Build RISC Zero zkVM guest code and provide handles to the host side.
In order for the host to execute guest code in the [RISC Zero
zkVM](https://docs.rs/risc0-zkvm), the host must be provided a compiled RISC-V ELF file and
the corresponding ImageID. This crate
contains the functions needed to take zkVM guest code, build a corresponding
ELF file and ImageID, and make the ImageID and a path to the ELF file
available for the host to use.
## Using risc0-build to build guest methods
Using this crate can be a bit delicate, so we encourage you to follow along
in our [RISC Zero Rust Starter repository](https://github.com/risc0/risc0-rust-starter).
In that repository, `risc0-build` is used in the
[`methods` directory](https://github.com/risc0/risc0-rust-starter/tree/main/methods).
Guest methods are embedded for the host to use by calling [embed_methods](crate::embed_methods)
(or [embed_methods_with_options](crate::embed_methods_with_options)) in a [build script](https://doc.rust-lang.org/cargo/reference/build-scripts.html).
An example `build.rs` file would look like
```no_run
fn main() {
risc0_build::embed_methods();
}
```
This requires including `risc0-build` as a _build_ dependency. You will also
need add a `[package.metadata.risc0]` section to your cargo file. In this
section, put a `methods` field with a list of relative paths containing the
guest code. For example, if your guest code is in the `guest` directory,
then `Cargo.toml` might include
```toml
[build-dependencies]
risc0-build = "0.17"
[package.metadata.risc0]
methods = ["guest"]
```
This builds a file `methods.rs` in your cargo output directory which you
must then include for the host to use. For example, you might make a file
`src/lib.rs` containing
```text
include!(concat!(env!("OUT_DIR"), "/methods.rs"));
```
This process will generate an image ID (`*_ID`) and the contents of an ELF
file (`*_ELF`). The names will be derived from the name of the ELF
binary, which will be converted to ALL_CAPS to comply with Rust naming
conventions. Thus, if a method binary is named `multiply`, the image ID
will be named `methods::MULTIPLY_ID` and the contents of the ELF file will
be named `methods::MULTIPLY_ELF`. These are included at the beginning
of the host-side code:
```text
use methods::{MULTIPLY_ELF, MULTIPLY_ID};
```