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v0.45.0

01 Nov 16:36
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v0.45.0 Latest
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In this release

  • Respect .cargo/config.toml build configuration, RUSTFLAGS, and RUSTDOCFLAGS
  • Spotlight: Ghosts in the Compilation

semver-mk3-optimized
✨ This adorable animation was made by @dekirisu ✨

This release requires Rust 1.89+ both to install (MSRV) and at runtime. Future releases will require Rust 1.90+.

Spotlight: Ghosts in the Compilation

Celebrating Halloween, Ghosts in the Compilation is the debugging story of how the functionality for respecting .cargo/config.toml configuration came to be.

We found a crate where:

  • cargo-semver-checks said the code didn't compile.
  • cargo check said it was fine.

As a result, we learned a fair bit about the edge cases of cargo, rustc, rustdoc, and compilation flags. Check it out here!

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Full Changelog: v0.44.0...v0.45.0

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v0.44.0

21 Sep 01:31
@obi1kenobi obi1kenobi
v0.44.0
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v0.44.0

In this release

  • 22 new lints, for a total of 200 🎉
  • Spotlight: The Road to 200 lints

semver-mk3-optimized
✨ This adorable animation was made by @dekirisu ✨

This release requires Rust 1.88+ both to install (MSRV) and at runtime. Future releases will require Rust 1.89+.

Spotlight: The Moore's Law of Lints

We have a kind of "Moore's Law" for lints: every year we run twice as many lints in the same total time.

  • 30 lints at the end of 2022
  • 57 lints at the end of 2023
  • 120 lints at the end of 2024
  • We'd need 240 lints at the end of 2025 to keep up the pace; with ~3.3 months to go, we're at 200 lints.
  • Meanwhile, as of the optimizations shipped 2.5 years ago, running lints on crates.io's largest library required only 8s or less.

This is rare and remarkable, and requires overcoming many complex socio-technical challenges! Here are a few examples from conference talks related to cargo-semver-checks:

We're also fortunate to have two extraordinary students working on cargo-semver-checks this year as part of Google Summer of Code! Their work is key to continuing this positive exponential trend, and we look forward to showcasing their impact more closely later this year.

New lints

The 22 new lints are grouped in several categories.

API or ABI breakage

  • function_const_generic_reordered
  • function_generic_type_reordered
  • function_now_returns_unit
  • inherent_method_const_generic_reordered
  • inherent_method_generic_type_reordered
  • inherent_method_now_returns_unit
  • repr_align_added
  • repr_align_changed
  • repr_align_removed
  • repr_packed_changed
  • trait_const_generic_reordered
  • trait_generic_type_reordered
  • trait_method_const_generic_reordered
  • trait_method_generic_type_reordered
  • trait_method_now_returns_unit
  • trait_method_return_value_added
  • type_const_generic_reordered
  • type_generic_type_reordered

Compatibility risks, such as changes that may require a SemVer major bump to revert, or may otherwise represent unintended API changes without being SemVer-major themselves:

  • pub_api_sealed_trait_method_return_value_added

Additive-only API changes (opt-in only; more work required):

  • function_must_use_removed
  • struct_must_use_removed
  • union_must_use_removed

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  • @nk9 made their first contribution in #1392

Full Changelog: v0.43.0...v0.44.0

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kornelski, obi1kenobi, and 4 other contributors
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v0.43.0

10 Aug 04:49
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v0.43.0
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v0.43.0

In this release

  • Spotlight: I'm giving a talk at Rust Forge!
  • Performance improvements
  • 1 new lint, for a total of 178

This release requires Rust 1.87+ both to install (MSRV) and at runtime. Future releases will require Rust 1.88+.

Spotlight: I'm giving a talk at Rust Forge!

Rust Forge is a creative new conference organized by Rust in Action author Tim McNamara. It's scheduled for 27-30 August 2025 in Wellington, New Zealand.

My talk is titled "The Past, Present, and Future of SemVer in Rust" and I'm extremely excited about it! Here's a short description:

At least 1-2 times per week, accidental breaking changes sneak into the new release of some popular Rust package, despite the maintainers' best efforts. Why is this still a problem 10 years after Rust 1.0? What will it take to finally stop such breakage, so we can have fearless cargo update?

It's not too late to grab a ticket! I hope to see you there!

Performance improvements

Supporting an exponentially-growing number of lints requires periodic maintenance in the form of mandatory performance engineering, Without it, things would quickly get out of hand, and large crates and large workspaces would be affected first. To prevent that, this release cycle focused on both internal and external improvements.

Externally-visible improvements include lint execution time improvements, courtesy of a new query profiling tool prototype developed by @CLIDragon. This allowed us to add targeted new indexes that speed up the execution of some lints by as much as 10x! After these optimizations, running the full suite of lints on Rust's largest crates needs only ~2s, down from around ~8s previously.

Internal improvements include a variety of optimizations aimed at reducing the time taken by our CI suite, from an original of ~7min for cargo test down to around ~1min.

If you're wondering why cargo test used to take 7min in the first place: our test suite runs ~250000 lint queries to ensure correctness and keep false-positives out! Even extremely cheap operations done 250k times add up very quickly! After the optimizations, we still run those 250k queries — we didn't sacrifice correctness at all, but merely cut out avoidable overhead.

New lints

We added only one new lint in this release, tracking an additive-only API change: enum_must_use_removed. This lint group remains opt-in-only.

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Full Changelog: v0.42.0...v0.43.0

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obi1kenobi, dependabot, and 3 other contributors
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v0.42.0

05 Jul 15:03
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v0.42.0
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v0.42.0

In this release

  • 13 new lints, for a total of 177
  • Spotlight: #[target_feature] lints
  • Bugfix: false-positive sealed trait lints

This release requires Rust 1.85+ both to install (MSRV) and at runtime. Future releases will require Rust 1.87+.

Spotlight: #[target_feature] lints

Check out this in-depth look at the SemVer and soundness surprises behind this attribute:
Unsoundness and accidental features in the #[target_feature] attribute

New lints

We added new 13 lints across several categories, including 12 lints around #[target_feature] and one lint about an additive-only change. Some of the new lints are error-by-default, while others are warnings meant to flag changes deserving closer review, or even opt-in only and disabled by default while we do more work to improve their user experience.

API breakage:

  • safe_function_requires_more_target_features
  • safe_function_target_feature_added
  • safe_inherent_method_requires_more_target_features
  • safe_inherent_method_target_feature_added
  • trait_method_target_feature_removed
  • unsafe_function_requires_more_target_features
  • unsafe_function_target_feature_added
  • unsafe_inherent_method_requires_more_target_features
  • unsafe_inherent_method_target_feature_added
  • unsafe_trait_method_requires_more_target_features
  • unsafe_trait_method_target_feature_added
  • pub_api_sealed_trait_method_target_feature_removed (warn-by-default)

Additive-only API changes (opt-in only; more work required):

  • exhaustive_enum_added

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kornelski, obi1kenobi, and 4 other contributors
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v0.41.0

22 Apr 17:49
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v0.41.0
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v0.41.0

In this release

  • 16 new lints, for a total of 164
  • Spotlight: Google Summer of Code
  • Bugfix: false-positive sealed trait lints

This release requires Rust 1.84+ both to install (MSRV) and at runtime. Future releases will require Rust 1.85+.

Spotlight: Google Summer of Code

cargo-semver-checks is participating in Google Summer of Code, a program aimed at bringing new contributors into open source software development.

This is our second year participating in the program under the Rust umbrella. Last summer, @suaviloquence joined us to design and build the ability for users to configure which lints run on their projects, on which SemVer changes, and with what effect such as raising an error vs warning. This is a foundational capability for our project! For example, it allows users to temporarily disable a lint that exhibits buggy behavior in their project, such as a false-positive.

The application process for this year's Google Summer of Code concluded two weeks ago, and we're currently in the evaluation process. The selected projects will be announced in early May. We thank everyone who applied for their hard work on their proposals, and wish everyone the best of luck!

New lints

We added new 16 lints across several categories. Some of the new lints are error-by-default, while others are warnings meant to flag changes deserving closer review, or even opt-in only and disabled by default while we do more work to improve their user experience.

API breakage:

  • enum_struct_variant_changed_kind
  • enum_non_exhaustive_tuple_variant_changed_kind

Item deprecations, which cause lints in downstream use:

  • enum_struct_variant_field_marked_deprecated
  • enum_tuple_variant_field_marked_deprecated
  • enum_variant_marked_deprecated

Compatibility risks, such as changes that may require a SemVer major bump to revert, or may otherwise represent unintended API changes without being SemVer-major themselves:

  • enum_no_longer_non_exhaustive
  • enum_variant_no_longer_non_exhaustive
  • enum_repr_int_added
  • function_no_longer_unsafe
  • function_now_const

Additive-only API changes (opt-in only; more work required):

  • enum_non_exhaustive_struct_variant_field_added
  • enum_non_exhaustive_tuple_variant_field_added
  • non_exhaustive_enum_added
  • pub_const_added
  • pub_static_added
  • union_added

Thanks to @shreyans413, @GlitchlessCode, @Frank-III, @qstommyshu, and @sandptel for contributing to this release!

Bugfix: false-positive sealed trait lints

In rare circumstances, cargo-semver-checks had the tendency to mis-classify traits as sealed even though that wasn't actually the case. This manifested as false-positive lints stating that traits have newly become sealed (a breaking change) or that a previously-sealed trait is no longer sealed (a future-compatibility warning).

After quite a bit of work, we were able to diagnose the issue as a logic error in the cycle-tracking code of our sealed trait analysis. #1200 has more information.

This portion of cargo-semver-checks has 2000+ lines of test code specifically dedicated to covering its edge cases. This bug made it through all of that without getting caught — that's how complex this space is.

We're grateful to the folks who contributed high-quality bug reproductions to help us diagnose the problem in #1200. We're also grateful to the generosity of our GitHub Sponsors who make it possible for cargo-semver-checks to continue powering through this very complex space.

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obi1kenobi, dependabot, and 6 other contributors
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v0.40.0

08 Mar 16:34
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v0.40.0

In this release

  • 21 new lints, for a total of 148!
  • Spotlight: #[doc(hidden)] and sealed lints

This release requires Rust 1.83+ both to install (MSRV) and at runtime. Future releases will require Rust 1.84+.

Spotlight: #[doc(hidden)] and sealed lints

When is "this trait can be implemented" part of the trait's public API?

At a high level, the answer is: when writing an impl of that trait for our own type doesn't require using any #[doc(hidden)] items. But you've been following cargo-semver-checks long enough to know the full answer is much more complex. And getting the right answer here affects the correctness of a ton of lints!

Good news! This cargo-semver-checks release ships with a much more sophisticated system for analyzing whether traits can be implemented or are instead "sealed." The new system is more thorough, more nuanced, and faster to boot!

Read more about it here: https://predr.ag/blog/when-is-trait-can-be-implemented-public-api/

New lints

We added new 21 lints across several categories. Some of the new lints are error-by-default, while others are merely warnings meant to flag changes deserving closer review.

API breakage:

  • static_became_unsafe

ABI and FFI breakage due to changed repr(C) type layout:

  • repr_c_enum_struct_variant_fields_reordered
  • repr_c_plain_struct_fields_reordered

Item deprecations, which cause lints in downstream use:

  • function_marked_deprecated
  • global_value_marked_deprecated
  • macro_marked_deprecated
  • proc_macro_marked_deprecated
  • struct_field_marked_deprecated
  • trait_associated_const_marked_deprecated
  • trait_associated_type_marked_deprecated
  • trait_marked_deprecated
  • trait_method_marked_deprecated
  • type_associated_const_marked_deprecated
  • type_method_marked_deprecated

Code behavior changes, such as changes to sort order of existing types:

  • partial_ord_enum_struct_variant_fields_reordered
  • partial_ord_enum_variants_reordered
  • partial_ord_struct_fields_reordered

Compatibility risks, such as changes that may require a SemVer major bump to revert, or may otherwise represent unintended API changes without being SemVer-major themselves:

  • pub_api_sealed_trait_became_unconditionally_sealed
  • unconditionally_sealed_trait_became_pub_api_sealed
  • unconditionally_sealed_trait_became_unsealed
  • pub_api_sealed_trait_became_unsealed

Thanks to @Frank-III, @geetanshjuneja, and @lovelindhoni for contributing lints to this release!

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obi1kenobi, dependabot, and 4 other contributors
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v0.39.0

28 Jan 16:00
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v0.39.0

In this release

  • 20 new lints, for a total of 127!
  • Spotlight: Merging into cargo

This release requires Rust 1.81+ both to install (MSRV) and at runtime. Future releases will require Rust 1.83+.

Spotlight: Merging into cargo

While cargo-semver-checks is currently a standalone tool, Rust's cargo team plans to eventually make it a built-in part of cargo itself. Here's how that would work.

When users run cargo publish today, cargo first runs a series of checks to make sure everything is in order. For example, it ensures that the code repository doesn't include any uncommitted changes, so as not to accidentally publish uncommitted code to crates.io:

$ cargo publish
    Updating crates.io index
error: 1 files in the working directory contain changes that were not yet committed into git:

src/lib.rs

to proceed despite this and include the uncommitted changes, pass the `--allow-dirty` flag

cargo-semver-checks is planned to become another such pre-publish check, equivalent to running cargo semver-checks && cargo publish today. Of course, there are situations where maintainers may prefer to intentionally publish breaking changes in a non-major version, such as for a sufficiently critical security fix. The automatic semver-check will be overridable by passing a flag analogous to the --allow-dirty flag for uncommitted changes.

While we're excited to merge cargo-semver-checks into cargo, we are proceeding with caution before enabling it by default. We want to ensure "merge day" is a day of celebration for the entire Rust community, instead of causing frustration and further breakage! Good news: you can help us do that! Consider:

New lints

Preventing breakage caused by changes in generic parameters (lifetimes, const generics, and generic types), changes in package features, union field changes, and more:

  • feature_not_enabled_by_default
  • function_abi_now_unwind
  • function_requires_different_const_generic_params
  • function_requires_different_generic_type_params
  • method_requires_different_const_generic_params
  • method_requires_different_generic_type_params
  • proc_macro_now_doc_hidden
  • trait_allows_fewer_const_generic_params
  • trait_allows_fewer_generic_type_params
  • trait_method_parameter_count_changed
  • trait_method_requires_different_const_generic_params
  • trait_method_requires_different_generic_type_params
  • trait_requires_more_const_generic_params
  • trait_requires_more_generic_type_params
  • type_allows_fewer_const_generic_params
  • type_allows_fewer_generic_type_params
  • type_requires_more_const_generic_params
  • type_requires_more_generic_type_params
  • union_field_added_with_all_pub_fields
  • union_field_added_with_non_pub_fields

Additionally, we've renamed one lint to align with the shift in preferred Rust terminology:

  • trait_no_longer_object_safe -> trait_no_longer_dyn_compatible

Thanks to @Frank-III for contributing lints to this release!

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Full Changelog: v0.38.0...v0.39.0

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obi1kenobi, dependabot, and Frank-III
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v0.38.0

11 Dec 16:24
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v0.38.0
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v0.38.0

In this release

  • 12 new lints, for a total of 107!
  • Bugfix for packages using the "SemVer trick"
  • Spotlight: Performance via parallelism

This release requires Rust 1.81+ both to install (MSRV) and at runtime.

Spotlight: Performance via parallelism

cargo-semver-checks is on a trajectory of doubling its number of lints every year. But more lints mean more code scanning work to be done! And nobody likes slow tools, so we had to parallelize!

We began taking a serious look at performance in early 2023. Compared to that point ~almost two years ago:

  • Lint checking time has reduced by 20%, in wall-clock terms & measured on the exact same hardware.
  • Yet, we run over 2.5x as many lints in that time!

We've aimed to parallelize as much of the execution of the tool as possible, with help from the rayon library. Today, both the lint execution and the rustdoc indexing steps use rayon to distribute the work across all available cores on your system. In practice, this ~halved linting time on the default GitHub Actions runners (which have only 2 cores), and it has an even bigger impact on more powerful hardware.

Parallelism is one of many tricks we employ to keep cargo-semver-checks fast while it's gaining ever more lints. More on this in future Spotlights!

New lints

Preventing breakage caused by macros, enum discriminants, mutable statics, and generic lifetime parameters:

  • function_like_proc_macro_missing
  • attribute_proc_macro_missing
  • derive_proc_macro_missing
  • derive_helper_attr_removed
  • macro_now_doc_hidden
  • macro_no_longer_exported
  • enum_repr_variant_discriminant_changed
  • enum_discriminants_undefined_non_exhaustive_variant
  • enum_discriminants_undefined_non_unit_variant
  • pub_static_now_mutable
  • type_mismatched_generic_lifetimes
  • trait_mismatched_generic_lifetimes

Thanks to @malenaohl and @orhun for contributing lints to this release!

Bugfix for self-referential crates

Crates that are about to release a new major version often use the "SemVer trick" to make the upgrade process more ergonomic for their users. This involves making the crate list a "future" version of itself as a dependency, which is quite unusual otherwise!

Unfortunately, this leads to having two packages by the same name in the dependency tree, which confused the new feature-scanning code shipped in cargo-semver-checks v0.37. The resulting crash was reported by @jonathanpallant, and is now fixed! Enjoy!

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Full Changelog: v0.37.0...v0.38.0

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obi1kenobi, jonathanpallant, and 2 other contributors
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v0.37.0

05 Dec 03:47
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v0.37.0
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v0.37.0

What's Changed

We shipped support for linting Cargo.toml files for breakage — learn more in the accompanying blog post!

This release requires Rust 1.80+ both to install (MSRV) and at runtime. Future releases will require Rust 1.81+.

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Full Changelog: v0.36.0...v0.37.0

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v0.36.0

18 Oct 14:43
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v0.36.0

What's changed

We're debuting a new "Spotlight" section in the release notes! We'll use it to shine a spotlight at aspects of cargo-semver-checks that don't usually get flashy headlines, even though they play a critical role.

In this release:

  • Spotlight: Supporting many Rust versions at once
  • 5 new lints (94 total, who will add lint number 💯?)
  • Performance upgrades
  • Support for comma-separated feature lists

This release requires Rust 1.77+ both to install (MSRV) and at runtime. Future releases will require Rust 1.80+.

Spotlight: Supporting many Rust versions at once

cargo-semver-checks uses the JSON output of Rust's rustdoc tool to analyze your APIs. This JSON format is not stable: between the release of Rust 1.77 on March 21, 2024 and today, we've had 9 mutually-incompatible format versions. That's more than one per month!

Most tools that rely on rustdoc JSON support only a single rustdoc format at a time. The user is required to install the version of the tool that matches the rustdoc format their Rust version provides. This works, but at the cost of pushing extra complexity onto the users.

Instead, we wanted to offer a seamless experience: use reasonably new Rust and newer cargo-semver-checks, and everything just works! Pulling this off required declarative queries, cutting-edge database technology, and the Trustfall query engine, and were described in depth in UA Rust 2024 and RustConf 2024 talks. The talk videos aren't online yet, so please follow along on social media (bsky, Mastodon, Twitter) or by subscribing to blog posts to make sure you don't miss them when they come out ✨

Today's cargo-semver-checks supports all stable Rust versions 1.77-1.82, as well as 1.83 beta and the most recent nightly. Today we set a new record in how many rustdoc formats we support at once! All the maintenance required to make this work is a challenge, and we are grateful to our GitHub Sponsors for supporting this work 💖

New lints

We're now at 94 lints! Who is going to add lint number 💯?

Performance upgrades

In any reasonably well-optimized system, ongoing performance wins usually come in the form of "5% here, 10% there" rather than "10x with this one weird trick." This is now true for cargo-semver-checks as well.

In coordination with the Rust project's T-rustdoc team, and with the tireless efforts of @jalil-salame, cargo-semver-checks users now benefit from:

  • ~6-7% smaller rustdoc JSON files, which consume less disk space to cache and are ~28% faster to load
  • more compact indexes which are ~12% faster to build

This translates to up to ~0.8s time savings for our largest projects, and even more speedup on resource-constrained CI hardware where disk and network I/O are usually much slower than on a desktop machine.

Support for comma-separated feature lists

Previously, our CLI did not support specifying multiple features as a comma-separated list — the following did not work:

cargo semver-checks --features foo,bar

Instead, users had to specify each feature separately, which was quite tedious:

cargo semver-checks --features foo --features bar

Thanks to @dmatos2012, the --features, --baseline-features, and --curent-features CLI options now all support comma-separated lists. What a lovely user experience win!

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miikka, obi1kenobi, and 7 other contributors
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