly, allows generic code to automatically be specialized based on more specific type information.

One example where this comes up in the standard library: conversion from a string slice (&str) to an owned String. One method, to_string, comes from a generic API which was previously relatively slow, while the custom to_owned implementation provided better performance. Using specialization, these two functions are now equivalent.

With this simple test of specialization under our belt, we have more performance improvements on the way in upcoming releases.

Library stabilizations

About 80 library functions and methods are now stable in 1.9. The most major is the std::panic module, described earlier, but there's a lot more too:


  • TcpStream, TcpListener, and UdpSocket gained a number of methods for configuring the connection.
  • SocketAddr and its variants gained set_ip() and set_port() conveniences.


  • BTreeSet and HashSet gained the take(), replace(), and get() methods, which make it possible to recover ownership of the original key.
  • OsString gained a few methods, bringing it closer to parity with String.
  • Slices gained copy_from_slice(), a safe form of memcpy.


  • char gained the ability to decode into UTF-16.


  • Raw pointers gained as_ref() and as_mut(), which returns an Option<&T>, translating null pointers into None.
  • ptr::{read,write}_volatile() allow for volatile reading and writing from a raw pointer.

Finally, many of the types in libcore did not contain a Debug implementation. This was fixed in the 1.9 release.

See the detailed release notes for more.

Cargo features

There were two major changes to Cargo:

First, Cargo can now be run concurrently.

Second, a new flag, RUSTFLAGS, was added. This flag allows you to specify arbitrary flags to be passed to rustc through an environment variable, which is useful for packagers, for example.

See the detailed release notes for more.

Contributors to 1.9

We had 127 individuals contribute to 1.9. Thank you so much!

  • Aaron Turon
  • Abhishek Chanda
  • Adolfo Ochagavía
  • Aidan Hobson Sayers
  • Alan Somers
  • Alejandro Wainzinger
  • Aleksey Kladov
  • Alex Burka
  • Alex Crichton
  • Amanieu d'Antras
  • Andrea Canciani
  • Andreas Linz
  • Andrew Cantino
  • Andrew Horton
  • Andrew Paseltiner
  • Andrey Cherkashin
  • Angus Lees
  • Ariel Ben-Yehuda
  • Benjamin Herr
  • Björn Steinbrink
  • Brian Anderson
  • Brian Bowman
  • Christian Wesselhoeft
  • Christopher Serr
  • Corey Farwell
  • Craig M. Brandenburg
  • Cyryl Płotnicki-Chudyk
  • Daniel J Rollins
  • Dave Huseby
  • David AO Lozano
  • David Henningsson
  • Devon Hollowood
  • Dirk Gadsden
  • Doug Goldstein
  • Eduard Burtescu
  • Eduard-Mihai Burtescu
  • Eli Friedman
  • Emanuel Czirai
  • Erick Tryzelaar
  • Evan
  • Felix S. Klock II
  • Florian Berger
  • Geoff Catlin
  • Guillaume Gomez
  • Gökhan Karabulut
  • JP Sugarbroad
  • James Miller
  • Jeffrey Seyfried
  • John Talling
  • Jonas Schievink
  • Jonathan S
  • Jorge Aparicio
  • Joshua Holmer
  • Kai Noda
  • Kamal Marhubi
  • Katze
  • Kevin Brothaler
  • Kevin Butler
  • Manish Goregaokar
  • Markus Westerlind
  • Marvin Löbel
  • Masood Malekghassemi
  • Matt Brubeck
  • Michael Huynh
  • Michael Neumann
  • Michael Woerister
  • Ms2ger
  • NODA, Kai
  • Nathan Kleyn
  • Nick Cameron
  • Niko Matsakis
  • Noah
  • Novotnik, Petr
  • Oliver Middleton
  • Oliver Schneider
  • Philipp Oppermann
  • Piotr Czarnecki
  • Pyfisch
  • Richo Healey
  • Ruud van Asseldonk
  • Scott Olson
  • Sean McArthur
  • Sebastian Wicki
  • Seo Sanghyeon
  • Simon Sapin
  • Simonas Kazlauskas
  • Steve Klabnik
  • Steven Allen
  • Steven Fackler
  • Stu Black
  • Sébastien Marie
  • Tang Chenglong
  • Ted Horst
  • Ticki
  • Tim Montague
  • Tim Neumann
  • Timon Van Overveldt
  • Tobias Bucher
  • Tobias Müller
  • Todd Lucas
  • Tom Tromey
  • Tshepang Lekhonkhobe
  • Ulrik Sverdrup
  • Vadim Petrochenkov
  • Valentin Lorentz
  • Varun Vats
  • Wang Xuerui
  • Wangshan Lu
  • York Xiang
  • arcnmx
  • ashleysommer
  • bors
  • ggomez
  • gohyda
  • ituxbag
  • mitaa
  • nicholasf
  • petevine
  • pierzchalski
  • pravic
  • srinivasreddy
  • tiehuis
  • ubsan
  • vagrant
  • vegai
  • vlastachu
  • Валерий Лашманов