That's not correct. cargo will fail to resolve your dependencies if that happens. For example (cargo_metadata 0.2.1 depends on serde ^1.0.2): [dependencie

That's not correct. cargo will fail to resolve your dependencies if that happens. For example (cargo_metadata 0.2.1 depends on serde ^1.0.2): [dependencies] serde = "=1.0.0" cargo_metadata = "0.2.1" Attempting to cargo fetch will result in this error message: error: failed to select a version for `serde` (required by `cargo_metadata`): all possible versions conflict with previously selected versions of `serde` This is an intentional design decision in cargo that isn't well known.