Running Little Rust Snippets

Cargo is a good, reliable way to build programs and libraries in Rust with versioned dependencies. Those who have worked with the Wild West practices of C++ development find this particularly soothing, and it's frequently given as one of the strengths of the Rust ecosystem.

However, it's not intended to make running little test programs straightforward - you have to create a project with all the dependencies you wish to play with, and then edit src/main.rs and do cargo run. A useful tip is to create a src/bin directory containing your little programs and then use cargo run --bin NAME to run them. But there is a better way; if you have such a project (say called 'cache') then the following compiler invocation will compile and link a program against those dependencies:

$ rustc -L /path/to/cache/target/debug/deps mytest.rs

Of course, you need to manually run cargo build on your cache project whenever new dependencies are added, or when the compiler is updated.

The runner tool helps to automate this pattern. It also supports snippets, which are little 'scripts' formatted like Rust documentation examples.

$ cat print.rs
println!("Hello, World!")

$ runner print.rs
Hello, World!

It adds the necessary boilerplate and creates a proper Rust program in temp, together with a editable prelude, which is initially:

#![allow(unused_imports)]
#![allow(dead_code)]
use std::fs;
use std::fs::File;
use std::io;
use std::io::prelude::*;
use std::env;
use std::path::{PathBuf,Path};
#[allow(unused_macros)]
macro_rules! debug {
    ($x:expr) => {
        println!(\"{} = {:?}\",stringify!($x),$x);
    }
}

After first invocation of runner, this is found in ~/.cargo/.runner/prelude.

debug! saves typing: debug!(my_var) is equivalent to println!("my_var = {:?}",my_var).

As you can see, runner is very much about playing with small code snippets. By default it links the snippet dynamically which is significantly faster. This hello-world snippet takes 0.337s to build on my machine, but building statically with runner -s print.rs takes 0.545s.

In both cases, the executable is temp/print - the dynamically-linked version can't be run standalone unless you make the Rust runtime available globally.

However, the static option is much more flexible. You can easily create a static cache with some common crates:

$ runner -c 'time json regex'

You can add as many crates if you like - number of available dependencies doesn't slow down the linker. Thereafter, you may refer to these crates in snippets:

// json.rs
extern crate json;

let parsed = json::parse(r#"

{
    "code": 200,
    "success": true,
    "payload": {
        "features": [
            "awesome",
            "easyAPI",
            "lowLearningCurve"
        ]
    }
}

"#)?;

println!("{}",parsed);

And then build statically and run (any extra arguments are passed to the program.)

$ runner -s json.rs
{"code":200,"success":true,"payload":{"features":["awesome","easyAPI","lowLearningCurve"]}}

You can use ? instead of the ubiquitous and awful unwrap, since the boilerplate encloses code in a function that returns Result<(),Box<Error>> - compatible with any error return.

runner provides various utilities for managing the static cache:

$ runner --help
Compile and run small Rust snippets
  -s, --static build statically (default is dynamic)
  -O, --optimize optimized static build
  -c, --create (string...) initialize the static cache with crates
  -e, --edit  edit the static cache
  -b, --build rebuild the static cache
  -d, --doc  display
  -P, --crate-path show path of crate source in Cargo cache
  -C, --compile  compile crate dynamically (limited)
  <program> (string) Rust program or snippet
  <args> (string...) arguments to pass to program

You can say runner -e to edit the static cache Cargo.toml, and runner -b to rebuild the cache afterwards. The cache is built for both debug and release mode, so using -sO you can build snippets in release mode. Documentation is also built for the cache, and runner -d will open that documentation in the browser. (It's always nice to have local docs, especially in bandwidth-starved situations.)

It would be good to provide such an experience for the dynamic-link case, since it is faster. There is in fact a dynamic cache as well but support for linking against external crates dynamically is very basic. It works fine for crates that don't have any external depdendencies, e.g. this creates a libjson.so in the dynamic cache:

$ runner -C json

But anything more complicated is hard; dynamic linking is not a priority for Rust tooling at the moment, and does not support it well enough without terrible hacking and use of unstable features.