Rust 1.19: what's new, what was stabilized

• [MutexGuard<T> may only be Sync if T is Sync.][41624]
• [-Z flags are now no longer allowed to be used on the stable
• [As a result of the -Z flag change, the cargo-check plugin no
• [Ending a float literal with ._ is now a hard error.
• [Any use of a private extern crate outside of its module is now a
• [use ::self::foo; is now a hard error.][36888] self paths are always
• [Floating point constants in match patterns is now a hard error][36890]
• [Struct or enum constants that don't derive PartialEq & Eq used
• [Lifetimes named '_ are no longer allowed.][36892] This was previously
• [From the pound escape, lines consisting of multiple #s are
• [It is an error to re-export private enum variants][42460]. This is
• [On Windows, if VCINSTALLDIR is set incorrectly, rustc will try

Language

• [Numeric fields can now be used for creating tuple structs.][41145] [RFC 1506] For example struct Point(u32, u32); let x = Point { 0: 7, 1: 0 };.
• [Macro recursion limit increased to 1024 from 64.][41676]
• [Added lint for detecting unused macros.][41907]
• [loop can now return a value with break.][42016] [RFC 1624] For example: let x = loop { break 7; };
• [C compatible unions are now available.][42068] [RFC 1444] They can only contain Copy types and cannot have a Drop implementation. Example: union Foo { bar: u8, baz: usize }
• [Non capturing closures can now be coerced into fns,][42162] [RFC 1558] Example: let foo: fn(u8) -> u8 = |v: u8| { v };

Compiler

• [Add support for bootstrapping the Rust compiler toolchain on Android.][41370]
• [Change arm-linux-androideabi to correspond to the armeabi official ABI.][41656] If you wish to continue targeting the armeabi-v7a ABI you should use --target armv7-linux-androideabi.
• [Fixed ICE when removing a source file between compilation sessions.][41873]
• [Minor optimisation of string operations.][42037]
• [Compiler error message is now aborting due to previous error(s) instead of aborting due to N previous errors][42150] This was previously inaccurate and would only count certain kinds of errors.
• [The compiler now supports Visual Studio 2017][42225]
• [The compiler is now built against LLVM 4.0.1 by default][42948]
• [Added a lot][42264] of [new error codes][42302]
• [Added target-feature=+crt-static option][37406] [RFC 1721] Which allows libraries with C Run-time Libraries(CRT) to be statically linked.
• [Fixed various ARM codegen bugs][42740]

Libraries

• [String now implements FromIterator<Cow<'a, str>> and Extend<Cow<'a, str>>][41449]
• [Vec now implements From<&mut [T]>][41530]
• [Box<[u8]> now implements From<Box<str>>][41258]
• [SplitWhitespace now implements Clone][41659]
• [[u8]::reverse is now 5x faster and [u16]::reverse is now 1.5x faster][41764]
• [eprint! and eprintln! macros added to prelude.][41192] Same as the print! macros, but for printing to stderr.

Stabilized APIs

• [OsString::shrink_to_fit]
• [cmp::Reverse]
• [Command::envs]
• [thread::ThreadId]

Cargo

• [Build scripts can now add environment variables to the environment the crate is being compiled in. Example: println!("cargo:rustc-env=FOO=bar");][cargo/3929]
• [Subcommands now replace the current process rather than spawning a new child process][cargo/3970]
• [Workspace members can now accept glob file patterns][cargo/3979]
• [Added --all flag to the cargo bench subcommand to run benchmarks of all the members in a given workspace.][cargo/3988]
• [Updated libssh2-sys to 0.2.6][cargo/4008]
• [Target directory path is now in the cargo metadata][cargo/4022]
• [Cargo no longer checks out a local working directory for the crates.io index][cargo/4026] This should provide smaller file size for the registry, and improve cloning times, especially on Windows machines.
• [Added an --exclude option for excluding certain packages when using the --all option][cargo/4031]
• [Cargo will now automatically retry when receiving a 5xx error from crates.io][cargo/4032]
• [The --features option now accepts multiple comma or space delimited values.][cargo/4084]
• [Added support for custom target specific runners][cargo/3954]

Misc

• [Added rust-windbg.cmd][39983] for loading rust .natvis files in the Windows Debugger.
• [Rust will now release XZ compressed packages][rust-installer/57]
• [rustup will now prefer to download rust packages with XZ compression][rustup/1100] over GZip packages.
• [Added the ability to escape # in rust documentation][41785] By adding additional #'s ie. ## is now #

Compatibility Notes

• MutexGuard<T> may only be Sync if T is Sync.
• -Z flags are now no longer allowed to be used on the stable compiler. This has been a warning for a year previous to this.
• As a result of the -Z flag change, the cargo-check plugin no longer works. Users should migrate to the built-in check command, which has been available since 1.16.
• Ending a float literal with ._ is now a hard error. Example: 42._ .
• Any use of a private extern crate outside of its module is now a hard error. This was previously a warning.
• use ::self::foo; is now a hard error. self paths are always relative while the :: prefix makes a path absolute, but was ignored and the path was relative regardless.
• Floating point constants in match patterns is now a hard error This was previously a warning.
• Struct or enum constants that don't derive PartialEq & Eq used match patterns is now a hard error This was previously a warning.
• Lifetimes named '_ are no longer allowed. This was previously a warning.
• From the pound escape, lines consisting of multiple #s are now visible
• It is an error to re-export private enum variants. This is known to break a number of crates that depend on an older version of mustache.
• On Windows, if VCINSTALLDIR is set incorrectly, rustc will try to use it to find the linker, and the build will fail where it did not previously