Document sys_common and sys

1 files changed, 22 insertions, 0 deletions

diff --git a/src/libstd/sys/mod.rs b/src/libstd/sys/mod.rs
index f7e1a0a075a..84f41a1c535 100644
--- a/src/libstd/sys/mod.rs
+++ b/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)]