cargo-public-api

List and diff the public API of Rust library crates between releases and commits. Detect breaking API changes and semver violations via CI or a CLI. Relies on and automatically builds rustdoc JSON, for which a recent version of the Rust nightly toolchain must be installed.

Installation

Install the cargo public-api subcommand with a recent regular stable Rust toolchain:

$ cargo install cargo-public-api --locked

Ensure nightly-2023-05-24 or later is installed (does not need to be the active toolchain) so cargo public-api can build rustdoc JSON for you:

$ rustup install nightly --profile minimal

Usage

List the Public API

This example lists the public API of the regex crate. First we clone the repo:

$ git clone https://github.com/rust-lang/regex ; cd regex

Now we can list the public API of regex by running

$ cargo public-api

which will print the public API of regex with one line per public item in the API:

Diff the Public API

… Against a Specific Published Version

To diff the public API of the regex crate in the current directory against published version 1.6.0 on crates.io:

$ cargo public-api diff 1.6.0

… Against the Latest Published Version

$ cargo public-api diff latest
Resolved `diff latest` to `diff 1.7.1`
[...]

… Between Git Refs

$ cargo public-api diff ref1..ref2
[...]

… as a CI Check

With a regular cargo test that you run in CI you will be able to

• prevent accidental changes to your public API
• review the public API diff of deliberate changes

First add the latest versions of the necessary libraries to your [dev-dependencies]:

$ cargo add --dev \
    rustup-toolchain \
    rustdoc-json \
    public-api \
    expect-test

Then add the following test to your project. As the author of the below test code, I hereby associate it with CC0 and to the extent possible under law waive all copyright and related or neighboring rights to it:

#[test]
fn public_api() {
    // Install a compatible nightly toolchain if it is missing
    rustup_toolchain::install(public_api::MINIMUM_NIGHTLY_RUST_VERSION).unwrap();

    // Build rustdoc JSON
    let rustdoc_json = rustdoc_json::Builder::default()
        .toolchain(public_api::MINIMUM_NIGHTLY_RUST_VERSION)
        .build()
        .unwrap();

    // Derive the public API from the rustdoc JSON
    let public_api = public_api::Builder::from_rustdoc_json(rustdoc_json)
        .build()
        .unwrap();

    // Assert that the public API looks correct
    expect_test::expect_file!["public-api.txt"].assert_eq(&public_api.to_string());
}

Before you run the test the first time you need to bless the current public API:

$ UPDATE_EXPECT=1 cargo test public_api

This creates a tests/public-api.txt file in your project that you git add together with your other project files. Whenever you change the public API, you need to bless it again with the above command. If you forget to bless, the test will fail, together with instructions on how to bless.

Less Noisy Output

For completeness, items belonging to Blanket Implementations, Auto Trait Implementations, and Auto Derived Implementations, such as

• impl<T, U> Into<U> for T where U: From<T>
• impl Sync for ...
• impl Debug for ... / #[derive(Debug)]

are included in the list of public items by default. Use

• --omit blanket-impls
• --omit auto-trait-impls
• --omit auto-derived-impls

respectively to omit such items from the output to make it much less noisy. For convenience you can also use -s (--simplified) to achieve the same thing:

$ cargo public-api --omit blanket-impls,auto-trait-impls,auto-derived-impls

$ cargo public-api -sss    # Shorter form of above command

Expected Output

Output aims to be character-by-character identical to the textual parts of the regular cargo doc HTML output. For example, this item has the following textual representation in the rendered HTML:

pub fn input_files<I, P>(&mut self, paths: I) -> &mut Self
where
    I: IntoIterator<Item = P>,
    P: AsRef<Path>,

and cargo public-api renders this item in the following way:

pub fn bat::PrettyPrinter::input_files<I, P>(&mut self, paths: I) -> &mut Self where I: IntoIterator<Item = P>, P: AsRef<Path>

If we remove newline characters and add some whitespace padding to get the alignment right for side-by-side comparison, we can see that they are exactly the same, except an irrelevant trailing comma:

pub fn                     input_files<I, P>(&mut self, paths: I) -> &mut Self where I: IntoIterator<Item = P>, P: AsRef<Path>,
pub fn bat::PrettyPrinter::input_files<I, P>(&mut self, paths: I) -> &mut Self where I: IntoIterator<Item = P>, P: AsRef<Path>

Compatibility Matrix

Contributing

See CONTRIBUTING.md.

Maintainers

• Enselic
• douweschulte
• Emilgardis