According to its manual, Cargo packages can have multiple executable targets, but only one library target is allowed.
A package can contain zero or one library crates and as many binary crates as you’d like. There must be at least one crate (either a library or a binary) in a package.
Why is it limited to one? What are the reasons and benefits?
Cargo is primarily a package manager. Thus, the primary role of a package is to define a library.
When we use a crate as a dependency, we only specify the package name in our Cargo.toml. Since there can be at most one library, Cargo doesn't need you to specify which one to use. If it were allowed to define multiple libraries in the same package, then we'd need to specify a way to define dependencies between them, so you'd have two ways to declare dependencies (external packages vs. internal crates), making the system more complex.
On the other hand, adding a dependency that doesn't provide a library doesn't make sense, at least not with Cargo, since Cargo only cares about the library target in that context. Thus, there is no reason to limit the other types of targets (binaries, examples, tests, etc.) to one each.
来源：https://stackoverflow.com/questions/54843118/why-can-a-cargo-package-only-have-one-library-target