Once installed, the following command prints out the result of macro expansion and #[derive] expansion applied to the current crate.

$ cargo expand

This is a wrapper around the more verbose compiler command:

$ cargo rustc --profile=check -- -Zunpretty=expanded

Install with cargo install cargo-expand.

This command optionally uses rustfmt to format the expanded output. The resulting code is typically much more readable than what you get from the compiler. If rustfmt is not available, the expanded code is not formatted. Install rustfmt with rustup component add rustfmt.

Cargo expand relies on unstable compiler flags so it requires a nightly toolchain to be installed, though does not require nightly to be the default toolchain or the one with which cargo expand itself is executed. If the default toolchain is one other than nightly, running cargo expand will find and use nightly anyway.

#[derive(Debug)]
struct S;

fn main() {
    println!("{:?}", S);
}

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std;
struct S;
#[automatically_derived]
#[allow(unused_qualifications)]
impl ::core::fmt::Debug for S {
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        match *self {
            S => {
                let mut debug_trait_builder = f.debug_tuple("S");
                debug_trait_builder.finish()
            }
        }
    }
}
fn main() {
    {
        ::std::io::_print(::core::fmt::Arguments::new_v1(
            &["", "\n"],
            &match (&S,) {
                (arg0,) => [::core::fmt::ArgumentV1::new(arg0, ::core::fmt::Debug::fmt)],
            },
        ));
    };
}

See cargo expand --help for a complete list of options, most of which are consistent with other Cargo subcommands. Here are a few that are common in the context of cargo expand.

To expand a particular test target:

$ cargo expand --test test_something

To expand without rustfmt:

$ cargo expand --ugly

To expand a specific module or type or function only:

$ cargo expand path::to::module

The cargo expand command reads the [expand] section of $CARGO_HOME/config.toml if there is one (usually ~/.cargo/config.toml).

Set the default syntax highlighting theme with the theme setting:

[expand]
theme = "TwoDark"

Run cargo expand --themes to print a list of available themes. Use theme = "none" to disable coloring.

Change the default coloring disposition (normally auto) with the color setting:

[expand]
color = "always"

Enable paging of the output with the pager setting:

[expand]
pager = true

Be aware that macro expansion to text is a lossy process. This is a debugging aid only. There should be no expectation that the expanded code can be compiled successfully, nor that if it compiles then it behaves the same as the original code.

For instance the following function returns 3 when compiled ordinarily by Rust but the expanded code compiles and returns 4.

fn f() -> i32 {
    let x = 1;

    macro_rules! first_x {
        () => { x }
    }

    let x = 2;

    x + first_x!()
}

Refer to The Book for more on the considerations around macro hygiene.