Syntax only changes

Allow type Foo: Ord syntactically.
Fuse associated and extern items up to defaultness.
Syntactically allow self in all fn contexts.
Merge fn syntax + cleanup item parsing.

item macro fragments can be interpolated into traits, impls, and extern blocks. For example, you may now write:

#![allow(unused)]
fn main() {
macro_rules! mac_trait {
    ($i:item) => {
        trait T { $i }
    }
}
mac_trait! {
    fn foo() {}
}
}

These are still rejected semantically, so you will likely receive an error but these changes can be seen and parsed by macros and conditional compilation.

Compiler

You can now pass multiple lint flags to rustc to override the previous flags. For example; rustc -D unused -A unused-variables denies everything in the unused lint group except unused-variables which is explicitly allowed. However, passing rustc -A unused-variables -D unused denies everything in the unused lint group including unused-variables since the allow flag is specified before the deny flag (and therefore overridden).
rustc will now prefer your system MinGW libraries over its bundled libraries if they are available on windows-gnu.
rustc now buffers errors/warnings printed in JSON.

Libraries

Arc<[T; N]>, Box<[T; N]>, and Rc<[T; N]>, now implement TryFrom<Arc<[T]>>,TryFrom<Box<[T]>>, and TryFrom<Rc<[T]>> respectively. Note These conversions are only available when N is 0..=32.
You can now use associated constants on floats and integers directly, rather than having to import the module. e.g. You can now write u32::MAX or f32::NAN with no imports.
u8::is_ascii is now const.
String now implements AsMut<str>.
Added the primitive module to std and core. This module reexports Rust's primitive types. This is mainly useful in macros where you want avoid these types being shadowed.
Relaxed some of the trait bounds on HashMap and HashSet.
string::FromUtf8Error now implements Clone + Eq.

Stabilized APIs

Cargo

You can now set config [profile]s in your .cargo/config, or through your environment.
Cargo will now set CARGO_BIN_EXE_<name> pointing to a binary's executable path when running integration tests or benchmarks. <name> is the name of your binary as-is e.g. If you wanted the executable path for a binary named my-programyou would use env!("CARGO_BIN_EXE_my-program").

Misc

Certain checks in the const_err lint were deemed unrelated to const evaluation, and have been moved to the unconditional_panic and arithmetic_overflow lints.

Compatibility Notes

Having trailing syntax in the assert! macro is now a hard error. This has been a warning since 1.36.0.
Fixed Self not having the correctly inferred type. This incorrectly led to some instances being accepted, and now correctly emits a hard error.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

Version 1.43.1 (2020-05-07)

Version 1.43.0 (2020-04-23)

Language