Well, there is mrustc[0], a Rust compiler that doesn't include a borrow-checker, so it's possible to compile (at least some versions of) Rust without a borrow checker, though it might not result in the most optimized code.

AFAIK there are some optimization like the infamous `noalias` optimization (which took several tries to get turned on[1]) that uses information established during borrow checking.

I'm also not sure what the relation with NLL (non-lexical lifetimes) is, where I would assume you would need at least a primitive borrow-checker to establish some information that the backend might be interested in. Then again, mrustc compiles Rust versions that have NLL features without a borrow-checker, so it's again probably more on the optimization side than being essential.

[0]: https://github.com/thepowersgang/mrustc

[1]: https://stackoverflow.com/a/57259339

I don’t think you need lifetimes for the noalias optimization, because it’s a guarantee of &mut, every single one of them, not their exact lifetimes.

Also the slowness is not because of lifetime checking. `cargo check` is very fast compared to `cargo build`.

The borrow checker determines the set of valid programs.

mrustc gets past not having a borrow checker by compiling invalid programs that would have otherwise had compiler errors. This, at best, results in runtime segfaults etc.

Just like you don't need to validate syntax if you just assume you're only ever fed valid syntax.

Nothing wrong with that, just want to clarify for people.

I don't think it strictly does, no.