Why `features = ["std"]` is not necessary in the `[dev-dependencies]` some crates like `sp-io`?

Question

We often use some basic substrate crates such as sp-core, and sp-io while doing pallet tests. In general, those dependencies are put under [dev-dependencies]. But I have an issue with whether we should add default-features = false or not. Seems both work.

For example:

# pallet crate level

[dependencies]
frame-system = = { default-features = false, git = "https://github.com/paritytech/substrate", branch = "polkadot-v0.9.37" }
# no sp-io here

[dev-dependencies]
# With`default-features = false`
sp-io = = { default-features = false, git = "https://github.com/paritytech/substrate", branch = "polkadot-v0.9.37" }

or

# Without default-features = false
sp-io = = { git = "https://github.com/paritytech/substrate", branch = "polkadot-v0.9.37" }

In the pallet tests (both ways can compile):

// mock.rs
use sp_io::TestExternalities;

Check from the substrate repo:

/// Type alias for Externalities implementation used in tests.
#[cfg(feature = "std")]  // this line
pub type TestExternalities = sp_state_machine::TestExternalities<sp_core::Blake2Hasher>;

As the code snippet shows, the TestExternalities is only available understd feature.So why the first way, With default-features = false can be compiled?

Answer 1

I think it's because the frame-system also depends on sp-io. And you enable the std for the frame-system in the test env, which also enables the sp-io/std for the whole scope.