Version 1.37.0 (2019-08-15)

Language

#[must_use] will now warn if the type is contained in a tuple, Box, or an array and unused.
You can now use the cfg and cfg_attr attributes on generic parameters.

You can now use enum variants through type alias. e.g. You can write the following:

#![allow(unused)]
fn main() {
type MyOption = Option<u8>;

fn increment_or_zero(x: MyOption) -> u8 {
    match x {
        MyOption::Some(y) => y + 1,
        MyOption::None => 0,
    }
}
}

You can now use _ as an identifier for consts. e.g. You can write const _: u32 = 5;.
You can now use #[repr(align(X)] on enums.
The ? Kleene macro operator is now available in the 2015 edition.

Compiler

You can now enable Profile-Guided Optimization with the -C profile-generate and -C profile-use flags. For more information on how to use profile guided optimization, please refer to the rustc book.
The rust-lldb wrapper script should now work again.

Libraries

mem::MaybeUninit<T> is now ABI-compatible with T.

Stabilized APIs

Cargo

Misc

Compatibility Notes

Using ... for inclusive range patterns will now warn by default. Please transition your code to using the ..= syntax for inclusive ranges instead.
Using a trait object without the dyn will now warn by default. Please transition your code to use dyn Trait for trait objects instead.