What are the use cases for zig? The website says general purpose and tool chain but people who have used it, what it excels at?
My understanding is that Zig has all of the power and modernity of Rust, without the strictness and borrow checker. Unlike Rust, it also has powerful compile-time evaluation and custom allocators, and probably more improvements I'm not familiar with (in Rust you can effectively emulate custom allocators, but you have to rewrite every allocating structure to use them; or you can use nightly, but most third-party libra…
This is an oxymoron :) The strictness and borrow checker are part of the power and modernity of Rust.
But even apart from that, Rust has automatic value-based destructors (destructor follows the value as it's moved across scopes and is only called in the final scope), whereas Zig only has scope-based destructors (defer) and you need to remember to write them and ensure they're called exactly once per value. Rust has properly composable Option/Result monads, whereas Zig has special-cased ! and ? which don't compose (no Option of Option or Result of Result) but do benefit from nice built-in syntax due to their special-cased-ness. Rust has typed errors whereas Zig only has integers, though again that allows Zig to have much simpler syntax for defining arbitary error sets which would require defining a combinatorial explosion of enums in Rust.
Of course from Zig's point-of-view these are features, not deficiences, which is completely understandable given what kind of language it wants to be. And the other advantages you listed like comptime (Rust's const-eval is very constrained and has been WIP for ages) and custom allocator support from day 1 (the way Rust is bolting it on will make most existing code unusable with custom allocators, including parts of Rust's own standard library) are indeed good advantages. Zig also has very nice syntax unification - generic types are type constructor functions fn(type) -> type, modules are structs, etc.
I hope that one day we'll have a language that combines the best of Rust's strictness and Zig's comptime and syntax.