cargo-eval

cargo-eval is a Cargo subcommand designed to let people quickly and easily run Rust "scripts" which can make use of Cargo's package ecosystem. It can also evaluate expressions and run filters.

Some of cargo-eval's features include:

• Reading Cargo manifests embedded in Rust scripts.
• Caching compiled artefacts (including dependencies) to amortise build times.
• Supporting executable Rust scripts via UNIX hashbangs and Windows file associations.
• Evaluating expressions on the command-line.
• Using expressions as stream filters (i.e. for use in command pipelines).
• Running unit tests and benchmarks from scripts.
• Custom templates for command-line expressions and filters.

Note: cargo-eval does not work when Cargo is instructed to use a target architecture different to the default host architecture.

Table of contents:

• Installation
  • Features
  • Manually Compiling and Installing
  • Self-Executing Scripts
• Usage
  • Scripts
  • Expressions
  • Stream Filters
  • Environment Variables
  • Templates
• Known Issues
• License
  • Contribution

Installation

The recommended method for installing cargo-eval is by using Cargo's install subcommand:

cargo install cargo-eval

If you have already installed cargo-eval, you can update to the latest version by using:

cargo install --force cargo-eval

Cargo Features

The following features are defined:

• suppress-cargo-output (default): if building the script takes less than 2 seconds and succeeds, cargo-eval will suppress Cargo's output. Note that this disabled coloured Cargo output on Windows.

Manually Compiling and Installing

cargo-eval requires Rust 1.11 or higher to build. Rust 1.4+ was supported prior to version 0.2.

Once built, you should place the resulting executable somewhere on your PATH. At that point, you should be able to invoke it by using cargo eval.

Self-Executing Scripts

On UNIX systems, you can use #!/usr/bin/env cargo eval -- as a hashbang line in a Rust script. If the script file is executable, this will allow you to execute a script file directly.

If you are using Windows, you can associate the .crs extension (which is simply a renamed .rs file) with cargo-eval. This allows you to execute Rust scripts simply by naming them like any other executable or script.

This can be done using the cargo eval file-association command. This command can also remove the file association. If you pass --amend-pathext to the file-assocation install command, it will also allow you to execute .crs scripts without having to specify the file extension, in the same way that .exe and .bat files can be used.

If you want to make a script usable across platforms, it is recommended that you use both a hashbang line and give the file a .crs file extension.

Usage

Generally, you will want to use cargo-eval by invoking it as cargo script (note the lack of a hypen). Doing so is equivalent to invoking it as cargo-eval script. cargo-eval supports several other subcommands, which can be accessed by running cargo-eval directly. You can also get an overview of the available options using the --help flag.

Scripts

The primary use for cargo-eval is for running Rust source files as scripts. For example:

$ echo 'fn main() { println!("Hello, World!"); }' > hello.rs
$ cargo eval hello.rs
Hello, World!
$ cargo eval hello # you can leave off the file extension
Hello, World!

The output of Cargo will be hidden unless compilation fails, or takes longer than a few seconds.

cargo-eval will also look for embedded dependency and manifest information in the script. For example, all of the following are equivalent:

• now.crs (code block manifest with UNIX hashbang and .crs extension):

  #!/usr/bin/env cargo eval --
  //! This is a regular crate doc comment, but it also contains a partial
  //! Cargo manifest.  Note the use of a *fenced* code block, and the
  //! `cargo` "language".
  //!
  //! ```cargo
  //! [dependencies]
  //! time = "0.1.25"
  //! ```
  extern crate time;
  fn main() {
      println!("{}", time::now().rfc822z());
  }
  

• now.rs (dependency-only, short-hand manifest):

  // cargo-deps: time="0.1.25"
  // You can also leave off the version number, in which case, it's assumed
  // to be "*".  Also, the `cargo-deps` comment *must* be a single-line
  // comment, and it *must* be the first thing in the file, after the
  // hashbang.
  extern crate time;
  fn main() {
      println!("{}", time::now().rfc822z());
  }
  

  Note: you can write multiple dependencies by separating them with commas. E.g. time="0.1.25", libc="0.2.5".

On running either of these, cargo-eval will generate a Cargo package, build it, and run the result. The output may look something like:

$ cargo eval now
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling winapi-build v0.1.1
   Compiling winapi v0.2.8
   Compiling libc v0.2.30
   Compiling kernel32-sys v0.2.2
   Compiling time v0.1.38
   Compiling now v0.1.0 (file:///C:/Users/drk/AppData/Local/Cargo/script-cache/file-now-37cb982cd51cc8b1)
    Finished release [optimized] target(s) in 49.7 secs
Sun, 17 Sep 2017 20:38:58 +1000

Subsequent runs, provided the script has not changed, will likely just run the cached executable directly:

$ cargo eval now
Sun, 17 Sep 2017 20:39:40 +1000

Useful command-line arguments:

• --bench: Compile and run benchmarks. Requires a nightly toolchain.
• --debug: Build a debug executable, not an optimised one.
• --features <features>: Cargo features to pass when building and running.
• --force: Force the script to be rebuilt. Useful if you want to force a recompile with a different toolchain.
• --gen-pkg-only: Generate the Cargo package, but don't compile or run it. Effectively "unpacks" the script into a Cargo package.
• --test: Compile and run tests.

Expressions

cargo-eval can also run pieces of Rust code directly from the command line. This is done by providing the --expr option; this causes cargo-eval to interpret the <script> argument as source code instead of as a file path. For example, code can be executed from the command line in a number of ways:

• cargo eval --dep time --expr "extern crate time; time::now().rfc822z().to_string()"
• cargo eval --dep time=0.1.38 --expr "extern crate time; ..." - uses a specific version of time
• cargo eval -d time -e "extern crate time; ..." - short form of above
• cargo eval -D time -e "..." - guess and inject extern crate time; this only works when the package and crate names of a dependency match.
• cargo eval -d time -x time -e "..." - injects extern crate time; works when the names do not match.

The code given is embedded into a block expression, evaluated, and printed out using the Debug formatter (i.e. {:?}).

Useful command-line arguments:

• -d/--dep: add a dependency to the generated Cargo.toml manifest.
• -t/--template: Specify a custom template for this expression (see section on templates).

Stream Filters

You can use cargo-eval to write a quick stream filter, by specifying a closure to be called for each line read from stdin, like so:

$ cat now.crs | cargo eval --loop \
    "let mut n=0; move |l| {n+=1; println!(\"{:>6}: {}\",n,l.trim_right())}"
   Compiling loop v0.1.0 (file:///C:/Users/drk/AppData/Local/Cargo/script-cache/loop-58079283761aab8433b1)
     1: // cargo-deps: time="0.1.25"
     2: extern crate time;
     3: fn main() {
     4:     println!("{}", time::now().rfc822z());
     5: }

You can achieve a similar effect to the above by using the --count flag, which causes the line number to be passed as a second argument to your closure:

$ cat now.crs | cargo eval --count --loop \
    "|l,n| println!(\"{:>6}: {}\", n, l.trim_right())"
   Compiling loop v0.1.0 (file:///C:/Users/drk/AppData/Local/Cargo/script-cache/loop-58079283761aab8433b1)
     1: // cargo-deps: time="0.1.25"
     2: extern crate time;
     3: fn main() {
     4:     println!("{}", time::now().rfc822z());
     5: }

Note that, like with expressions, you can specify a custom template for stream filters.

Environment Variables

The following environment variables are provided to scripts by cargo-eval:

• CARGO_EVAL_BASE_PATH: the base path used by cargo-eval to resolve relative dependency paths. Note that this is not necessarily the same as either the working directory, or the directory in which the script is being compiled.

• CARGO_EVAL_PKG_NAME: the generated package name of the script.

• CARGO_EVAL_SAFE_NAME: the file name of the script (sans file extension) being run. For scripts, this is derived from the script's filename. May also be "expr" or "loop" for those invocations.

• CARGO_EVAL_SCRIPT_PATH: absolute path to the script being run, assuming one exists. Set to the empty string for expressions.

Templates

You can use templates to avoid having to re-specify common code and dependencies. You can view a list of your templates by running cargo eval templates list, or show the folder in which they should be stored by running cargo eval templates show. You can dump the contents of a template using cargo-eval templates dump NAME.

Templates are Rust source files with two placeholders: #{prelude} for the auto-generated prelude (which should be placed at the top of the template), and #{script} for the contents of the script itself.

For example, a minimal expression template that adds a dependency and imports some additional symbols might be:

// cargo-deps: itertools="0.6.2"
#![allow(unused_imports)]
#{prelude}
use std::io::prelude::*;
use std::mem;
use itertools::Itertools;

fn main() {
    let result = {
        #{script}
    };
    println!("{:?}", result);
}

If stored in the templates folder as grabbag.rs, you can use it by passing the name grabbag via the --template option, like so:

$ cargo eval -t grabbag -e "mem::size_of::<Box<Read>>()"
16

In addition, there are three built-in templates: expr, loop, and loop-count. These are used for the --expr, --loop, and --loop --count invocation forms. They can be overridden by placing templates with the same name in the template folder. If you have not overridden them, you can dump the contents of these built-in templates using the templates dump command noted above.

License

Licensed under either of

• MIT license (see LICENSE or http://opensource.org/licenses/MIT)
• Apache License, Version 2.0 (see LICENSE or http://www.apache.org/licenses/LICENSE-2.0)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.