`OsRng` in Plonky2 breaks no-std

[nikitamasych]

Hello guys!

Prerequisite

I see in field/src/types.rs following usage:

use rand::rngs::OsRng;

For that, you add dependency as below:

rand = { workspace = true, features = ["getrandom"] }

Problem

I am working on a project, where strict no-std compatibility is required. After adding plonky2 with no default features, it came to my surprise that there was a compilation failure regarding getrandom being dependent on std.

Upon investigating, I discovered, that getrandom does not support my target and, as a result, an error was thrown causing linking to std.

In my specific case, I don't actually need sampling functionality.

Proposed Solution

I suggest adding a feature-flag, where default will remain as it was with OsRng, however, as an opt-in path for projects, as mine, that might not have a need or support for OsRng, seeded solution.