Update documentation

Author: CDirkx

library/std/src/sys/common/alloc.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)]
-
 use crate::alloc::{GlobalAlloc, Layout, System};
 use crate::cmp;
 use crate::ptr;

library/std/src/sys/common/mod.rs

@@ -1 +1,13 @@
+// This module contains code that is shared between all platforms, mostly utility or fallback code.
+// This explicitly does not include code that is shared between only a few platforms,
+// such as when reusing an implementation from `unix` or `unsupported`.
+// In those cases the desired code should be included directly using the #[path] attribute,
+// not moved to this module.
+//
+// Currently `sys_common` contains a lot of code that should live in this module,
+// ideally `sys_common` would only contain platform-independent abstractions on top of `sys`.
+// Progress on this is tracked in #84187.
+
+#![allow(dead_code)]
+
 pub mod alloc;

library/std/src/sys_common/mod.rs

@@ -8,9 +8,11 @@
 //! 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.
+//! This is because `sys_common` not only contains platform-independent code,
+//! but also code that is shared between the different platforms in `sys`.
+//! Ideally all that shared code should be moved to `sys::common`,
+//! and the dependencies between `std`, `sys_common` and `sys` all would form a dag.
+//! Progress on this is tracked in #84187.
 
 #![allow(missing_docs)]
 #![allow(missing_debug_implementations)]