Rollup merge of #82492 - CDirkx:sys_common_alloc, r=m-ou-se

Move `std::sys_common::alloc` to new module `std::sys::common`

https://github.com/rust-lang/rust/blob/6b56603e35b39c9f6cc76782330e5e415f9e43d5/library/std/src/sys_common/mod.rs#L7-L13

It was my impression that the goal for `std::sys` has changed from extracting it into a separate crate to making std work with features. However the fact remains that there is a lot of interdependence between `sys` and `sys_common`, this is because `sys_common` contains two types of code:

- abstractions over the different platform implementations in `std::sys` (for example [`std::sys_common::mutex`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/mutex.rs))
- code shared between platforms (for example [`std::sys_common::alloc`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys_common/alloc.rs))

This PR attempts to address this by adding a new module `common` to `std::sys` which will contain code shared between platforms, `alloc.rs` in this case but more can be moved over in the future.

library/std/src/sys_common/alloc.rs → 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

@@ -0,0 +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/mod.rs

@@ -22,6 +22,8 @@
 
 #![allow(missing_debug_implementations)]
 
+mod common;
+
 cfg_if::cfg_if! {
     if #[cfg(target_os = "vxworks")] {
         mod vxworks;

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

@@ -1,6 +1,6 @@
 use crate::alloc::{GlobalAlloc, Layout, System};
 use crate::ptr;
-use crate::sys_common::alloc::{realloc_fallback, MIN_ALIGN};
+use crate::sys::common::alloc::{realloc_fallback, MIN_ALIGN};
 
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
 unsafe impl GlobalAlloc for System {

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

@@ -5,7 +5,7 @@ use crate::ffi::c_void;
 use crate::ptr;
 use crate::sync::atomic::{AtomicPtr, Ordering};
 use crate::sys::c;
-use crate::sys_common::alloc::{realloc_fallback, MIN_ALIGN};
+use crate::sys::common::alloc::{realloc_fallback, MIN_ALIGN};
 
 #[cfg(test)]
 mod tests;

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)]
@@ -46,7 +48,6 @@ macro_rules! rtunwrap {
     };
 }
 
-pub mod alloc;
 pub mod at_exit_imp;
 pub mod backtrace;
 pub mod bytestring;