alt Hacker NewsThe lack of use cases in that document is a concern. They're all "nice to have" features, but is the payoff there for real work? The "effects" section mentions properties useful for a proof system. But it's not part of a proof system. If it were, most of those could be found automatically by static analysis, without bothering the programmer. (I was doing that decades ago in a very early proof of correctness system.) Getting programmers to annotate items is tough. Just getting C++ programmers to use "const" enough is hard.
"View types" are interesting. But how much of that generality is really needed? We already have it for arrays, with "split_at_mut" and its friends. That's a library function which uses "unsafe" but exports a safe interface. The compiler will already let you pass two different fields of a struct as "&mut". That covers the two most pressing cases.
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"View types" addresses one of the top pain points of the borrow checker, which is disjoint borrows on a single value. The compiler can understand values where only some fields are borrowed, so e.g. you can mutably borrow the currently-unborrowed fields, but there's no way to write these partial borrows and so you cannot write functions that either take or return these borrows. This often means having to restructure code to split up one struct into several child structs just to be able to borrow those substructs independently.
I would also have liked to see some motivational examples, but I think the most interesting upside of an effect system is composability.
Rust is actually really unique among imperative languages in its general composability - things just compose really well across most language features.
The big missing pieces for composability are higher-kinded types (where you could be generic over Option, Result, etc.), and effects (where you could be generic over async-fn, const-fn, hypothetical nopanic-fn, etc.)
The former becomes obvious with the amount of interface duplication between types like Option and Result. The latter becomes obvious with the number of variants of certain functions that essentially do the same thing but with a different color.