This change provides a common convention for using allocator-dependent
features with:
#![cfg(feature = "alloc")]
When available, `Vec` is imported consistently as `prelude::Vec`, which
means modules that need access to `Vec` can simply do:
use prelude::*;
and if an allocator is available, `Vec` will be in the crate prelude.
This allows all `alloc` vs `std` gating to be handled in `lib.rs`,
`build.rs`, and `prelude.rs` so the rest of the codebase doesn't have to
do any gating whatsoever.
Each backend can now be selected by an individual feature:
- `u32_backend` for `backend::u32`;
- `u64_backend` for `backend::u64`;
- `avx2_backend` for `backend::avx2`;
The `u64_backend` is selected by default, since most people use X64 and we have
no way to select based on target (see discussion in #126). However, these
changes mean that it is possible to select the backend explicitly, and if we
had the ability to select target-default features, we could do so easily.
Since Criterion can only benchmark public API, these changes just drop
all internal benchmarks (e.g., benchmarks for field operations). But
those are usually microbenchmarks whose meaning is kind of questionable
anyways, so I don't think this is a big loss.
The `bench` feature disappears, since Criterion works on stable Rust.
libcollections was recently merged into liballoc:
https://github.com/rust-lang/rust/pull/42648
I went ahead and also added an "alloc" feature which no_std users can use to opt
into liballoc features (i.e. any code using Vec). This should have no effect on
anything but no_std usage. It does make it possible for people without
allocators to use curve25519-dalek if they want though. Might be nice for
"bare metal" development.
All that said, from what I can gather liballoc, while not "stable", should
likely stick around for the forseeable future.
Some backstory on the liballoc/libcollections merge here:
https://github.com/rust-lang/rust/pull/42565
