Overbroad Version Requirements

Overbroad version requirements such as *, >=1, or unconstrained upper bounds invite future SemVer-incompatible releases into normal resolution.

The mistake

The mistake is writing dependency requirements that are much wider than the API compatibility you have tested or intend to support.

Examples include crate = "*", crate = ">=1.0", crate = "1.*" when a normal caret requirement is enough, or crate = ">=0.6,<2" for a public dependency without a strong reason.

Why it happens

Broad requirements look flexible, but they transfer risk to downstream users. Cargo generally prefers the highest matching version. If the requirement admits a future major version, a routine update can select it.

Broad ranges also make duplicate versions more likely. If one crate asks for rand = "0.7" and another asks for rand = ">=0.6", Cargo may select both 0.7 and a newer incompatible line for the broad requirement.

The correct alternative is usually the default caret form with the minimum version your code actually needs, such as serde = "1.0.219".

The resolver does not know which APIs your code actually calls. It can only compare version requirements and package metadata. A broad range says “any of these versions are acceptable,” so Cargo is allowed to pick a future release you never tested.

This is especially risky when a dependency’s types appear in your public API. If your crate accepts time::OffsetDateTime, then accidentally allowing a future incompatible time major line can split downstream type identity.

Example

mod v1 {
    pub struct Id(pub u64);
}
 
mod v2 {
    pub struct Id(pub u64);
}
 
fn takes_v1(id: v1::Id) -> u64 {
    id.0
}
 
fn main() {
    let id = v1::Id(7);
    assert_eq!(takes_v1(id), 7);
    let _other_major_is_a_different_type = v2::Id(7);
}

Two SemVer-incompatible crate versions behave like these modules: same names do not make their types interchangeable.

Bad and good manifest examples

# Bad: admits future breaking releases.
[dependencies]
serde = ">=1"
regex = "*"
time = ">=0.3, <1"
 
# Good: state the minimum compatible version you tested.
[dependencies]
serde = "1.0.219"
regex = "1.11.1"
time = "0.3.41"

The “good” lines are still flexible: Cargo may choose newer compatible releases, and Cargo.lock records the exact chosen versions for an application build.

Common errors

error: failed to select a version for `some-crate`
  previously selected package `dep v2.0.0`
  ... which satisfies dependency `dep = ">=1"` ...

Fix: narrow the broad requirement to the compatible major version your code supports, such as dep = "1.4.2".

error[E0308]: mismatched types
  expected `dep::Id`, found a different `dep::Id`

Fix: inspect cargo tree -d. Duplicate incompatible versions can produce same-named but distinct public types.

Best practice

• ✅ Use default caret requirements with all three components for normal dependencies.
• ✅ Update the minimum version when you begin using newer dependency APIs.
• ✅ Use exact requirements only for tightly coupled packages, such as a parent crate and companion proc macro.
• ✅ Let Cargo.lock pin application builds instead of encoding pins into library manifests.
• ✅ Treat public dependency types as part of your own SemVer surface and keep their ranges conservative.
• ✅ Use cargo update -p name --precise version for temporary application triage rather than publishing broad or exact library requirements.

Pitfalls

• ⚠️ Bare * is not allowed on crates.io and is still risky in unpublished packages.
• ⚠️ >=1.0 can admit 2.0, 3.0, or later breaking releases.
• ⚠️ Overly narrow ranges can be bad too; ~1.3 may conflict with another crate that correctly accepts 1.4.
• ⚠️ Combining broad lower bounds with no upper compatibility boundary can make a previously valid lockfile update fail months later.
• ⚠️ Broad dev-dependency ranges can still break CI, examples, benches, and publish dry runs.

See also

Cargo & Dependencies · Dependencies and Version Requirements · Semantic Versioning · Cargo.lock · cargo-audit and cargo-deny · Minimizing Dependencies · Cargo Workspaces · Feature Flags · Publishing to crates.io

Sources

• The Cargo Book, “Version requirement syntax” — cargo-book, https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#version-requirement-syntax
• The Cargo Book, “Dependency Resolution: Recommendations” — cargo-book, https://doc.rust-lang.org/cargo/reference/resolver.html#recommendations