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rust-semverver is a tool to check semver-compliance in Rust library crates. The core of the tool has been developed as a student project during the Google Summer of Code 2017.

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 Keep in mind that only the pinned version(in [rust-toolchain]) of the nightly toolchain is supported at any given time.

It's recommended to use nightly-2022-08-03 toolchain. You can install it by using rustup install nightly-2022-08-03 if you already have rustup. Then you can do:

$ rustup component add rustc-dev llvm-tools-preview --toolchain nightly-2022-08-03
$ cargo +nightly-2022-08-03 install --git

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

Manual installation and more details
$ git clone
$ 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 to the one last published on, 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]

    -h, --help          print this message and exit
    -V, --version       print version information and exit
    -e, --explain       print detailed error explanations
    -q, --quiet         suppress regular cargo output, print only important
        --show-public   print the public types in the current crate given by
                        -c or -C and exit
    -d, --debug         print command to debug and exit
    -a, --api-guidelines
                        report only changes that are breaking according to the
        --features FEATURES
                        Space-separated list of features to activate
        --all-features  Activate all available features
                        Do not activate the `default` feature
        --compact       Only output the suggested version on stdout for
                        further processing
    -j, --json          Output a JSON-formatted description of all collected
                        data on stdout.
    -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
        --target <TRIPLE>
                        Build for the target triple

This means that you can compare any two crates' specified versions, as long as they are available on 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

# install a current version of rust-semverver
cargo +nightly-2022-08-03 install --git
# 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.

JSON output

By passing the -j flag, all output on standard out is formatted as a machine-readable JSON blob. This can be useful for integration with other tools, and always generates all possible output (ignoring other output-related flags). The output format is defined as follows:

The top level object contains the keys old_version, new_version and changes. The former two hold a version number in the format major.minor.patch, the latter an object describing changes between the crate versions, which contains two arrays in the keys path_changes and changes.

The path_changes array contains objects describing item additions and removals, which have the following keys:

  • name: The name of the item.
  • def_span: An object describing the location of the item in one of the crates.
  • additions: An array of spans that describe locations where the item has been added.
  • removals: An array of spans that describe locations where the item has been removed.

An example object might look like this:

  "name": "NFT_META_CGROUP",
  "def_span": {
    "file": "/path/to/libc-0.2.48/src/unix/notbsd/linux/other/",
    "line_lo": 776,
    "line_hi": 776,
    "col_lo": 0,
    "col_hi": 40
  "additions": [
      "file": "/path/to/libc-0.2.48/src/",
      "line_lo": 195,
      "line_hi": 195,
      "col_lo": 16,
      "col_hi": 23
  "removals": []

The changes array contains objects describing all other changes, which have the following keys:

  • name: The name of the item
  • max_category: the most severe change category for this item, as a string.
    • Possible values are Patch, NonBreaking, TechnicallyBreaking, and Breaking.
  • new_span: an object describing the location of the item in the new crate (see example).
  • changes: an array of 2-element sequences containing an error message and an optional sub-span (null if none is present)

An example object might look like this:

  "name": "<new::util::enumerate::Enumerate<T> as new::prelude::Stream>",
  "max_category": "TechnicallyBreaking",
  "new_span": {
    "file": "/path/to/tokio-0.1.17/src/util/",
    "line_lo": 46,
    "line_hi": 63,
    "col_lo": 0,
    "col_hi": 1
  "changes": [
      "trait impl generalized or newly added",

For reference, all objects describing spans have the same keys:

  • file: A file name.
  • line_lo: The line the span starts on.
  • line_hi: The line the span ends on.
  • col_lo: The column the span starts on.
  • col_hi: The column the span ends on.


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 (possibly 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

Keep in mind however that the results presented to the user are merely an approximation of the required versioning policy.


Please see


rust-semverver is distributed under the terms of the 3-clause BSD license.

See LICENSE for details.


Automatic checking for semantic versioning in library crates



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