#semver #plugin

nightly bin+lib semverver

Automatic verification of SemVer adhrence in rust library crates

37 releases

✓ Uses Rust 2018 edition

new 0.1.37 Jun 15, 2019
0.1.32 May 29, 2019
0.1.27 Mar 26, 2019
0.1.22 Dec 8, 2018
0.1.7 Mar 30, 2018

#8 in Cargo plugins

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This repository is hosting a proof-of-concept implementation of an automatic tool checking rust library crates for semantic versioning adherence, developed during the Google Summer of Code 2017. The goal is to provide an automated command akin to cargo clippy that analyzes the current crate's source code for changes compared to the most recent version on crates.io.

Details on the work done during GSoC 2017 can be found here.


The approach taken is to compile both versions of the crate to rlibs and to link them as dependencies of a third, empty, dummy crate. Then, a custom compiler driver is run on the said dummy and all necessary analysis is performed in that context, where type information and other resources are available.

More information on the inner workings of the tool can be found here.


The tool is implemented as a cargo plugin. As of now, it can be obtained from this git repository and compiled from source or installed from crates.io. Keep in mind that only the newest version of the nighly toolchain is supported at any given time.

If you are already using Rust nightly and have successfully installed tools like cargo add and cargo clippy, just do:

$ rustup update nightly
$ cargo +nightly install semverver

You'd also need cmake for some dependencies, and a few common libraries (if I you hit build failures because of missing system-wide dependencies, please open an issue, so they can be added here).

You can also install the newest version of the tool from git:

$ rustup update nightly
$ cargo +nightly install --git https://github.com/rust-dev-tools/rust-semverver
Manual installation and more details
# using rustup is recommended
$ rustup update nightly
$ rustup default nightly

$ git clone https://github.com/rust-dev-tools/rust-semverver
$ cd rust-semverver
$ cargo install

At this point, the current development version can be invoked using cargo semver in any directory your project resides in. If you prefer not to install to ~/.cargo/bin, you can invoke it like so after building with a regular cargo build:

$ PATH=/path/to/repo/target/debug:$PATH cargo semver <args>

If you have built using cargo build --release instead, change the path to point to the release subdirectory of the target directory.


By default, running cargo semver in directory with a Cargo project will try to compare the local version the one last published on crates.io, and display warnings or errors for all changes found.

Invoking cargo semver -h gives you the latest help message, which outlines how to use the cargo plugin:

$ cargo semver -h
usage: cargo semver [options] [-- cargo options]

    -h, --help          print this message and exit
    -V, --version       print version information and exit
    -e, --explain       print detailed error explanations
    -d, --debug         print command to debug and exit
    -s, --stable-path PATH
                        use local path as stable/old crate
    -c, --current-path PATH
                        use local path as current/new crate
    -S, --stable-pkg NAME:VERSION
                        use a `name:version` string as stable/old crate
    -C, --current-pkg NAME:VERSION
                        use a `name:version` string as current/new crate

This means that you can compare any two crates' specified versions, as long as they are available on crates.io or present on your filesystem.

CI setup

Assuming you use a CI provider that gives you access to cargo, you can use the following snippet to check your build for semver compliance, and enforce that version bumps are carried out correctly with regards to the current version of your crate on crates.io:

# install a current version of rust-semverver
cargo install semverver
# fetch the version in the manifest of your crate (adapt this to your usecase if needed)
eval "current_version=$(grep -e '^version = .*$' Cargo.toml | cut -d ' ' -f 3)"
# run the semver checks and output them for convenience
cargo semver | tee semver_out
# fail the build if necessary
(head -n 1 semver_out | grep "\-> $current_version") || (echo "versioning mismatch" && return 1)

Make sure you do the above with access to a nightly toolchain. Check your CI provider's documentation on how to do that.


The guideline used to implement semver compatibility is the API evolution RFC, which applies the principles of semantic versioning to the Rust language's semantics. According to the RFC, most changes are already recognized correctly, even though some type checks still behave incorrectly in edge-cases. A longterm goal is to fix this in the compiler.

At the time of writing, the following types of changes are recognized and classified correctly:

  • items moving from pub to non-pub and vice-versa
  • items changing their kind, i.e. from a struct to an enum
  • additions and removals of region parameters to and from an item's declaration
  • additions and removals of (possibly defaulted) type parameters to and from an item's declaration
  • changes to the variance of type and region parameters
  • additions and removals of enum variants
  • additions and removals of enum variant- or struct fields
  • changes from tuple structs or variants to struct variants and vice-versa
  • changes to a function or method's constness
  • additions and removals of a self-parameter on methods
  • additions and removals of (posslibly defaulted) trait items
  • correct handling of "sealed" traits
  • changes to the unsafety of a trait
  • type changes of all toplevel items, as well as associated items in inherent impls and trait definitions
  • additions and removals of inherent impls or methods contained therein
  • additions and removals of trait impls

Yet, the results presented to the user are merely an approximation of the required versioning policy, especially at such an early stage of development.


~675K SLoC