Document sys_common and sys

src/libstd/sys/mod.rs

@@ -8,6 +8,28 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Platform-dependent platform abstraction
+//!
+//! The `std::sys` module is the abstracted interface through which
+//! `std` talks to the underlying operating system. It has different
+//! implementations for different operating system families, today
+//! just Unix and Windows.
+//!
+//! The centralization of platform-specific code in this module is
+//! enforced by the "platform abstraction layer" tidy script in
+//! `tools/tidy/pal.rs`.
+//!
+//! This module is closely related to the platform-independent system
+//! integration code in `std::sys_common`. See that module's
+//! documentation for details.
+//!
+//! In the future it would be desirable for the indepedent
+//! implementations of this module to be extracted to their own crates
+//! that `std` can link to, thus enabling their implementation
+//! out-of-tree via crate replacement. Though due to the complex
+//! inter-dependencies within `std` that will be a challenging goal to
+//! achieve.
+
 pub use self::imp::*;
 
 #[cfg(unix)]

src/libstd/sys_common/mod.rs

@@ -8,6 +8,20 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Platform-independent platform abstraction
+//!
+//! This is the platform-independent portion of the standard libraries
+//! platform abstraction layer, whereas `std::sys` is the
+//! platform-specific portion.
+//!
+//! The relationship between `std::sys_common`, `std::sys` and the
+//! rest of `std` is complex, with dependencies going in all
+//! directions: `std` depending on `sys_common`, `sys_common`
+//! depending on `sys`, and `sys` depending on `sys_common` and `std`.
+//! Ideally `sys_common` would be split into two and the dependencies
+//! between them all would form a dag, facilitating the extraction of
+//! `std::sys` from the standard library.
+
 #![allow(missing_docs)]
 
 use sync::Once;