What is being stabilized

• ! is now a full-fledged type and can now be used in any type position (eg. RFC 1216). The ! type can coerce into any other type, see https://github.com/rust-lang/rust/tree/master/src/test/run-fail/adjust_never.rs for an example.

• Type inference will now default unconstrained type variables to ! instead of (). The resolve_trait_on_defaulted_unit lint has been retired. An example of where this comes up is if you have something like:

  // We didn't specify the type of `x`. Under some circumstances, type inference
  // will pick a type for us rather than erroring
  let x = Deserialize::deserialize(data);

  Under the old rules this would deserialize a (), whereas under the new rules it will deserialize a !.

• The never_type feature gate is stable, although some of the behaviours it used to gate now live behind the new exhaustive_patterns feature gate (see below).

What is not being stabilized

• Exhaustive pattern-matching for uninhabited types. eg.

  let x: Result<u32, !> = ...;
  let Ok(y) = x;

  This code will still complain that Ok(_) is a refutable pattern. This can be fixed by using the exhaustive_patterns feature gate. See RFC 1872 for progress on this issue. See https://github.com/rust-lang/rust/tree/master/src/test/ui/feature-gate-exhaustive-patterns.rs for the testcase which confirms that this behaviour is still gated.