This Development-cycle in Cargo: 1.79

May 7, 2024 · Ed Page on behalf of The Cargo Team

This Development-cycle in Cargo: 1.79

This is a summary of what has been happening around Cargo development for the last 6 weeks which is approximately the merge window for Rust 1.79.

Plugin of the cycle

Cargo can't be everything to everyone, if for no other reason than the compatibility guarantees it must uphold. Plugins play an important part of the Cargo ecosystem and we want to celebrate them.

Our plugin for this cycle is cargo-outdated which gives an overview of out-of-date dependencies. As of Cargo 1.78, we include some of this information in the cargo-update output (#13372). Try giving cargo update --dry-run --verbose a try! As for how we could further improve our reporting of outdated dependencies, see #4309.

Thanks to LukeMathWalker for the suggestion!

Please submit your suggestions for the next post.



Update from 1.78

weihanglo dug into the cargo code base to enumerate official and unofficial deprecations and recorded them in #13629.

The deprecations ended up being divided into the following categories:

Deprecate, remove on next Edition: including #13747, #13804, and #13839.

Deprecate but never remove: This is targeted in areas like the CLI or .cargo/config.toml which don't have an Edition mechanism to evolve them.

Remove, breaking compatibility: This is focused on bugs with minimized impact to users.

An easy example is badges.workspace = true allowing inheritance from package.badges. This was not in the RFC, undocumented, and didn't follow the standard pattern for inheritance making it harder to discover. We removed support for this in #13788.

Cargo also allowed dependencies without a source (e.g. dep = {}). This was originally removed 3 years ago in #9686 but was reverted after it was reported to have broken an old version of the bit-set crate which was used by libusb which has gone unmaintained (see #9885). We revisited this and decided to remove support for it again (see #13775) and soon after a user of libusb noticed again (#13824). After looking at this more carefully, we decided to stick with our original decision. We broke people 3 years ago, been warning since that it will be removed, and there are two maintained replacement packages (rusb and nusb).

Re-evaluate in the future: In particular, for #4797, we want to wait until there is a stable mechanism to replace it.

User-controlled cargo diagnostics

Update from 1.78. In summary, this aims to add user-controlled cargo lints that look like rustc and are controlled through the [lints] table

Muscraft started off this development cycle with a rough sketch of lint system (#13621) and fleshed it out and polished it up including

  • Reporting why a lint is being shown (#13801)
  • Handling forbid's special behavior (#13797)
  • Support for unstable lints (#13805)

Original lint names were written using kebab-case. In #13635, they were switched to also support snake_case to match rustc. After we had to deal with deprecating snake_case fields in Cargo.toml, Muscraft brought up whether we should initially only support one case. A couple of the participants stylistically preferred kebab-case, especially to match the rest of the manifest. However, rustc considers snake_case to be the canonical form and we decided that would be a good starting point (#13837). We can always add a second style later, if we so wished.

Our test case for this functionality is deprecating implicit features in Edition 2024. We modeled this as a deprecation warning for implicit features in existing Editions while Edition 2024 will report the optional dependency as unused (#13778). We discussed how we wanted to model unused optional dependemncies. At a high level, the most direct way is we change how we internally enumerate features to be based on the edition. However, this doesn't play well with registry packages. We resolve them off of the Index which doesn't have the full Cargo.toml, particularly the Edition, and prior versions of Cargo would read these Index entries and generate implicit features, breaking on upgrade of Cargo without extra care. Maybe we should work to support the Edition in the Index but we don't need to do that now. We ended up stripping unused optional dependencies from the published Cargo.toml and the Index. The way this was done also means they won't show up in Cargo.lock like unused workspace.dependencies. As a side effect, some lints may not run against these dependencies.

rendering of unused optional dependency lint

MSRV-aware Cargo

Update from 1.78

The subset needed for Edition 2024 is effectively code complete! Feel free to try it out and leave feedback.

We continued to iterate on how we report lockfile changes, including

  • Reporting that dependencies changed on any command, not just update (#13561, #13759)

We've continued to iterate on the MSRV resolver's behavior, including

  • Defaulting to rustc -V when your package.rust-version is unset (#13743)
  • Tweaked the behavior when a dependency's package.rust-version is unset (#13791)
  • Avoiding it for cargo install (#13790)

As for controlling the resolver policy, we've implemented:

  • --ignore-rust-version disables MSRV dependency resolution (#13738)
  • We added --ignore-rust-version to cargo update and cargo generate-lockfile (#13742)
  • We added a placeholder config field so it can be forced on or off (#13769). We still need final names for this, see #13540.
  • We added package.resolver = "3" (#13776)
  • We made this the default resolver for Edition 2024 (#13785)
Edition 2024

Update from 1.76

In addition to the above, work on Editions draws more attention to cargo fix. This includes #13728 and #13792 by weihanglo.

We also discussed on Zulip if there are Cargo.toml changes that should be made by cargo fix --edition, like updating the package.edition or the package.rust-version.

The challenge with updating package.edition is cargo fix only runs for one set of build targets, platforms, and feature combinations and so we don't know when an entire project is fully converted over to the new edition. The user might need to make multiple calls to migrate and updating package.edition too early can get in the way of that.

Normalizing Published Package Files

Update from 1.78

After much work ( #13666 #13693 #13701 #13713 #13729 ), published and vendored Cargo.toml files will now include all build targets explicitly enumerated.


  • You cannot bypass the checksum in adding a build target to a vendored dependency by dropping a file
  • When all build targets have an explicit path, you now get a warning if one is excluded when packing, helping to catch mistakes
  • You can now intentionally exclude a build target from publishing without having to set the path
  • It is easier to audit changes to the build targets across versions
  • We hope this opens the door to more performance improvements when parsing large dependency trees

As a side effect, the output from cargo vendor will vary by Cargo version. We try to minimize this kind of churn but felt it was justified in this case.

cargo info

Update from 1.76

There was some recent discussion on an issue for how cargo add should render features (#10681). epage figured cargo info could be a good place to try out their proposal (cargo-information#140). A question aspect of this was to apply the same rendering to dependencies to distinguish between required, activated-optional, and deactivated-optional dependencies.

epage also made the auto-selection of what version to show a little smarter. Instead of showing the latest when a version is unspecified, cargo info tries to be smart and show you a version that is relevant. Before, that was a version from your lockfile or a MSRV-compatible version. With cargo-information#137, we don't just check the lockfile but first check the direct dependencies of the package you are in and then the direct dependencies of all workspace members, making it more likely what will be shown is what you will be using.

rendering of cargo-info's verbose output using SVG (verbose output, normally dependencies are hidden)

At this point, cargo-information feels like it could be ready to merge into cargo. Please give it a try and let us know what you think!

Design discussions

Applying patch files to dependencies

Update from 1.76

Previously, we discussed closing this RFC, asking for an experimental implementation to help flesh out the design. weihanglo stepped in with a proof of concept in #13779. High level design discussions are on-going on that PR.

Cargo script

Update from 1.77

T-lang has approved RFC #3503 for the syntax of embedding manifests. This still leaves RFC #3502.

While cargo script is primarily focused on exploration, there will be times people want to do heavy analysis and want release builds (see RFC comment).

We could add cargo --release <script>. This runs into a common challenge of #! processing being limited in processing of interpreter arguments which can't be resolved in a fully cross-platform way.

Unfortunately, you can't just do:

inherits = "release"

as that is disallowed for built-in profiles. This also infects anything else you do on the script, like cargo test.

We could introduce a run profile as that is reserved for cargo's use. The name implies we should also update cargo run to use it which would cause the target directory to change from cargo build, causing extra builds and a larger target/.

We could add a config for default-profile. That would be stored in a separate config file that needs to go with the script. It also would shift the emphasis to the user setting it, rather than the script author. Likely, the script author knows best what to be done. We'd likely want to put this in the manifest.

Knowing that there are ways to improve this in the future in a backwards compatible way, we were fine deferring this out of the RFC.

Another challenge is the nuance of how to interpret cargo <something>. It could be a built-in command, an alias, a third-party command, or a script. How do we decide which to try first? This is the continuation of this RFC thread. epage had more details on prior art in epage/cargo-script-mvs#154. In looking at the different cases, the main problem we found is that a script named cargo test would run cargo-test. To run the script, you'd need to run cargo ./test.

An obvious way of solving this problem is a separate cargoscript binary. The RFC goes into some trade offs on this. Its particularly an issue if the binary name starts with cargo- as that would be picked up as a third-party command.

However, this only really happens with non-standard PATHs on Linux and Mac (Windows is a separate case) or having too-generic of script names in your PATH. This lowered the priority for us.

We could possibly take advantage of AT_EXECFN where supported so cargo can detect its being run as a #! interpreter and bypass the precedence rules.

Talking about this brought up a new concern: cargo <script> loads the config file based on <script>, rather than your current working directory. This is like cargo install and unlike all other cargo commands. The big concern that was raised was with running a script out of a temp directory or a downloads directory and picking up an unexpected config file. Now, the RFC specifies cargo <script> to load the config only from CARGO_HOME.

With these things resolved the RFC has been proposed for merging.


Update from 1.76

Though the RFC is not yet approved, justahero has created a proof-of-concept of it in #13709.

As a team, we recently discussed the configuration side of this RFC. Currently, the RFC lists this as a build.sbom = <bool> config field. When we previously discussed RFC #2801 (cargo auditable) and shifted our focus to a full SBOM, the core of the idea was that we could model SBOMs like we model debug info. We could start it as a "split" "full" SBOM and evolve from there. This would imply that this should be a profile setting, rather than a general build setting. Especially if we eventually support embedding this in the binary, it seems like something that is dependent on the type of build (release vs development) than something to do unconditionally. However, the debuginfo analogy falls down in that a project might want both a final SBOM and cargo auditable embedded SBOM.

If we set aside the future possibilities, it is assumed that this will mostly be set by wrapper tools that generate a proper SBOM out of the fragment we generate. They will be setting it on a per-run basis and not expect users to enable this setting, making the focus be on a environment variable, rather than config. In this framing, we aren't tying it to a specific profile and requiring people to do so could add some extra hurdles.

We could explore having a build setting for now and allow for layering a profile setting in the future. The main issue to be worked out is if there is a reasonable precedence policy to this.

During the discussion, we also came up with another framing. This could be viewed as a build report that could be used for more than just SBOMs but for external tools to analyze builds, including what was actually built, is being fingerprinted as part of the build, etc. In this case, its more of a transient build setting.

We were not yet able to reach a conclusion and will need to revisit this.

Nested packages

Update from 1.78

Last time the Cargo team discussed this, we didn't come to any firm conclusion and felt we needed more time to dig into the RFC text.

The core question to the discussion is "do we expect the benefits to be worth the cost?".

An additional use case that came up during the discussion was to combine this with artifact dependencies so you could have build scripts that were full packages that didn't require the overhead of publishing a full workspace.

Nested packages would create a footgun when it comes to workspaces. If you have a nested package that is a public dependency of multiple workspace members, you could start passing types between the packages. This will work locally because they all use the same instance of the nested package as a dependency. When you publish, each package will have its own instance of the nested package and the types will become incompatible.

In general, there was unease that nested packages could encourage people to vendor dependencies more often in a way that is less traceable and allows the code to drift in uncontrolled ways while we want to have the workflow encourage people to work directly with upstream.

As a side note, during the discussion, the idea came up for a new pub(scope) to allow access to private APIs within a namespace.

Workspace inheritance of deps

LukeMathWalker recently announced cargo autoinherit which will consolidate your dependencies of workspace members sources to [workspace.dependencies].

While discussing the announcement, a footgun came up with regards to default-features. If you set default-features = true in the workspace dependencies then default-features = false in your package dependencies is ignored (#12162). This might not be too bad except default-features = true is the default including when using the dep = "1.0" syntax. You'd then have to explicitly re-enable default-features when needed, for example:

foo = { version = "1.0", default-features = false }

foo = { workspace = true, default-features = true }

While discussing workspace inheritance for the public dependency field (#13125), one thought that epage had was that maybe dependency inheritance should focus exclusively on inheriting the source of the dependency and not anything else. When trying to not repeat oneself, it can be easy to go overboard and de-duplicate logic that reflects more of the current implementation, and not the requirements. As the implementation changes, you then have to shift things in and out between the shared and specific state.

Applying that concept to default-features, maybe we should disallow it in workspace dependencies (on a new Edition), and have the package dependencies' default-features always win. As workspaces don't have an Edition, we could frame this from the package's perspective as it inherits a field, making it so the package's Edition applies.

Going to the full extreme of removing default-features and features from workspace dependencies might be too large of departure from today. For example, likely a maintainer would find it convenient to share serde's derive feature throughout their workspace. An incremental step could be to treat the lack of default-features in a workspace dependency as unset.

We discussed this in the Cargo team meeting. To better communicate this, we borrowed a table from a prior discussion on inheriting default-features

Workspace Member 1.64 behaviour 1.69 behavior proposed 202X Edition behavior Reasoning
nothing df=false Enabled Enabled, warning that it is ignored Disabled Basically means let the package control default features when not specified
df=true df=false Enabled Enabled, warning Enabled, warning Should have been an error in the past since the field is ignored. May change to an error in the future.
nothing df=true Enabled Enabled Enabled There is no conflict about default being enabled.
df=true df=true Enabled Enabled Enabled ^Same
nothing nothing Enabled Enabled Enabled No ambiguity.
df=true nothing Enabled Enabled Enabled Enabled
df=false df=false Disabled Disabled Disabled No ambiguity.
df=false df=true Disabled Enabled Enabled This allows members to decide whether or not they want default features. The workaround of features=["default"] seems unnecessary.
df=false df=nothing Disabled Disabled Disabled Natural way to write "give me whatever was written in workspace".

Some points that were discussed

  • Documenting this as this deviates from the intended mental model
  • Ecosystem churn in switching to this
  • If we discourage features in workspace dependencies, it might preclude people who are doing it as low effort workspace hack. We felt that we should exercise caution and not remove features, yet.

As this was after the cut off for the 2024 Edition, this will likely be stuck waiting on the 2027 Edition.


  • Thank to PaulDance, we are now publishing cargo-test-support and cargo-test-macro which can be useful for developing tools that integrate with cargo #13418. The focus is on Cargo's needs, making the APIs and documentation a little rough for others to adopt.
  • Thanks for linyihai, we now have unstable support for cargo update foo --precise <prerelease> (#13626), last discussed in 1.76.

Focus areas without progress

These are areas of interest for Cargo team members with no reportable progress for this development-cycle.


Needs design and/or experimentation:


How you can help

If you have ideas for improving cargo, we recommend first checking our backlog and then exploring the idea on Internals.

If there is a particular issue that you are wanting resolved that wasn't discussed here, some steps you can take to help move it along include:

  • Summarizing the existing conversation (example: Better support for docker layer caching, Change in Cargo.lock policy, MSRV-aware resolver )
  • Document prior art from other ecosystems so we can build on the work others have done and make something familiar to users, where it makes sense
  • Document related problems and solutions within Cargo so we see if we are solving to the right layer of abstraction
  • Building on those posts, propose a solution that takes into account the above information and cargo's compatibility requirements (example)

We are available to help mentor people for S-accepted issues on zulip and you can talk to us in real-time during Contributor Office Hours. If you are looking to help with one of the bigger projects mentioned here and are just starting out, fixing some issues will help familiarize yourself with the process and expectations, making things go more smoothly. If you'd like to tackle something without a mentor, the expectations will be higher on what you'll need to do on your own.