<p align="center">
  <img alt="semver-checks" src="./brand/banner.png" width="800" />
</p>

# cargo-semver-checks

Lint your crate API changes for semver violations.

- [Quick Start](#quick-start)
- [FAQ](#faq)
- [Configuration](#configuration)
- [Troubleshooting](#troubleshooting)
- [Contributing](https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md)

## Quick Start

```sh
# If you already use `cargo-binstall` for faster tool installations:
$ cargo binstall cargo-semver-checks

# Otherwise:
$ cargo install cargo-semver-checks --locked

# Lint a new release for SemVer breakage before `cargo publish`:
$ cargo semver-checks
```

Or use as a [GitHub Action](https://github.com/obi1kenobi/cargo-semver-checks-action) (used in `.github/workflows/ci.yml` in this repo):

```yaml
- name: Check semver
  uses: obi1kenobi/cargo-semver-checks-action@v2
```

![image](https://user-images.githubusercontent.com/2348618/180127698-240e4bed-5581-4cbd-9f47-038affbc4a3e.png)

Each failing check references specific items in the
[Cargo SemVer reference](https://doc.rust-lang.org/cargo/reference/semver.html)
or other reference pages, as appropriate. It also includes the item name
and file location that are the cause of the problem, as well as a link
to the implementation of that query in the current version of the tool.

## FAQ
- [What Rust versions does `cargo-semver-checks` support?](#what-rust-versions-does-cargo-semver-checks-support)
- [Can I use `cargo-semver-checks` with `nightly` Rust?](#can-i-use-cargo-semver-checks-with-nightly-rust)
- [What if my project needs stronger guarantees around supported Rust versions?](#what-if-my-project-needs-stronger-guarantees-around-supported-rust-versions)
- [Does the crate I'm checking have to be published on crates.io?](#does-the-crate-im-checking-have-to-be-published-on-cratesio)
- [What features does `cargo-semver-checks` enable in the tested crates?](#what-features-does-cargo-semver-checks-enable-in-the-tested-crates)
- [My crate uses `--cfg` conditional compilation. Can `cargo-semver-checks` scan it?](#my-crate-uses---cfg-conditional-compilation-can-cargo-semver-checks-scan-it)
- [Does `cargo-semver-checks` have false positives?](#does-cargo-semver-checks-have-false-positives)
- [Will `cargo-semver-checks` catch every semver violation?](#will-cargo-semver-checks-catch-every-semver-violation)
- [Can I configure individual lints?](#can-i-configure-individual-lints)
- [If I really want a new feature to be implemented, can I sponsor its development?](#if-i-really-want-a-new-feature-to-be-implemented-can-i-sponsor-its-development)
- [How is `cargo-semver-checks` similar to and different from other tools?](#how-is-cargo-semver-checks-similar-to-and-different-from-other-tools)
- [Why is it sometimes `cargo-semver-check` and `cargo-semver-checks`?](#why-is-it-sometimes-cargo-semver-check-and-cargo-semver-checks)
- [What is the MSRV policy with respect to semver?](#what-is-the-msrv-policy-with-respect-to-semver)

### What Rust versions does `cargo-semver-checks` support?

`cargo-semver-checks` uses the rustdoc tool to analyze the crate's API.
Rustdoc's JSON output format isn't stable, and can have breaking changes in new Rust versions.

When each `cargo-semver-checks` version is released, it will at minimum include support
for the then-current stable and beta Rust versions. It may, but is not guaranteed to,
additionally support some nightly Rust versions.

[The GitHub Action](https://github.com/obi1kenobi/cargo-semver-checks-action) by default uses
the most recent versions of both `cargo-semver-checks` and stable Rust,
so it should be unaffected. Users using `cargo-semver-checks` in other ways
are encouraged to update `cargo-semver-checks` when updating Rust versions
to ensure continued compatibility.

### Can I use `cargo-semver-checks` with `nightly` Rust?

Support for `nightly` Rust versions is on a best-effort basis.
It will work _often, but not always_. If you _must_ use `nightly`,
it's strongly recommended to pin to a specific `nightly` version to avoid broken workflows.

`cargo-semver-checks` relies on the rustdoc JSON format, which is unstable and changes often.
After a new rustdoc JSON format version gets shipped in `nightly`, it usually takes
several days to several weeks for it to be supported in a new `cargo-semver-checks`,
during which time it is not possible to use `cargo-semver-checks` with those `nightly` versions.

It's also possible that support for some `nightly` versions may be dropped even while older
stable versions are still supported. This usually happens when a rustdoc format gets superseded by
a newer version before becoming part of any stable Rust. In that case, we may drop support for
that format to conserve maintenance bandwidth and speed up compile times.
For example, `cargo-semver-checks` v0.24
[supported](https://github.com/obi1kenobi/cargo-semver-checks/blob/93baddc2a65a5055117ca670fe156dc761403fa5/Cargo.toml#L18) rustdoc formats v24, v26, and v27, but did not support the nightly-only v25 format.

### What if my project needs stronger guarantees around supported Rust versions?

If you'd like extended support for older Rust versions,
or an SLA on supporting new `nightly` releases, we're happy to offer those _on a commercial basis_.
It could be in the form of a formal support contract,
or something as simple as discussing expectations over email
and setting up a recurring GitHub sponsorship for an agreed-upon amount.

Please reach out at the email in the
[Cargo.toml](https://github.com/obi1kenobi/cargo-semver-checks/blob/93baddc2a65a5055117ca670fe156dc761403fa5/Cargo.toml#L5)
and let us know about what projects this is for and what their needs are.

### Does the crate I'm checking have to be published on crates.io?

No, it does not have to be published anywhere. You'll just need to use a flag to help
`cargo-semver-checks` locate the version to use as a baseline for semver-checking.

By default, `cargo-semver-checks` uses crates.io to look up the previous version of the crate,
which is used as the baseline for semver-checking the current version of the crate.
The following flags can be used to explicitly specify a baseline instead:

```
--baseline-version <X.Y.Z>
    Version from registry to lookup for a baseline

--baseline-rev <REV>
    Git revision to lookup for a baseline

--baseline-root <MANIFEST_ROOT>
    Directory containing baseline crate source

--baseline-rustdoc <JSON_PATH>
    The rustdoc json file to use as a semver baseline
```

Custom registries are not currently supported
([#160](https://github.com/obi1kenobi/cargo-semver-checks/issues/160)), so crates published on
registries other than crates.io should use one of the other approaches of generating the baseline.

### What features does `cargo-semver-checks` enable in the tested crates?

By default, checking is done on all features except features named `unstable`, `nightly`, `bench`, `no_std`, or ones with prefix `_`, `unstable-`, or `unstable_`, as such names are commonly used for private or unstable features.

This behaviour can be overriden. Checked feature set can be changed to:

- _all_ the features, selected with `--all-features`,
- only the crate's default features, selected with `--default-features`,
- empty set, selected with `--only-explicit-features`.

Additionally, features can be enabled one-by-one, using flags `--features`, `--baseline-features` and `--current-features`.

For example, consider crate [serde](https://github.com/serde-rs/serde), with the following features (per v1.0.163):

- `std` - the crate's only default feature,
- `alloc`, `derive`, `rc` - optional features,
- `unstable` - a feature that possibly breaks semver.

| used flags                                     | selected feature set                       | explanation                                                        |
| ---------------------------------------------- | ------------------------------------------ | ------------------------------------------------------------------ |
| none                                           | `std`, `alloc`, `derive`, `rc`             | Feature `unstable` is excluded by the default heuristic.           |
| `--features unstable`                          | `std`, `alloc`, `derive`, `rc`, `unstable` | The flag explicitly adds `unstable` to the heuristic's selections. |
| `--all-features`                               | `std`, `alloc`, `derive`, `rc`, `unstable` | All the features are used, disabling the default heuristic.        |
| `--default-features`                           | `std`                                      | The crate has only one default feature.                            |
| `--default-features --features derive`         | `std`, `derive`                            | Feature `derive` is used along with crate's default features.      |
| `--only-explicit-features`                     | none                                       | No explicit features are passed.                                   |
| `--only-explicit-features --features unstable` | `unstable`                                 | All features can be added explicitly, regardless of their name.    |

### My crate uses `--cfg` conditional compilation. Can `cargo-semver-checks` scan it?

Yes! You can configure the `--cfg` options that `cargo-semver-checks` will use
when scanning your crate by setting them in the `RUSTDOCFLAGS` environment variable.

For example, you can ask `cargo-semver-checks` to enable the `some-option` config
by invoking it as:
```
RUSTDOCFLAGS="--cfg some-option" cargo semver-checks
```

### Does `cargo-semver-checks` have false positives?

"False positive" means that `cargo-semver-checks` reported a semver violation incorrectly.
A design goal of `cargo-semver-checks` is to not have false positives.
If they do occur, they are considered bugs.

When `cargo-semver-checks` reports a semver violation, it should always point to a specific
file and approximate line number where the specified issue occurs; failure to specify
a file and line number is also considered a bug.

If you think `cargo-semver-checks` might have a false-positive but you aren't sure, please
[open an issue](https://github.com/obi1kenobi/cargo-semver-checks/issues/new?assignees=&labels=C-bug&template=bug_report.yml).
Semver in Rust has [many non-obvious and tricky edge cases](https://predr.ag/blog/toward-fearless-cargo-update/),
especially [in the presence of macros](https://github.com/obi1kenobi/cargo-semver-checks/issues/167).
We'd be happy to look into it together with you to determine if it's a false positive or not.

### Will `cargo-semver-checks` catch every semver violation?

No, it will not — not yet!
There are many ways to break semver, and `cargo-semver-checks`
[doesn't yet have lints for all of them](https://github.com/obi1kenobi/cargo-semver-checks/issues/5).
New lints are added frequently, and
[we'd be happy to mentor you](https://github.com/obi1kenobi/cargo-semver-checks/blob/main/CONTRIBUTING.md)
if you'd like to contribute new lints!

Append `--verbose` when semver-checking your crate to see the full list of performed semver checks.

Here are some example areas where `cargo-semver-checks` currently will not catch semver violations:

- breaking type changes, for example in the type of a field or function parameter
- breaking changes in generics or lifetimes
- breaking changes that exist when only a subset of all crate features are activated

### Can I configure individual lints?

Yes! See [lint-level configuration](#lint-level-configuration).

### If I really want a new feature to be implemented, can I sponsor its development?

Depending on the feature, possibly yes!

Please reach out to us _ahead of time_
[over email](https://github.com/obi1kenobi/cargo-semver-checks/blob/93baddc2a65a5055117ca670fe156dc761403fa5/Cargo.toml#L5)
to discuss the scope of the feature and sponsorship.

It's possible that the feature might be deemed out of scope, too complex to build or maintain,
or otherwise unsuitable. In such cases we'd like to let you know that _before_ you've sent us money,
since [there are no refunds on GitHub Sponsors](https://docs.github.com/en/billing/managing-billing-for-github-sponsors/downgrading-a-sponsorship#:~:text=When%20you%20downgrade%20or%20cancel,for%20payments%20for%20GitHub%20Sponsors.).

If the feature is viable and the work involved in building is commensurate to
the sponsorship amount, we'd be happy to build it.
At your option, we'd also be happy to give you a shout-out for sponsoring the feature when
it is announced in the release notes.

### How is `cargo-semver-checks` similar to and different from other tools?

[rust semverver](https://github.com/rust-lang/rust-semverver) builds on top of
rustc internals to build rlib's and compare their metadata. This strips the
code down to the basics for identifying changes. However, is tightly coupled
to specific nightly compiler versions and [takes work to stay in
sync](https://github.com/rust-lang/rust-semverver/search?q=Rustup+to&type=commits).
As of April 17, 2023, it appears to be
[deprecated and no longer maintained](https://github.com/rust-lang/rust-semverver/pull/390).

[cargo breaking](https://github.com/iomentum/cargo-breaking) effectively runs
`cargo expand` and re-parses the code using
[`syn`](https://crates.io/crates/syn) which requires re-implementing large
swaths of rust's semantics to then lint the API for changes.
This is limited to the feature and target the crate was compiled for.
As of November 22, 2022, it appears to be
[archived and no longer maintained](https://github.com/iomentum/cargo-breaking).

`cargo-semver-checks` sources its data from rustdoc's json output. While the
json output format is unstable, the rate of change is fairly low, reducing the
churn in keeping up. The lints are also written as queries for `trustfall`
["query everything" engine](https://github.com/obi1kenobi/trustfall), reducing
the work for creating and maintaining them. Because of the extra data that
rustdoc includes, some level of feature/target awareness might be able to be
introduced.

There is interest in
[hosting rustdoc JSON](https://github.com/rust-lang/docs.rs/issues/1285) on `docs.rs` meaning
that semver-checking could one day download the baseline rustdoc JSON file instead of generating it.
Also, generally speaking, inspecting JSON data is likely going to be faster than full compilation.

[`cargo-public-api`](https://crates.io/crates/cargo-public-api) uses rustdoc,
like `cargo-semver-checks`, but focuses more on API diffing (showing which
items has changed) and not API linting (explaining why they have changed and
providing control over what counts).

### Why is it sometimes `cargo-semver-check` and `cargo-semver-checks`?

This crate was intended to be published under the name `cargo-semver-check`, and may indeed one
day be published under that name. Due to
[an unfortunate mishap](https://github.com/rust-lang/crates.io/issues/728#issuecomment-1182760954),
it remains `cargo-semver-checks` for the time being.

The `cargo_semver_check` name is reserved on crates.io but all its versions
are intentionally yanked. Please use the `cargo-semver-checks` crate instead.

### What is the MSRV policy with respect to semver?

MSRV bumps are _not_ considered major changes.

`cargo-semver-checks` has two Rust version bounds, since it depends on Rust
both at compile-time and at runtime:

- The MSRV for _compiling_ `cargo-semver-checks` (_"compile MSRV"_) is currently Rust 1.70.
  This is primarily determined by our dependencies' MSRVs.
- The MSRV for _checking crates_ (_"runtime MSRV"_) is currently Rust 1.71.
  This is determined based on the rustdoc JSON format versions and
  known bugs in older rustdoc versions.

As much as practically possible, changes to the _runtime MSRV_ will come in bumps
of the _middle_ number in the version, e.g. `0.24.1 -> 0.25.0` or `1.2.3 -> 1.3.0`.

Changes to the _compile MSRV_ may happen in any kind of version bump.
As much as practically possible, we'll aim to make them
simultaneously with _runtime MSRV_ bumps.

## Configuration

### Lint-level configuration

`cargo-semver-checks` offers the ability to customize which lints are enforced, what SemVer versions they require, and whether violations of that lint produce `deny / warn / allow` behavior.

As a reminder, a "lint" or "check" is a rule in `cargo-semver-checks` that looks for a specific kind of issue. For example, the `function_missing` lint looks for functions that no longer exist in a crate's public API. `cargo-semver-checks` has many dozens of such lints.

Lints may be configured in two ways:
- **lint level:** When a lint finds issues, the lint level controls how `cargo-semver-checks` responds. The `deny` level makes the lint a hard error: `cargo-semver-checks` will exit with an error and will require a version bump to resolve. The `warn` level will print a warning describing the issue, but will not cause `cargo-semver-checks` to exit with an error code — meaning it _will not_ block CI runs. The `allow` level means the findings of the lint should be silently ignored, so the check doesn't even need to be run.
- **required update:** This sets the kind of version bump this check should require (for `deny`-level) or suggest (for `warn`) when it spots an issue. For example, the `function_missing` lint is `major` by default, so if a public function is removed between versions, the version needs a major version bump (e.g., 1.2.3 to 2.0.0 or 0.5.2 to 0.6.0). This can be configured to `major` or `minor` (1.2.3 to 1.3.0 or 0.5.2 to 0.5.3). There is no "patch" setting since that is equivalent to setting an `allow` lint level.

To configure the level and/or required update for a lint, first find its name. This will be in `snake_case` and is reported in the CLI on errors/warnings, and can also be found as the file name in the [lints folder](https://github.com/obi1kenobi/cargo-semver-checks/tree/main/src/lints).

- [Example: Downgrading an error-level lint to a warning](#example-changing-the-semver-requirement-for-a-lint)
- [Example: Changing the SemVer requirement for a lint](#example-downgrading-an-error-level-lint-to-a-warning)
- [Example: Configuring lints for an entire workspace](#example-configuring-lints-for-an-entire-workspace)
- [Example: Overriding workspace configuration](#example-overriding-workspace-configuration)
- [Common configurations: Make `#[must_use]` lints warn-only](#common-configurations-make-must_use-lints-warn-only)
- [Common configurations: Disable `#[must_use]` lints entirely](#common-configurations-disable-must_use-lints-entirely)
- [Implementation details & limitations](#implementation-details--limitations)

#### Example: Downgrading an error-level lint to a warning

`cargo-semver-checks` by default considers adding `#[must_use]` on an existing function to require a `minor` version bump, and has level `deny`. Let's say our package considers that too strict, and would prefer adding `#[must_use]` to produce a warning instead of an error.

We can accomplish that by adding the following to the `Cargo.toml` manifest for the package:

```toml
[package.metadata.cargo-semver-checks.lints]
function_must_use_added = "warn"
```

That option is shorthand notation for:
```toml
[package.metadata.cargo-semver-checks.lints]
function_must_use_added = { level = "warn" }
```

If we wanted to configure other lints simultaneously, we could add their configuration there as well: one lint per line.

#### Example: Changing the SemVer requirement for a lint

Say we instead wanted to mandate a major version bump if `#[must_use]` is added.

We'd add the following to our package's `Cargo.toml` file:
```toml
[package.metadata.cargo-semver-checks.lints]
function_must_use_added = { required-update = "major" }
```

Of course, it's possible to combine `level` and `required-update` settings.
For example, the following will make the `function_must_use_added` cause warnings (instead of errors) whenever `#[must_use]` is added without a major version bump:
```toml
[package.metadata.cargo-semver-checks.lints]
function_must_use_added = { level = "warn", required-update = "major" }
```

#### Example: Configuring lints for an entire workspace

`cargo-semver-checks` allows defining your lint configuration at the workspace level, and reusing it in each of your crates.

First, add your configuration to the workspace root `Cargo.toml`, noting the `workspace.metadata` prefix instead of `package.metadata`:
```toml
[workspace.metadata.cargo-semver-checks.lints]
function_must_use_added = { level = "warn" }
```

Then, to opt into the workspace configuration in individual packages, add either one of these keys to that package's `Cargo.toml`:

```toml
[package.metadata.cargo-semver-checks.lints]
workspace = true
```

or, if your workspace already configures workspace-level lints (e.g. for clippy) at `[workspace.lints]`,

```toml
[lints]
workspace = true
```

Using the `lints.workspace` key will cause a cargo error if it is set and there is no `[workspace.lints]` table in the workspace Cargo.toml. We support the `lints.workspace` key to ease our transition toward [merging `cargo-semver-checks` into `cargo` itself](https://github.com/obi1kenobi/cargo-semver-checks/issues/61).

Setting `workspace = false` is not valid configuration for either of these keys. To have a package opt-out from the workspace's lints configuration, omit both keys entirely from the package's `Cargo.toml`.

#### Example: Overriding workspace configuration

When `workspace = true` is set, it is still possible to override individual lint settings in a package.

Rule of thumb: for each config option of each lint,
- the workspace configuration overrides the lint's defaults, and
- the package's configuration overrides both of the above.

For example, if we have in the *workspace* `Cargo.toml`:

```toml
[workspace.metadata.cargo-semver-checks.lints]
function_missing = { level = "warn", required-update = "minor" }
trait_now_doc_hidden = "warn"
function_must_use_added = "warn"
```

and in the *package* `Cargo.toml`:

```toml
[package.metadata.cargo-semver-checks.lints]
workspace = true
function_missing = "deny"
trait_now_doc_hidden = { required-update = "minor" }
```

Here's the final configuration for that package:
- For `function_missing`:
  - The [built-in defaults](https://github.com/obi1kenobi/cargo-semver-checks/blob/1e0c82331cf26cf5ea52f3787d07eb583556cf79/src/lints/function_missing.ron#L5-L6) are `level = "deny", required-update = "major"`.
  - The workspace overrides both to `level = "warn", required-update = "minor"`.
  - The package opts into the workspace's configuration with `workspace = true`, then overrides to `level = "deny"`.
  - The final config: `level = "deny", required-update = "minor"`.
- For `trait_now_doc_hidden`:
  - The [built-in defaults](https://github.com/obi1kenobi/cargo-semver-checks/blob/1e0c82331cf26cf5ea52f3787d07eb583556cf79/src/lints/trait_now_doc_hidden.ron#L5-L6) are `level = "deny", required-update = "major"`.
  - The workspace overrides `level = warn"`, leaving `required-update` unchanged.
  - The package opts into the workspace's configuration with `workspace = true`, then sets `required-update = "minor"`.
  - The final config: `level = "deny", required-update = "minor"`.
- For `function_must_use_added`:
  - The [built-in defaults](https://github.com/obi1kenobi/cargo-semver-checks/blob/1e0c82331cf26cf5ea52f3787d07eb583556cf79/src/lints/function_must_use_added.ron#L5-L6) are `level = "deny", required-update = "minor"`.
  - The workspace overrides `level = "warn"`, leaving `required-update` unchanged.
  - The package opts into the workspace's configuration with `workspace = true`.
  - The final config: `level = "warn", required-update = "minor"`.

#### Common configurations: Make `#[must_use]` lints warn-only

In the default configuration, `cargo-semver-checks` considers it an error to add `#[must_use]` attributes in patch versions.
The rationale is that such an addition risks introducing new lints in downstream projects, and many projects consider lints as errors and may be broken as a result.
(Whether that setting is good practice for widespread use or not is outside the scope of `cargo-semver-checks`.)

To downgrade all lints related to `#[must_use]` from error to warnings, add the following lines to the `cargo-semver-checks` configuration in your package or workspace:
```toml
function_must_use_added = "warn"
inherent_method_must_use_added = "warn"
struct_must_use_added = "warn"
enum_must_use_added = "warn"
trait_must_use_added = "warn"
union_must_use_added = "warn"
```

#### Common configurations: Disable `#[must_use]` lints entirely

To skip checking `#[must_use]`-related lints entirely, apply the following configuration to your package or workspace:

```toml
function_must_use_added = "allow"
inherent_method_must_use_added = "allow"
struct_must_use_added = "allow"
enum_must_use_added = "allow"
trait_must_use_added = "allow"
union_must_use_added = "allow"
```

#### Implementation details & limitations

When checking a package, `cargo-semver-checks` uses the manifest of the current subject's `Cargo.toml` file (plus its workspace configuration, if opted in).

Configuration set in the _baseline version_ of the package (the version being compared _against_, such as an existing version published on crates.io) is not read and has no effect. This is because when publishing a new version, it makes sense to use the new version's configuration instead of any prior configuration.

When the `--manifest-path` option is used to specify the subject package's `Cargo.toml` file, that's also the file from which configuration is loaded. If that CLI flag is not specified, `cargo-semver-checks` will by default attempt to find and use a `Cargo.toml` file that belongs to the current directory.

If `cargo-semver-checks` is executed in a way that skips reading the current manifest (such as with the `--current-rustdoc` flag), it is currently not possible to configure lints. Interest in, and progress toward resolving this limitation is tracked in [this issue](https://github.com/obi1kenobi/cargo-semver-checks/issues/827).

## Troubleshooting

This section documents common issues and the best ways to resolve them.

### Running `cargo install cargo-semver-checks --locked` produces an error

**Recommendation**: use [`cargo-binstall`](https://github.com/cargo-bins/cargo-binstall) to download a prebuilt binary if one is available for your platform, rather than compiling from scratch.

Specific errors and their resolutions:
- ["error: failed to run custom build command for `libz-ng-sys vX.Y.Z`"](https://github.com/obi1kenobi/cargo-semver-checks/issues/841)
  - **Preferred resolution**: use a prebuilt binary to avoid this problem entirely.
  - This error is caused by missing `cmake` on your system, which is required by a transitive dependency of `cargo-semver-checks`. `cargo` does not currently offer a mechanism for such a binary dependency to be declared as required, nor automatically installed when needed.
  - You can usually install it via a command like `apt install cmake` or `brew install cmake`, depending on your platform.

***

### Visual Design

Logo by [NUMI](https://github.com/numi-hq/open-design):

[<img src="https://raw.githubusercontent.com/numi-hq/open-design/main/assets/numi-lockup.png" alt="NUMI Logo" style="width: 200px;"/>](https://numi.tech/?ref=semver-checks)

### License

Available under the Apache License (Version 2.0) or the MIT license, at your option.

Copyright 2022-present Predrag Gruevski and Project Contributors.
The present date is determined by the timestamp of the most recent commit in the repository.
Project Contributors are all authors and committers of commits in the repository.
