Nightly feature usage

[dhardy]

KAS currently requires may use unstable language features when using nightly Rust.

Features enabled by the nightly feature flag (optional but recommended):

• nightly-diagnostics — used in kas-macros for better diagnostics
• min_specialization — used by the Label widget to select appropriate draw routine no longer used
• generic_associated_types — used in several places where associated types require a lifetime parameter, and desired to use in several more places. unsafe and boxing/cloning used as workarounds. This feature is now stable!

Additional (not generally recommended) nightly features:

• spec — the TryFormat utility uses specialisation to impl Debug where available on a generic type
• doc_cfg — enables annotation of feature flags in documentation

Features not used but desired:

• if-let-chains Now stable
• permit _ in type aliases
• Equality constraints in where clauses
• Extended GATs (object-safe traits using GATs for lifetimes)
• Some way to simulate &mut reborrows in user code
• Direct support for anonymous trait object constructions (i.e. a custom, single-instance type definition, trait implementation, and instantiation). Related: structural records (but support for attribute macros and custom trait implementations is required).
• Inherent associated types

[dhardy]

new_uninit is now behind a nightly feature gate.

[dhardy]

Since #57 we depend on two more nightly features:

• unsize — tracking issue
• (optional) generic_associated_types — tracking issue

[dhardy]

#101 enables compatibility with stable Rust 🎉

This issue should remain open to track usage of nightly features.

[dhardy]

proc_macro_hygiene was stabilised in Rust 1.45.0.

[dhardy]

Updated: we now use min_specialization and no longer use new_uninit.

[dhardy]

Updated:

• const_generics is stable now
• (full) specialization is used, but only for a utility
• unsize is no longer used

[dhardy]

Data models could be simplified with return position impl trait in traits.

Edit: type_alias_impl_trait doesn't allow simplification of the traits, but it does allow simplified impls:

type RowKeyIter<'b> = impl Iterator<Item = u8>;

[dhardy]

Proc-macro lints (see rust-lang/libs-team#524) are highly desirable:

• Currently, proc-macros can only show warnings on nightly using the nightly-diagnostics feature
• Without this or nightly-diagnostics, all proc-macro emitted errors are "fatal", resulting in various secondary errors which are not useful (e.g. error[E0432]: unresolved import event_config::EventConfig due to an error in the EventConfig widget)
• Without this, warnings cannot be ignored; e.g. the #[autoimpl] attribute on trait Tile emits a warning which is generally valid but should in this case be allowed