# 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.
## Configuration
```
USAGE:
cargo inspect [OPTIONS] <INPUT_FILE>
FLAGS:
-h, --help Prints help information
--plain Don't highlight output
-V, --version Prints version information
-v, --verbose Print the original code as a comment above the desugared code
OPTIONS:
--unpretty <unpretty> rustc "unpretty" parameters [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 the compiler backend easier to maintain.
The goal is to make the compiler more approachable for mere.
Mystery! Exploration! Discovery!
Read more on the background of `cargo-inspect` [on my blog](https://matthias-endler.de/2018/cargo-inspect/).
## Code Examples
### `If-let` gets desugared into `match`
Consider the following code snippet:
```rust
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.
### 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`](https://github.com/mre/prettyprint), for syntax-highlighting,
which is just a wrapper around the awesome
[syntect](https://github.com/trishume/syntect/blob/master/examples/syncat.rs)
and [bat](https://github.com/sharkdp/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. :blush:
## 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.
[rustup]: https://rustup.rs/