The interfaces to custom allocators are more idiomatic, standardized, and normalized in Zig, but there's nothing systematically unsafer about the Rust equivalent. You can use custom allocators all you want in Rust, as long as you're okay using third-party crates like Bumpalo.

On that topic, worth mentioning that Rust's long-awaited `Allocator` trait is perilously close to stabilizing; watch for https://github.com/rust-lang/rust/pull/157428 to be merged, then the stabilization PR to progress here: https://github.com/rust-lang/rust/pull/156882