getrandom VS rand

The crypto source in Go is great, no complaints there. Lints like gosec even recommend using it when generating crypto entropy. Go did a good job here, and I expect Rust will do the same sometime after getrandom reaches 1.0 so the API questions are settled, plus whatever makes sense for the future-proofing the standard library needs.

I would wait for the getrandom crate to reach 1.0, which will answer many of the questions around what an API like this can look like, and then maybe the standard library discussion will be on firmer footing because at least we'll know what API we want to immortalize. Rushing that now just to save people importing a small crate does not seem to be the way to go.

I'd caution against using /dev/random directly, and instead recommend using getrandom. It's effectively the same thing on Haiku and Redox, but is cross-platform and will upgrade to better sources on various platforms as available (such as using the getrandom() call on Linux and Android, or getentropy() on macOs, if avaialable).

getrandom

It's a default, but overwritable behavior, see the #[path] attribute. You still have to create N files for each supported platform, but at the top level you will see only one module. On of the crates which uses this approach in practice is getrandom.

IIUC the main problem which prevents from moving HashMap & co to alloc is lack of API to get system entropy which is required for DOS protection. Ideally we would have a #[global_allocator]-like functionality for retrieving system entropy. Relevant issue: https://github.com/rust-random/getrandom/issues/21

Making compatibility promises for distributions means they cannot take advantage of potential advancements in the field.

Defining an error type that is meaningful, portable, and compatible with no-std isn't straightforward. If the std lib's getrandom requires std, then just like that, rand and many other crates won't use it anyway. Using io::Result seems to me to face this challenge.

Some wasm targets can’t generate random numbers at all but in the case of the book because you are using wasm in a browser you can use JS to generate random numbers. I believe there’s a way to get the rand crate to use JS as the backend for generating rand but its a bit more convoluted than the easy one-liner that the book suggests.

rand

This uses rand and xcomplex to handle the mathematics, png to write image files, and dialoguer and indicatif for some pretty prompts and progress bars.

You can often achieve this without any unsafe by putting an assert!() on the length before the hot loop. For example, I got rid of some unsafe in rand that way.

This is a widely used method to map random integers to floating point numbers, but it has the disadvantage of wasting 1 bit of float mantissa precision.

On modern CPUs, its computational advantage over full-precision mapping methods, such as multiplication by a float, is not always clear [1].

[1] https://github.com/rust-random/rand/issues/416

In fact, there are a lot of crates in Rust where in other programming languages, it would be included in the standard library. Examples are regex, random number generators, additional iterator methods, macros for other collections, num traits, loggers, HTTP libraries, error handling, async runtimes, serialization and deserialization, date and time, and many more.