add nightly cargo to prerequisites in README.md

Building a project with a custom toolchain requires nightly cargo. And the only way to get nightly cargo is rustup toolchain install nightly.(though there may be alternatives.)

Without nightly cargo, you can't build core library. If you don't have it, cargo build on your project will throw a bunch of errors, something like

error[E0425]: cannot find function, tuple struct or tuple variant `Some` in this scope
   --> /Users/DicklessGreat/.cargo/registry/src/index.crates.io-6f17d22bba15001f/version_check-0.9.4/src/lib.rs:292:31
    |
292 |         Some(false) => return Some(false),
    |                               ^^^^ not found in this scope

You will find other basic types and literals missing. You can reproduce these errors by 1, rustup toolchain uninstall nightly(at anywhere) 2, cargo +custom-rv32ec build(on your project)

The reason why you can't build core lib is that stable cargo ignores build-std in .cargo/config.toml. Because build-std is unstable cargo feature, we have to use nightly cargo. https://doc.rust-lang.org/cargo/reference/unstable.html#build-std That's why we need to rustup toolchain install nightly and it is a prerequisites. (And as you already notice, it installs unnecessary toolchain, something like nightly-x86_64-apple-darwin on my machine.Is there any way to install nightly cargo only? I'm not 100% sure the best way to get nightly Cargo.)

You can check which version your Cargo is called bycargo --version. And here is the interesting experiment that makes me confirm what caused the core library build failure.

$ cargo --version
cargo 1.74.0 (ecb9851af 2023-10-18)
$ cargo +custom-rv32ec --version
cargo 1.77.0-nightly (84976cd69 2024-01-12)
$ rustup toolchain uninstall nightly
info: uninstalling toolchain 'nightly-x86_64-unknown-linux-gnu'
info: toolchain 'nightly-x86_64-unknown-linux-gnu' uninstalled
$ cargo +custom-rv32ec --version
cargo 1.74.0 (ecb9851af 2023-10-18)

As mentioned in the rustup book, rustup seeks cargo from the toolchain, and if there isn't, seeks your channels.

Because the rust-lang/rust tree does not include Cargo, when cargo is invoked for a custom toolchain and it is not available, rustup will attempt to use cargo from one of the release channels, preferring 'nightly', then 'beta' or 'stable'.

https://rust-lang.github.io/rustup/concepts/toolchains.html

ch32-rs / ch32v00x-hal

add nightly cargo to prerequisites in README.md #5