rust-lang/cargo · error · anyhow::Error

failed to parse the version requirement

Error message

failed to parse the version requirement `{}` for dependency `{}`

What it means

`Dependency::parse` constructs a `Dependency` from a manifest entry: a name, an optional version string, and a `SourceId`. When `version` is `Some(v)`, it calls `SemVer::VersionReq::parse(v)`; on failure it wraps the parse error with `.context(...)` interpolating both the raw version requirement string and the dependency name. This surfaces malformed version requirements (e.g. invalid operators, bad comparator syntax) with full context about which dependency failed.

Solutions

Correct the version requirement to valid Cargo/SemVer syntax (e.g. `"1"`, `"1.2"`, `"=1.2.3"`, `"^1.2"`, `">=1.0, <2.0"`).
Refer to the Cargo SemVer requirement grammar and rewrite the specifier accordingly.
Remove the version key entirely if any version is acceptable (Cargo treats omission as `*`).

Example fix

# before
[dependencies]
serde = { version = "^=>1.0.0" }

# after
[dependencies]
serde = { version = "1.0.0" }

Defensive patterns

Strategy: validation

Validate before calling

use semver::VersionReq;
fn valid_version_req(v: &str) -> bool { VersionReq::parse(v).is_ok() }
// validate dependency version strings before writing them into Cargo.toml
assert!(valid_version_req("1.0.0"));

Prevention

Validate version requirements with `semver::VersionReq::parse` before committing manifests.
Run `cargo check`/`cargo metadata` in CI to catch manifest parse errors.
Keep a reference of Cargo SemVer syntax and avoid cross-ecosystem specifiers.

When it happens

Trigger: Declaring a dependency with an unparseable version requirement in `Cargo.toml`, e.g. `serde = { version = "^=>1.0" }` or `req = "1.0.0-alpha.."`. `VersionReq::parse` returns `Err`, triggering the context-wrapped error.

Common situations: Typos in version specifiers, mixing npm/cargo version syntax, trailing operators, incomplete ranges, copy-pasting a version from another ecosystem, or using prerelease/metadata syntax that SemVer rejects.

Understand the failure class

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of rust-lang/cargo@98a09e7e7d (2026-08-11). Data as JSON: /api/errors/1c133e1a8b39a79d. Report an issue: GitHub.

Appendix: source

Thrown at src/workspace/dependency.rs:129

            DepKind::Build => Some("build"),
        }
        .serialize(s)
    }
}

impl Dependency {
    /// Attempt to create a `Dependency` from an entry in the manifest.
    pub fn parse(
        name: impl Into<InternedString>,
        version: Option<&str>,
        source_id: SourceId,
    ) -> CargoResult<Dependency> {
        let name = name.into();
        let (specified_req, version_req) = match version {
            Some(v) => match VersionReq::parse(v) {
                Ok(req) => (true, OptVersionReq::Req(req)),
                Err(err) => {
                    return Err(anyhow::Error::new(err).context(format!(
                        "failed to parse the version requirement `{}` for dependency `{}`",
                        v, name,
                    )));
                }
            },
            None => (false, OptVersionReq::Any),
        };

        let mut ret = Dependency::new_override(name, source_id);
        {
            let ptr = Arc::make_mut(&mut ret.inner);
            ptr.only_match_name = false;
            ptr.req = version_req;
            ptr.specified_req = specified_req;
        }
        Ok(ret)
    }

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