Rust in zcashd

zcashd is primarily a C++ codebase, but most new code is being written in Rust where possible.

Adding new dependencies in online-Rust mode

The zcashd build system pins all dependencies, and in order to facilitate deterministic builds, cargo is configured to run in offline mode with vendored crates. This means that if, for example, you add the foobar crate to Cargo.toml, you will likely see an error similar to this:

$ cargo check
error: no matching package named `foobar` found
location searched: registry `https://github.com/rust-lang/crates.io-index`
required by package `librustzcash v0.2.0 (/path/to/zcash)`

Instead, you first need to build zcashd in online-Rust mode:

CONFIGURE_FLAGS=--enable-online-rust ./zcutil/build.sh

After doing so, you can add a new dependency as follows:

1. Add the new dependency to Cargo.toml.
2. Run cargo check to update the Cargo.lock file.
3. Commit Cargo.toml and Cargo.lock.

Using a local Rust dependency

During development, you can use a locally checked out version of a dependency by applying a cargo patch.

For example, to use a local version of the orchard crate that includes a new API, add the following patch to Cargo.toml:

[dependencies]
# This dependency is listed with a version, meaning it comes from crates.io; the
# patch goes into a [patch.crates-io] section.
orchard = "0.0"
...

[patch.crates-io]
# Comment out any existing patch, if present.
# orchard = { git = "https://github.com/zcash/orchard.git", rev = "..." }

# Add this patch (both relative and absolute paths work):
orchard = { path = "../relative/path/to/orchard" }

Usually you can apply a patch to use a locally checked out dependency without needing to build zcashd in online-Rust mode. However, if your local changes include a new dependency, you will need to ensure you are in online-Rust mode.