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 ~/.cargo/.runner/bin, prefixed with a 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 goes into the same directory as the expanded code - but the dynamically-linked version can't be run standalone unless you make the Rust runtime available globally.

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

$ runner --create '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 -h
Compile and run small Rust snippets
  -s, --static build statically (default is dynamic)
  -O, --optimize optimized static build
  -e, --expression evaluate an expression
  -i, --iterator evaluate an iterator
  -n, --lines evaluate expression over stdin; 'line' is defined
  -x, --extern... (string) add an extern crate to the snippet

  Cache Management:
  --create (string...) initialize the static cache with crates
  --add  (string...) add new crates to the cache (after --create)
  --edit  edit the static cache Cargo.toml
  --build rebuild the static cache
  --doc  display
  --edit-prelude edit the default prelude for snippets

  Dynamic compilation:
  -P, --crate-path show path of crate source in Cargo cache
  -C, --compile  compile crate dynamically (limited)

  <program> (string) Rust program, snippet or expression
  <args> (string...) arguments to pass to program

You can say runner --edit to edit the static cache Cargo.toml, and runner --build 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 --doc 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

And then you can run the json.rs example without -s.

The --compile action takes three kinds of argument:

• a crate name that is already loaded and known to Cargo
• a Cargo directory
• a Rust source file - the crate name is the file name without extension.

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.

There are a few Perl-inspired features. The -e flag compiles and evaluates an expression. You can use it as an unusually strict desktop calculator:

$ runner -e "10 + 20*4.5"
error[E0277]: the trait bound `{integer}: std::ops::Mul<{float}>` is not satisfied
  --> temp/tmp.rs:20:22
   |
20 |     let res = 10 + 20*4.5;
   |                      ^ no implementation for `{integer} * {float}`

Likewise, you have to say 1.2f64.sin() because 1.2 has ambiguous type.

--expression is very useful if you quickly want to find out how Rust will evaluate an expression - we do a debug print for maximum flexibility.

$ runner -e 'PathBuf::from("bonzo.dog").extension()'
Some("dog")

(This works because we have a use std::path::PathBuf in the runner prelude.)

-i (or --iterator) evaluates iterator expressions and does a debug dump of the results:

$ runner -i '(0..5).map(|i| (10*i,100*i))'
(0, 0)
(10, 100)
(20, 200)
(30, 300)
(40, 400)

Any extra command-line arguments are available for these commands, so:

$ runner -i 'env::args().enumerate()' one 'two 2' 3
(0, "/home/steve/.cargo/.runner/bin/tmp")
(1, "one")
(2, "two 2")
(3, "3")

And finally -n (or --lines) evaluates the expression for each line in standard input:

$ echo "hello there" | runner -n 'line.to_uppercase()'
"HELLO THERE"

The -x flag (--extern) allows you to insert an extern crate into your snippet. This is particularly useful for these one-line shortcuts. For example, my easy-shortcuts crate has a couple of helper functions:

$ runner -xeasy_shortcuts -e 'easy_shortcuts::argn_err(1,"gimme an arg!")' 'an arg'
"an arg"
$ runner -xeasy_shortcuts -e 'easy_shortcuts::argn_err(1,"gimme an arg!")'
/home/steve/.cargo/.runner/bin/tmp error: no argument 1: gimme an arg!

This also applies to --iterator:

$ runner -xeasy_shortcuts -i 'easy_shortcuts::files(".")'
"json.rs"
"print.rs"

By default, runner -e does a dynamic link, and there are known limitations. By also using --static, you can evaluate expressions against crates that can only be compiled as static libraries. So, assuming that we have time in the static cache (runner --add time will do that for you):

$ runner -s -xtime -e "time::now()"
Tm { tm_sec: 34, tm_min: 4, tm_hour: 9, tm_mday: 28, tm_mon: 6, tm_year: 117,
tm_wday: 5, tm_yday: 208, tm_isdst: 0, tm_utcoff: 7200, tm_nsec: 302755857 }

If you can get away with dynamic linking, then runner can make it easy to test a module interactively. In this way you get much of the benefit of a fully interactive interpreter (a REPL):

$ cat universe.rs
pub fn answer() -> i32 {
    42
}
$ runner -C universe.rs
$ runner -xuniverse -e "universe::answer()"
42