Write user documentation for Rust contracts

README.md

@@ -90,7 +90,7 @@ One of the [design goals](https://github.com/ewasm/design/blob/master/rationale.
 At present, we've developed support for the following languages and toolchains:
 
 - [C/C++ (LLVM) WebAssembly tutorial](./clang.md)
-- Rust: documentation pending
+- [Rust](./rust.md)
 - [AssemblyScript](https://github.com/AssemblyScript/assemblyscript), a subset of TypeScript, which uses the JavaScript toolchain: see the [etherts org](https://github.com/etherts/docs) for more information on writing contracts in AssemblyScript.
 
 If you're interested in adding support for another language, framework, or toolset, see the Contributing section above and reach out.

rust.md

@@ -0,0 +1,146 @@
+# Writing Ewasm contracts in Rust
+
+Rust is a modern systems programming language. 
+Its type system and memory model allow for strong assumptions about memory safety at little or no runtime cost.
+Furthermore, its sophisticated compiler optimizations minimize the overhead of high-level language features.
+
+Such a combination of performance and safety lends itself to use in developing Ethereum smart contracts. 
+This document serves as a comprehensive guide to using Rust in your Ewasm project.
+
+### Assumptions
+
+This document assumes:
+* A Unix-like OS (although hopefully some of this document is applicable to Windows as well)
+* The presence of a working Rust and `cargo` installation via `rustup`
+* Basic familiarity with the command line, Rust, and Ethereum.
+
+## 1. Toolchain Setup
+
+In order to compile to WebAssembly, Rust needs the WebAssembly backend installed.
+
+`rustup` makes this extremely simple. Just run:
+```console
+$ rustup target add wasm32-unknown-unknown
+```
+Make sure you are installing the target `wasm32-unknown-unknown` and not `wasm32-unknown-emscripten`.
+
+## 2. Configuration and Build
+
+Now that the WebAssembly backend is installed, it can be used to compile almost any Rust project to a Wasm binary.
+However, Ewasm also specifies a set of imports and exports for contracts. These are roughly analogous to symbols in a native binary.
+
+In order to properly export and import said symbols, the project must be compiled as a "shared library."
+While this has no meaning in WebAssembly (all binaries are modules), the flag ensures that the compiler will export and import the correct symbols.
+
+To do this, ensure that the project uses `lib.rs` as the crate root (not `main.rs`), and add the following lines to `Cargo.toml`:
+```toml
+[lib]
+crate-type = ["cdylib"]
+```
+Compile the project using the `--target` flag:
+```console
+$ cargo build --target=wasm32-unknown-unknown
+```
+Alternatively, to set WebAssembly as a default target, one can specify it in `.cargo/config`. 
+Create a `.cargo` directory in the project root, if it does not exist already:
+```console
+$ mkdir .cargo
+```
+Proceed to create or modify `.cargo/config` and add the following:
+```toml
+[build]
+target = "wasm32-unknown-unknown"
+```
+Once this is done, `cargo build` will compile the project to WebAssembly by default.
+
+### Optimizing Ewasm binaries
+
+By default, the Rust compiler can generate huge WebAssembly binaries, even in release mode. 
+These include lots of unnecessary cruft and are not suitable for use as Ewasm contracts.
+
+The following steps will show how to optimize for size and remove unnecessary code segments from the resulting Wasm binary.
+
+#### Compiler optimizations
+
+The simplest way to slim down Rust-generated binaries is by enabling certain compiler optimizations:
+
+* Enabling `lto`, or link-time optimizations, allows the compiler to prune or inline parts of the code at link-time.
+* Setting `opt-level` to `'s'` tells the compiler to optimize for binary size rather than speed. `opt-level = 'z'` will optimize more aggressively for size, although at the cost of performance.
+
+Enable these options in the `release` profile by adding the following to `Cargo.toml`:
+```toml
+[profile.release]
+lto = true
+opt-level = 's'
+```
+
+#### Using [wasm-snip](https://github.com/rustwasm/wasm-snip)
+
+The Rust compiler, by default, includes panicking and formatting code in generated binaries. For the purpose of Ewasm contracts, this is useless.
+Using the `wasm-snip` tool, it is possible to replace the function bodies of such code with a single `unreachable` opcode, further shrinking the binary size.
+
+`wasm-snip` can be easily installed using `cargo`:
+```console
+$ cargo install wasm-snip
+```
+`wasm-snip` depends on the name section in the Wasm binary being present. Therefore, we must build with `debug` enabled. Add the following to the `[profile.release]` section in `Cargo.toml`:
+```toml
+debug = true
+```
+After build, run `wasm-snip` on the resulting binary. It should be located in `target/wasm32-unknown-unknown/release`:
+```console
+$ wasm-snip --snip-rust-panicking-code --snip-rust-fmt-code input.wasm -o output.wasm
+```
+
+## 3. Writing Ewasm contracts
+
+In addition to the normal constraints of the blockchain, the Ewasm [specification](https://github.com/ewasm/design) makes writing code for Ewasm contracts a little different from writing plain Rust.
+
+The following is a list of rules and best practices for writing Ewasm contracts.
+
+### `lib.rs`, not `main.rs`
+
+As stated earlier, using `main.rs` as a crate root will produce an executable rather than a library.
+
+### Export `main`
+
+Every Ewasm contract is required to export a `main` function as the main entry point for execution. In Rust, simply add the `pub` and `extern "C"` qualifiers to do this:
+```rust
+#[no_mangle]
+pub extern "C" fn main() {
+	// function body goes here
+}
+```
+The `no_mangle` directive should be included as well, to ensure that the compiler will not mangle the `main` identifier and invalidate it.
+
+### Use `finish` and `revert` to end execution
+
+When writing code for a native platform, ending execution properly is usually handled behind the scenes. In Ewasm, there are two EEI functions that signal to end execution:
+
+* [`finish`](https://github.com/ewasm/design/blob/master/eth_interface.md#finish) behaves like EVM's `return`. Simply put, it sets the output data and halts the VM.
+* [`revert`](https://github.com/ewasm/design/blob/master/eth_interface.md#revert) is the exact same thing as EVM's `revert`. It halts the VM and refunds any gas.
+
+### Minimize the use of the standard library
+
+Many constructs in Rust's standard library are not optimized for usage in Ewasm, or are outright unavailable due to dependence on a native syscall interface.
+
+Minimizing the use of non-primitive standard library constructs will improve the size, performance, and gas efficiency of resulting Wasm binaries. Where possible, follow these rules of thumb:
+* Minimize the usage of the heap, in general. Don't use `Box<T>` or `Rc<T>`, or any other smart pointers.
+* Avoid using types with implicitly heap-allocated data. Use arrays instead of `Vec`, and `str` instead of `String`.
+* Borrowing is your friend. Stop using `clone()` and passing non-trivial structs by value, use lifetime parameters and references instead.
+
+### Use the [ewasm-rust-api](https://github.com/ewasm/ewasm-rust-api)
+
+The `ewasm-rust-api` is a convenient library providing safe wrappers around the native EEI functions, in addition to a set of useful EVM-style types.
+
+To use it in your contract, simply add `ewasm_api` as a dependency to `Cargo.toml` and include it in your project.
+
+## 4. Deploying Ewasm contracts
+
+There is one final step that must be done before contract creation: post-processing. 
+
+The Rust compiler, at the moment, does not have fine-grained control over the imports and exports generated other than by their identifiers in the source. As a result, unnecessary exports must be removed and malformed imports corrected.
+
+This can be done using an automated tool such as [wasm-chisel](https://github.com/wasmx/wasm-chisel), or by hand in the `.wast` file produced by disassembling the Wasm binary.
+
+If done by hand, the most common errors will be import names being prefixed with `ethereum_` and globals or tables being unnecessarily exported.