Re-export optional cargo feature that can be turned off - rust

Question

With the following directory structure:
tree hello_dep
.
├── Cargo.lock
├── Cargo.toml
├── dep_a
│ ├── Cargo.toml
│ └── src
│ └── main.rs
├── dep_b
│ ├── Cargo.toml
│ └── src
│ └── main.rs
└── src
└── main.rs
And the following dependency chain: hello_dep -> dep_a -> dep_b -> (optional feature) rustc-serialize,
I would like to create a feature in dep_a that re-exports the optional rustc-serialize feature in dep_b.
At the bottom, I have dep_b, which has rustc-serialize as an optional default feature:
# dep_b/Cargo.toml
[package]
name = "dep_b"
version = "0.1.0"
[dependencies]
rustc-serialize = { version = "0.3.19", optional = true }
[features]
default = ["rustc-serialize"]
I would like to create a feature in dep_a to optionally reexport "rustc-serialize". Here is the attempt:
# dep_a/Cargo.toml
[package]
name = "dep_a"
version = "0.1.0"
[dependencies]
dep_b = { version = "0.1.0", path = "../dep_b" }
[features]
rustc-serialize = ["dep_b/rustc-serialize"]
default = ["rustc-serialize"]
However, when I try to add this as a dependency with the default off using the following Cargo.toml:
# hello_dep/Cargo.toml
[package]
name = "hello_dep"
version = "0.1.0"
[dependencies]
dep_a = { version = "0.1.0", path = "dep_a", default-features = false, optional = true }
cargo build still yields rustc-serialize in Cargo.lock. But directly depending on dep_b correctly avoids pulling in rustc-serialize with the following line
dep_b = { version = "0.1.0", path = "dep_b", default-features = false }
Is this a bug in Cargo, or am I doing something wrong? Here is a related question

Answer 1

In dep_a/Cargo.toml, you did not specify default-features = false on the dep_b dependency. Therefore, the rustc-serialize feature in dep_b is enabled by default. The fact that you included a feature in dep_a to enable dep_b's rustc-serialize doesn't change the fact that it's still enabled when dep_a's feature is not enabled.
Thus, in dep_a/Cargo.toml, you should have:
[dependencies]
dep_b = { version = "0.1.0", path = "../dep_b", default-features = false }