Why the standard library uses extension traits (and not cfg-guarded items)

A common pattern in the standard library is to put target-specific methods into extension traits, rather than providing them as cfg-guarded methods directly on objects themselves. For example, the many extension traits in std::os::unix::prelude provide UNIX-specific methods on standard types.

The standard library could, instead, provide these methods directly on the standard types, guarded by #[cfg(unix)]. However, it does not do so, and PRs adding cfg-guarded methods are often rejected.

Providing these methods via extension traits forces code to explicitly use those extension traits in order to access the methods. This, effectively, requires code to declare whether it depends on target-specific functionality, either because the code is target-specific, or because it has appropriately cfg-guarded code for different targets. Without these extension traits, code could more easily use target-specific functionality "accidentally".

This policy may change in the future if Rust develops better mechanisms for helping code explicitly declare its portability, or lack of portability, before accessing target-specific functionality.