Continue resolving cargo-semver-checks blockers for merging into cargo

Summary

Design and implement cargo-semver-checks functionality that lies on the critical path for merging the tool into cargo itself. Continues the work of the 2025h1 goal.

Motivation

Cargo assumes that all packages adhere to semantic versioning (SemVer). However, SemVer adherence is quite hard in practice: research shows that accidental SemVer violations are relatively common (lower-bound: in 3% of releases) and happen to Rustaceans of all skill levels. Given the significant complexity of the Rust SemVer rules, improvements here require better tooling.

cargo-semver-checks is a linter for semantic versioning (SemVer) in Rust. It is broadly adopted by the Rust community, and the cargo team has expressed interest in merging it into cargo itself as part of the existing cargo publish workflow. By default, cargo publish would require SemVer compliance, but offer a flag (analogous to the --allow-dirty flag for uncommitted changes) to override the SemVer check and proceed with publishing anyway.

The cargo team has identified a set of milestones and blockers that must be resolved before cargo-semver-checks can be integrated into the cargo publish workflow. Our goal here is to make steady progress toward resolving them.

The status quo after the 2025h1 goal period

The 2025h1 goal targeted work in two major areas:

• Checking of cross-crate items
• SemVer linting of type information

While meaningful progress was achieved, significant work remains to be done.

'static bounds were discovered to be able to be implied, meaning that the bound is not syntactically present at the public API definition site, but is nevertheless part of the public API via being implied by another bound (or by a bound implied by such a bound, etc.). Conversely, ?Sized was discovered to possibly not apply even when syntactically present, as an implied bound elsewhere may imply Sized and take precedence over ?Sized. SemVer linting requires precise—not just syntactic—information in both of these cases. Failure to obtain precise information would lead to both false-positives and false-negatives, which would be an unacceptable user experience. Rustdoc JSON today does not include such precise information; the addition of such information is currently under discussion with the rustdoc team.

Outside of these special cases of type-checking lints, some progress was made on a concrete plan for more general type-checking lints. The current plan is to make use of Rust's only stable API: "please compile this program for me." Whenever types in any public API location might appear to have changed, cargo-semver-checks would generate a witness program and check it via cargo check, such that compilation would only succeed if the type change is backward-compatible. This is an extension of the same technique we used several years ago in our survey of SemVer compliance and breakage in the Rust ecosystem. One of Rust's Google Summer of Code participants is working toward making witness generation work end-to-end, and we're thrilled to be working together!

As part of the All Hands at RustWeek, we also made progress toward enabling cross-crate linting. We identified a set of changes in Rust tooling that, when implemented, will result in cargo-semver-checks being able to uniquely determine which crate and version a given item came from. While all stakeholders are aligned on the idea and have reasonable confidence that it will work as planned, much more implementation work remains to be done and it's possible additional obstacles may be identified along the way.

Checking of cross-crate items

This section is background information and is unchanged from the 2024h2 goal.

Currently, cargo-semver-checks performs linting by only using the rustdoc JSON of the target package being checked. However, the public API of a package may expose items from other crates. Since rustdoc no longer inlines the definitions of such foreign items into the JSON of the crate whose public API relies on them, cargo-semver-checks cannot see or analyze them.

This causes a massive number of false-positives ("breakage reported incorrectly") and false-negatives ("lint for issue X fails to spot an instance of issue X"). In excess of 90% of real-world false-positives are traceable back to a cross-crate item, as measured by our SemVer study!

For example, the following change is not breaking but cargo-semver-checks will incorrectly report it as breaking:

#![allow(unused)]
fn main() {
// previous release:
pub fn example() {}

// in the new release, imagine this function moved to `another_crate`:
pub use another_crate::example;
}

This is because the rustdoc JSON that cargo-semver-checks sees indeed does not contain a function named example. Currently, cargo-semver-checks is incapable of following the cross-crate connection to another_crate, generating its rustdoc JSON, and continuing its analysis there.

Resolving this limitation will require changes to how cargo-semver-checks generates and handles rustdoc JSON, since the set of required rustdoc JSON files will no longer be fully known ahead of time. It will also require CLI changes in the same area as the changes required to support manifest linting.

While there may be other challenges on rustc and rustdoc's side before this feature could be fully implemented, we consider those out of scope here since there are parallel efforts to resolve them. The goal here is for cargo-semver-checks to have its own story straight and do the best it can.

SemVer linting of type information

This section is background information and is unchanged from the 2024h2 goal.

In general, at the moment cargo-semver-checks lints cannot represent or examine type information. For example, the following change is breaking but cargo-semver-checks will not detect or report it:

#![allow(unused)]
fn main() {
// previous release:
pub fn example(value: String) {}

// new release:
pub fn example(value: i64) {}
}

Analogous breaking changes to function return values, struct fields, and associated types would also be missed by cargo-semver-checks today.

The main difficulty here lies with the expressiveness of the Rust type system. For example, none of the following changes are breaking:

#![allow(unused)]
fn main() {
// previous release:
pub fn example(value: String) {}

// new release:
pub fn example(value: impl Into<String>) {}

// subsequent release:
pub fn example<S: Into<String>>(value: S) {}
}

Similar challenges exist with lifetimes, variance, trait solving, async fn versus fn() -> impl Future, etc.

While some promising preliminary work has been done toward resolving this challenge, more in-depth design work is necessary to determine the best path forward.

The next 6 months

• Expose precise information regarding 'static and ?Sized, then make use of it in lints.
• Audit our current set of lints related to trait and lifetime bounds, aiming to create a comprehensive list of what lints we still need to add.
• Support our GSoC contributor in building witness program generation and type-checking infrastructure.
• Support the rustdoc team in exposing additional information we require, by contributing ideas and pull requests as needed while seeking to minimize the review and maintenance burden thus applied.

The "shiny future" we are working towards

This section is unchanged from the 2024h2 goal.

Accidentally publishing SemVer violations that break the ecosystem is never fun for anyone involved.

From a user perspective, we want a fearless cargo update: one's project should never be broken by updating dependences without changing major versions.

From a maintainer perspective, we want a fearless cargo publish: we want to prevent breakage, not to find out about it when a frustrated user opens a GitHub issue. Just like cargo flags uncommitted changes in the publish flow, it should also quickly and accurately flag breaking changes in non-major releases. Then the maintainer may choose to release a major version instead, or acknowledge and explicitly override the check to proceed with publishing as-is.

To accomplish this, cargo-semver-checks needs the ability to express more kinds of lints (including manifest and type-based ones), eliminate false-positives, and stabilize its public interfaces (e.g. the CLI). At that point, we'll have lifted the main merge-blockers and we can consider making it a first-party component of cargo itself.

Ownership and team asks

Owner: Predrag Gruevski, as maintainer of cargo-semver-checks

I (Predrag Gruevski) will be working on this effort. The only other resource request would be occasional discussions and moral support from the cargo and rustdoc teams, of which I already have the privilege as maintainer of a popular cargo plugin that makes extensive use of rustdoc JSON.

Task	Owner(s) or team(s)	Notes
Expose precise 'static and ?Sized info	Predrag Gruevski
Lints for 'static and ?Sized	Predrag Gruevski
Audit lints for lifetime and trait bounds	Predrag Gruevski
Support our GSoC contributor's work on witness programs and type-checking infra	Predrag Gruevski
Discussion and moral support	cargo rustdoc

Frequently asked questions

This section is unchanged from the 2024h2 goal.

Why not use semverver instead?

Semverver is a prior attempt at enforcing SemVer compliance, but has been deprecated and is no longer developed or maintained. It relied on compiler-internal APIs, which are much more unstable than rustdoc JSON and required much more maintenance to "keep the lights on." This also meant that semverver required users to install a specific nightly versions that were known to be compatible with their version of semverver.

While cargo-semver-checks relies on rustdoc JSON which is also an unstable nightly-only interface, its changes are much less frequent and less severe. By using the Trustfall query engine, cargo-semver-checks can simultaneously support a range of rustdoc JSON formats (and therefore Rust versions) within the same tool. On the maintenance side, cargo-semver-checks lints are written in a declarative manner that is oblivious to the details of the underlying data format, and do not need to be updated when the rustdoc JSON format changes. This makes maintenance much easier: updating to a new rustdoc JSON format usually requires just a few lines of code, instead of "a few lines of code apiece in each of hundreds of lints."