rust-lang/cargo · error

match ensured element is present

Error message

match ensured element is present

What it means

This panic is in infer_package_for_git_source(). When packages.len() == 1 (exactly one package found in the git source), the code calls packages.pop().expect("match ensured element is present"). The match statement on line 1045 guarantees len is exactly 1 in this arm, so pop() must return Some. This is a pure structural invariant.

Source

Thrown at src/ops/cargo_add/mod.rs:1050

                    anyhow::bail!(
                        "unexpectedly found multiple copies of crate `{dependency}` at `{source}`"
                    )
                }
            }
        }
    }
}

fn infer_package_for_git_source(
    mut packages: Vec<Package>,
    src: &dyn std::fmt::Display,
) -> CargoResult<Package> {
    let package = match packages.len() {
        0 => unreachable!(
            "this function should only be called with packages from `GitSource::read_packages` \
            and that call should error for us when there are no packages"
        ),
        1 => packages.pop().expect("match ensured element is present"),
        _ => {
            let mut names: Vec<_> = packages
                .iter()
                .map(|p| p.name().as_str().to_owned())
                .collect();
            names.sort_unstable();
            anyhow::bail!(
                "multiple packages found at `{src}`:\n    {}\nTo disambiguate, run `cargo add --git {src} <package>`",
                names
                    .iter()
                    .map(|s| s.to_string())
                    .coalesce(|x, y| if x.len() + y.len() < 78 {
                        Ok(format!("{x}, {y}"))
                    } else {
                        Err((x, y))
                    })
                    .into_iter()
                    .format("\n    "),

View on GitHub (pinned to 0e07a15537)

Solutions

1. Re-run cargo add — if it was a transient memory issue it won't recur.
2. Run a memory diagnostic (memtest86) if this panic recurs consistently.
3. Report as a cargo bug with full reproduction steps, as it indicates a deeper system issue.

// This is effectively unreachable; catch as a last resort
use std::panic;
let result = panic::catch_unwind(|| {
    infer_package_for_git_source(packages, &src)
});
if result.is_err() {
    eprintln!("internal error in package inference; retrying");
    // retry or fallback
}

When it happens

Trigger: Theoretically unreachable — if packages.len() == 1 then Vec::pop always returns Some. This could only fire due to memory corruption, a concurrent mutation of the packages vector (which is owned and not shared), or an extremely unlikely allocator bug.

Common situations: Effectively never fires in practice; if it does, suspect memory corruption, a faulty RAM module, or a severe allocator bug. No normal cargo usage should trigger this.

Related errors

• manifest validated
• latest always has a source
• as `None` are compatible, we can't be here
• source should be resolved before here
• only none when there is 1

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/71302d7f26bc8a3d.json. Report an issue: GitHub.