Cargo Workspace Features by Rust Version

Which Rust/Cargo release stabilised each manifest and workspace feature this skill relies on. Cargo ships with Rust, so "1.64" means "Cargo shipped with Rust 1.64". Use this to choose an MSRV, or to explain why an old toolchain cannot parse a manifest. The manifest_lint script's rule CW020 checks the most common mismatches.

#Contents

• Quick matrix
• Timeline
• Edition 2021 Cargo changes
• Edition 2024 Cargo changes
• Lock file versions
• Choosing an MSRV for manifest features
• Migrating an older workspace

#Quick matrix

#Timeline

#1.12 - 1.44: the foundations

• 1.12: [workspace] with a shared Cargo.lock and target/ directory; members by path.
• 1.31: Rust 2018 edition; dependency renaming via package = "...".
• 1.34: alternate registries.
• 1.37: cargo vendor for offline/vendored builds.
• 1.41: profile overrides: [profile.dev.package.<name>], [profile.dev.package."*"] for all dependencies, and [profile.dev.build-override] for build scripts and proc-macros.
• 1.44: cargo tree built in (previously the cargo-tree plugin).

#1.51 - 1.60: features grow up

• 1.51: feature resolver 2 (resolver = "2"): build-dependency, dev-dependency and target-specific features stop leaking into normal builds. Opt-in until edition 2021.
• 1.56: edition 2021 (resolver 2 by default for packages, but not for virtual manifests, which have no edition); rust-version key; [env] in config.
• 1.57: custom profiles with inherits.
• 1.59: strip in profiles.
• 1.60: dep: namespaced features and ?/ weak dependency features; --timings stable.

#1.62 - 1.75: workspace ergonomics

• 1.62 / 1.66: cargo add / cargo remove.
• 1.64: workspace inheritance: [workspace.package] and [workspace.dependencies]; members use key.workspace = true. Manifests using it fail to parse on older Cargo.
• 1.68 / 1.70: sparse registry protocol; default for crates.io in 1.70 (much faster index updates in CI).
• 1.71: debug = "line-tables-only".
• 1.74: [lints] and [workspace.lints]; --keep-going; credential providers for registries.
• 1.75: package.version may be omitted (defaults to 0.0.0, not publishable).

#1.77 - 1.90: MSRV awareness and edition 2024

• 1.77: cargo::KEY=VALUE build-script directive syntax; release profiles strip debuginfo by default when they have no debuginfo of their own.
• 1.78: Cargo can read lock file version 4.
• 1.80: --check-cfg is on by default; unknown cfg names/values (including feature names) warn via unexpected_cfgs; expected custom cfgs are declared in [lints.rust] unexpected_cfgs.
• 1.82: cargo info <crate> shows crate metadata from the registry.
• 1.83: new lock files use version 4.
• 1.84: MSRV-aware resolver: resolver = "3" in the manifest, or incompatible-rust-versions = "fallback" under [resolver] in .cargo/config.toml. Version selection prefers dependencies whose rust-version is compatible with your packages' rust-version.
• 1.85: Rust 2024 edition (see below).
• 1.90: cargo publish can publish multiple packages in one invocation (--workspace), handling the order; rust-lld becomes the default linker on x86_64-unknown-linux-gnu.

#Edition 2021 Cargo changes

• Default resolver = "2" for packages on edition 2021.
• Virtual manifests are unaffected: they still need resolver = "2" explicitly; Cargo warns when members are on 2021 but the virtual workspace defaults to resolver 1.

#Edition 2024 Cargo changes

Edition 2024 (Rust 1.85) changes three Cargo behaviours for packages that opt in:

1. Rust-version aware resolver: edition 2024 implies resolver = "3". Virtual manifests still need the key (resolver = "3").
2. Table and key name consistency: the underscore aliases are rejected: dev_dependencies, build_dependencies, default_features, crate_type, proc_macro and friends, as is [project] instead of [package] (CW017). Use the hyphenated forms.
3. Unused inherited default-features rejected: { workspace = true, default-features = false } is an error when the workspace entry does not itself disable default features (CW016). Before 2024 it was a warning and was ignored.

Migration: cargo fix --edition per crate (it handles code; review manifests by hand), then set edition = "2024" in [workspace.package]. Edition is per crate, so a workspace can migrate crate by crate, but CW005 flags the mixed state so it does not become permanent.

#Lock file versions

Cargo keeps the version of an existing lock file unless it has to change it. An MSRV CI job on a toolchain older than 1.78 cannot read a v4 lock file; either keep the file at version 3 or regenerate it with the MSRV toolchain.

#Choosing an MSRV for manifest features

The manifest itself sets a floor below which your declared MSRV is meaningless:

Dependencies set their own floors: if tokio requires 1.70, your MSRV is at least 1.70 regardless of your manifest. With resolver 3, cargo update picks dependency versions that respect your rust-version where possible; cargo msrv find or cargo hack check --rust-version confirms the real value.

#Pinning the toolchain

A rust-toolchain.toml at the workspace root makes rustup use the same toolchain for every developer and CI job that runs cargo in the repository:

[toolchain]
channel = "1.85.0"              # or "stable", or a dated nightly such as "nightly-2025-06-01"
components = ["clippy", "rustfmt"]
profile = "minimal"

• Pinning the development toolchain (for reproducible clippy and rustfmt output) is independent of the MSRV: develop on a recent stable, test the MSRV in a separate CI job with cargo +1.xx or cargo hack check --rust-version.
• Bump the pinned toolchain in its own PR; new clippy lints appear with new toolchains.
• rustc --version and cargo --version print what is actually used; rustup show explains why (override, toolchain file, default).

#Migrating an older workspace

1. Add resolver = "2" (or "3" if MSRV >= 1.84) to a virtual manifest; check that nothing relied on resolver 1 feature leaking (build with --workspace and with each -p).
2. Introduce [workspace.package] and [workspace.dependencies] (needs 1.64); move versions there one dependency at a time; members switch to .workspace = true.
3. Add [workspace.lints] (1.74) and [lints] workspace = true in every member; move #![deny(...)] crate attributes and RUSTFLAGS lint flags into it.
4. Declare rust-version and add an MSRV CI job.
5. Move to edition 2024 when MSRV allows 1.85: cargo fix --edition, fix manifests (underscore keys, inherited default-features), set the edition at the root.
6. Run /tmp/manifest_lint . --strict and /tmp/dep_audit . --strict after each step.