cargo-semver-checks

Lint your crate API changes for semver violations.

• Quick Start
• FAQ
• Configuration
• Troubleshooting
• Contributing

Quick Start

# 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 (used in .github/workflows/ci.yml in this repo):

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

Each failing check references specific items in the Cargo SemVer reference 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?
• Can I use cargo-semver-checks with nightly Rust?
• What if my project needs stronger guarantees around supported Rust versions?
• Does the crate I'm checking have to be published on crates.io?
• What features does cargo-semver-checks enable in the tested crates?
• Does cargo-semver-checks have false positives?
• Will cargo-semver-checks catch every semver violation?
• Can I configure individual lints?
• 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?
• Why is it sometimes cargo-semver-check and cargo-semver-checks?
• 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 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 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 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), 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, 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.

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. Semver in Rust has many non-obvious and tricky edge cases, especially in the presence of macros. 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. New lints are added frequently, and we'd be happy to mentor you 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.

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 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.

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 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. As of April 17, 2023, it appears to be deprecated and no longer maintained.

cargo breaking effectively runs cargo expand and re-parses the code using 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.

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, 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 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 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, 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.

• Example: Downgrading an error-level lint to a warning
• Example: Changing the SemVer requirement for a lint
• Example: Configuring lints for an entire workspace
• Example: Overriding workspace configuration
• Common configurations: Make #[must_use] lints warn-only
• Common configurations: Disable #[must_use] lints entirely
• 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:

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

That option is shorthand notation for:

[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:

[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:

[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:

[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:

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

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

[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.

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:

[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:

[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 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 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 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:

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:

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.

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 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"
  • 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.

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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.