> You can, but it is an extreme pain to do it, It is not extreme, and it isn't even necessary: just reuse one of the many arenas others have already written,

> You can, but it is an extreme pain to do it, It is not extreme, and it isn't even necessary: just reuse one of the many arenas others have already written, e.g. https://crates.io/crates/typed-arena https://crates.io/crates/typed-arena . Copy-on-write is fairly ease to write in Rust, e.g. the even core Rc and Arc types have a get_mut function that does copy-on-write. > You cannot implement many data structures in a decent performance way without unsafe, so Rust guarantee is weak. You can't implement anything in C or C++ without unsafe: Rust's main strength is the ability to write high performance code with a safe interface, so that only one library has to muddle through the `unsafe` problems (if they have to touch `unsafe` at all). Most code one writes is not implementing data structures. As a demonstration of this strength, even very low-level projects like operating systems can get away with not using `unsafe` everywhere by packaging it up into safe interfaces, e.g. http://os.phil-opp.com/modifying-page-tables.html http://os.phil-opp.com/modifying-page-tables.html