When building with --no-default-features the compiler emits:
warning: unused import: `mem`
The call site is feature gated so we either need to feature gate the
import or use a fully qualified path. Since 'core' is quite short elect
to use the fully qualified path.
This feature was not useful for Cargo users, since Cargo does not give you
the kind of fine-grained control over C library linkage that you need. So
it was just unnecessarily confusing and would cause the build to break if
you enabled it accidentally, say, with --all-features.