Running Little Rust Snippets

Leaving the Comfort (and Restrictions) of Cargo

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 invocation will compile and link a program against those dependencies (rustc is an unusually intelligent compiler)

$ 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!

You can even - on Unix platforms - add a 'shebang' line to invoke runner:

$ cat hello
#!/usr/bin/env runner
println!("Hello, World!");

$ ./hello
Hello, World!

runner 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; you can edit it later with runner --edit-prelude.

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

Adding External Crates

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 0.34s to build on my machine, but building statically with runner -s print.rs takes 0.55s.

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 ? in snippets instead of the ubiquitous and awful unwrap, since the boilerplate encloses code in a function that returns Result<(),Box<Error>> which is 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
  -X, --wild... (string) like -x but implies wildcard use

  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 documentation
  --edit-prelude edit the default prelude for snippets
  --alias (string...) crate aliases in form alias=crate_name (used with -x)

  Dynamic compilation:
  -P, --crate-path show path of crate source in Cargo cache
  -C, --compile  compile crate dynamically (limited)
  --cfg... (string) pass configuration variables to rustc
  --libc  link dynamically against libc (special case)

  <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.)

Dynamic Linking

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 arguments:

• 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 harder, because dynamic linking is not a priority for Rust tooling at the moment. So we have to build more elaborate libraries without the help of Cargo. (The following assumes that you have already brought in regex for a Cargo project, so that the Cargo cache is populated, e.g. with runner --add regex)

runner -C --cfg 'feature="default"' --cfg 'feature="use_std"' libc
runner -C --libc memchr
runner -C --libc thread-id
runner -C --cfg 'feature="std"'  void
runner -C utf8-ranges
runner -C unreachable
runner -C aho-corasick
runner -C lazy_static
runner -C thread_local
runner -C regex-syntax
runner -C regex

This script drives home how tremendously irritating life in Rust would be without Cargo. We have to track the dependencies, ensure that the correct default features are enabled in the compilation, and special-case crates which directly link to libc.

However, the results feel worthwhile. Compiling the first regex documented example:

extern crate regex;
use regex::Regex;
let re = Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
assert!(re.is_match("2014-01-01"));

With a static build (-s) I get 0.90s on this machine, and 0.47s with dynamic linking. On my souped-up office machine, it's 0.62s versus 0.32s.

A useful trick - if you want to look at the Cargo.toml of an already downloaded crate to find out dependencies and features, then this command will open it for you:

favorite-editor $(runner -P some-crate)/Cargo.toml

Rust on the Command-line

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"

With long crate names like this, you can define aliases:

$ runner --alias es=easy_shortcuts
$ runner -xes -e 'es::argn_err(1,"gimme an arg!")'
...

By default, runner -e does a dynamic link, and there are known limitations. By also using --static, you can evaluate expressions against crates 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 }

'-X' is like '-x' except it brings all the crate's symbols into scope. Not something you would overdo in regular code, but it makes for shorter command lines - the last example becomes (note how short flags can be combined):

$ runner -sXtime -e "now()"
...

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