cargo-inspect

What is Rust doing behind the scenes?

There are only two ways to live your life.
One is as though nothing is a miracle. The other is as though everything is a miracle. -- Albert Einstein

Installation

You need Rust nightly and rustfmt to get started.
You can install those via rustup:

rustup install nightly
rustup component add rustfmt

All set? Let's get cracking!

cargo install cargo-inspect

Usage

Call it on any Rust file:

cargo inspect main.rs

If you don't specify a file, the current crate will be analyzed instead.

cargo inspect

Depending on the size of the crate, this might take a while.
Please be patient.

It can also compare two file outputs! Try this:

cargo inspect --diff examples/range.rs,examples/range_inclusive.rs --plain

Configuration

USAGE:
    cargo inspect [FLAGS] [OPTIONS] [INPUT_FILE]

FLAGS:
    -h, --help
            Prints help information

        --list-themes
            Should we list all pretty printer themes?

        --plain
            Don't highlight output

    -V, --version
            Prints version information

    -v, --verbose
            Print the original code as a comment above the desugared code


OPTIONS:
        --theme <THEME>
            Specify a theme override for the pretty printer

        --diff <files>
            Diff input files

        --format <format>
            Override for the format that gets outputted when the `unpretty` mode is set to `flowgraph` [default: svg]

        --unpretty <unpretty>
            rustc "unpretty" parameters

            *Note*: For `--unpretty=flowgraph=[symbol]` you need to have `dot` on your PATH. [default: hir]

ARGS:
    <INPUT_FILE>
            Input file

Background

Rust allows for a lot of syntactic sugar, that makes it a pleasure to write. It is sometimes hard, however, to look behind the curtain and see what the compiler is really doing with our code.

To quote @tshepang, "It is good to know what these conveniences are, to avoid being mystified by what's going on under the hood... the less magical thinking we have of the world, the better."

• lifetime elisions
• type inference
• syntactic sugar
• implicit dereferencing
• type coercions
• hidden code (e.g. the prelude)

I was always interested in how programming languages work in the background, how my code was unrolled to make the compiler backend easier to maintain.

The goal is to make the compiler more approachable for mere mortals.
Mystery! Exploration! Discovery!

Read more on the background of cargo-inspect on my blog.

Code Examples

If-let gets desugared into match

Consider the following code snippet:

fn main() {
    if let Some(x) = Some(1) {
        // Do something with x
    }
}

When you compile it, the first thing Rust does is desugar it. To see what the code looks like after this step, run

cargo inspect examples/if_let.rs

This produces the following output:

You can see that the if let was desugared into a match statement.

To change the colorscheme, try cargo-inspect --list-themes, e.g.

cargo inspect examples/if_let.rs --theme GitHub

Oh, and if you have graphviz installed, you can also print a pretty flowgraph from your code:

cargo inspect --unpretty=flowgraph=main examples/if_let.rs

More examples

Please find more examples in the examples folder. You can also contribute more.

The Magic Sauce

The best things in the world are assembled from simple building blocks. This tool stands on the shoulders of giants. To work its magic, it runs the following commands:

1. rustc -Zinspect=hir, for retrieving the HIR.
2. rustfmt, for formatting the output.
3. prettyprint, for syntax-highlighting, which is just a wrapper around the awesome syntect and bat crates.

Contributing

This is a young project, which has downsides and upsides.

• Everything is in flux and things can break at any time. 😫
• There's plenty of opportunity to shape and form the project. 😊

Thus, become a contributor today!

Known issues

As of now, this is a very fragile tool. If it fails, it might will produce horrible output. You have been warned. That said, it won't eat your code, of course. 😊

License

Licensed under either of

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

at your option.

Credits

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