Make std an optional dependency. Require alloc instead.

[briansmith]

This raises the minimum version of Rust required to 1.36.

[josephlr]

This is great! If you're making backwards-incompatible changes to the Cargo features, it might be worth reconsidering the dev_urandom_fallback feature.

If I'm understanding the motivation correctly, the main reason to not have the dev_urandom_fallback feature is to avoid depending on libstd. However, if we now have separate std and alloc features, it might make sense to have the std feature control if we read from /dev/urandom on Linux.

Once #558 is fixed, there's not a good security reason to disable the fallback. However, this might be better addressed in a followup PR.

[josephlr]

Just a few cfg related nits.

Edit: I now see that the commit messages that said alloc is required for testing. If that's the case then adding the following code to lib.rs might help:

#[cfg(all(test, not(feature = "alloc")))]
compile_error!("Testing requires the alloc feature");

[josephlr]

feature = "std"

[josephlr]

This still needs a #[cfg(test)] in order for cargo build --no-default-features to work.

[josephlr]

Actually, #[cfg(feature = "alloc")] is what you want here, so that the integration tests keep working.

[josephlr]

#[cfg(feature = "alloc")] is all that's needed here, as test requires alloc.

In general, there a lot of #[cfg(any(test, feature = "alloc"))] in the code. These should be simplified to #[cfg( feature = "alloc")]

[josephlr]

#[cfg(feature = "std")] is what's needed here, otherwise the current cfg make tests require std.

[josephlr]

If #[cfg(feature = "alloc")] is added to lib.rs then these annotations are not needed in this file (here and elsewhere).

[briansmith]

Thanks. I updated the PR to address most of your concerns.

There is more work that needs to be done to properly support testing for no_std environments, e.g. adding the non-default configurations to CI. Those changes will have to wait until my pipeline is cleared.

[josephlr]

Looks awesome. cargo test --no-default-features --features=alloc and cargo build --no-default-features work w/o errors.

[briansmith]

Thanks for the review!