Auto merge of #77380 - fusion-engineering-forks:unbox-the-mutex, r=dtolnay

Unbox mutexes and condvars on some platforms

Both mutexes and condition variables contained a Box containing the actual os-specific object. This was done because moving these objects may cause undefined behaviour on some platforms.

However, this is not needed on Windows[1], Wasm[2], cloudabi[2], and 'unsupported'[3], were the box was only needlessly making them less efficient.

This change gets rid of the box on those platforms.

On those platforms, `Condvar` can no longer verify it is only used with one `Mutex`, as mutexes no longer have a stable address. This was addressed and considered acceptable in #76932.

[1]\: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
[2]\: These are just a single atomic integer together with futex wait/wake calls/instructions.
[3]\: The `unsupported` platform doesn't support multiple threads at all.

15 files changed, 33 insertions, 0 deletions

diff --git a/library/std/src/sys/cloudabi/condvar.rs b/library/std/src/sys/cloudabi/condvar.rs
index dabdc0c9b51..82d89b260fa 100644
--- a/library/std/src/sys/cloudabi/condvar.rs
+++ b/library/std/src/sys/cloudabi/condvar.rs
@@ -15,6 +15,8 @@ pub struct Condvar {
     condvar: UnsafeCell<AtomicU32>,
 }
 
+pub type MovableCondvar = Condvar;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/cloudabi/mutex.rs b/library/std/src/sys/cloudabi/mutex.rs
index 580ab0e8ad8..66839e05bf0 100644
--- a/library/std/src/sys/cloudabi/mutex.rs
+++ b/library/std/src/sys/cloudabi/mutex.rs
@@ -15,6 +15,8 @@ extern "C" {
 // implemented identically.
 pub struct Mutex(RWLock);
 
+pub type MovableMutex = Mutex;
+
 pub unsafe fn raw(m: &Mutex) -> *mut AtomicU32 {
     rwlock::raw(&m.0)
 }
diff --git a/library/std/src/sys/hermit/condvar.rs b/library/std/src/sys/hermit/condvar.rs
index 52c8c3b17e8..b45e8718f08 100644
--- a/library/std/src/sys/hermit/condvar.rs
+++ b/library/std/src/sys/hermit/condvar.rs
@@ -14,6 +14,8 @@ pub struct Condvar {
     sem2: *const c_void,
 }
 
+pub type MovableCondvar = Box<Condvar>;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/sgx/condvar.rs b/library/std/src/sys/sgx/condvar.rs
index ed6dbcf4971..dfe22e6a1b3 100644
--- a/library/std/src/sys/sgx/condvar.rs
+++ b/library/std/src/sys/sgx/condvar.rs
@@ -7,6 +7,8 @@ pub struct Condvar {
     inner: SpinMutex<WaitVariable<()>>,
 }
 
+pub type MovableCondvar = Box<Condvar>;
+
 impl Condvar {
     pub const fn new() -> Condvar {
         Condvar { inner: SpinMutex::new(WaitVariable::new(())) }
diff --git a/library/std/src/sys/sgx/mutex.rs b/library/std/src/sys/sgx/mutex.rs
index 4911c2f5387..8874517dac6 100644
--- a/library/std/src/sys/sgx/mutex.rs
+++ b/library/std/src/sys/sgx/mutex.rs
@@ -8,6 +8,8 @@ pub struct Mutex {
     inner: SpinMutex<WaitVariable<bool>>,
 }
 
+pub type MovableMutex = Box<Mutex>;
+
 // Implementation according to “Operating Systems: Three Easy Pieces”, chapter 28
 impl Mutex {
     pub const fn new() -> Mutex {
diff --git a/library/std/src/sys/unix/condvar.rs b/library/std/src/sys/unix/condvar.rs
index 9f1847943f3..e38f91af9f0 100644
--- a/library/std/src/sys/unix/condvar.rs
+++ b/library/std/src/sys/unix/condvar.rs
@@ -6,6 +6,8 @@ pub struct Condvar {
     inner: UnsafeCell<libc::pthread_cond_t>,
 }
 
+pub type MovableCondvar = Box<Condvar>;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/unix/mutex.rs b/library/std/src/sys/unix/mutex.rs
index 45c600f75f5..ebc737b75ae 100644
--- a/library/std/src/sys/unix/mutex.rs
+++ b/library/std/src/sys/unix/mutex.rs
@@ -5,6 +5,8 @@ pub struct Mutex {
     inner: UnsafeCell<libc::pthread_mutex_t>,
 }
 
+pub type MovableMutex = Box<Mutex>;
+
 #[inline]
 pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
     m.inner.get()
diff --git a/library/std/src/sys/unsupported/condvar.rs b/library/std/src/sys/unsupported/condvar.rs
index a578eee8ccc..35d12a69c8a 100644
--- a/library/std/src/sys/unsupported/condvar.rs
+++ b/library/std/src/sys/unsupported/condvar.rs
@@ -3,6 +3,8 @@ use crate::time::Duration;
 
 pub struct Condvar {}
 
+pub type MovableCondvar = Condvar;
+
 impl Condvar {
     pub const fn new() -> Condvar {
         Condvar {}
diff --git a/library/std/src/sys/unsupported/mutex.rs b/library/std/src/sys/unsupported/mutex.rs
index 9ef8af52eb5..06ea9a1e2c1 100644
--- a/library/std/src/sys/unsupported/mutex.rs
+++ b/library/std/src/sys/unsupported/mutex.rs
@@ -4,6 +4,8 @@ pub struct Mutex {
     locked: UnsafeCell<bool>,
 }
 
+pub type MovableMutex = Mutex;
+
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {} // no threads on this platform
 
diff --git a/library/std/src/sys/vxworks/condvar.rs b/library/std/src/sys/vxworks/condvar.rs
index 5a77966d974..b4724be7c7c 100644
--- a/library/std/src/sys/vxworks/condvar.rs
+++ b/library/std/src/sys/vxworks/condvar.rs
@@ -6,6 +6,8 @@ pub struct Condvar {
     inner: UnsafeCell<libc::pthread_cond_t>,
 }
 
+pub type MovableCondvar = Box<Condvar>;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/vxworks/mutex.rs b/library/std/src/sys/vxworks/mutex.rs
index 103d87e3d2f..dd7582c21a7 100644
--- a/library/std/src/sys/vxworks/mutex.rs
+++ b/library/std/src/sys/vxworks/mutex.rs
@@ -5,6 +5,8 @@ pub struct Mutex {
     inner: UnsafeCell<libc::pthread_mutex_t>,
 }
 
+pub type MovableMutex = Box<Mutex>;
+
 #[inline]
 pub unsafe fn raw(m: &Mutex) -> *mut libc::pthread_mutex_t {
     m.inner.get()
diff --git a/library/std/src/sys/wasm/condvar_atomics.rs b/library/std/src/sys/wasm/condvar_atomics.rs
index d86bb60507b..a96bb18e6ef 100644
--- a/library/std/src/sys/wasm/condvar_atomics.rs
+++ b/library/std/src/sys/wasm/condvar_atomics.rs
@@ -9,6 +9,8 @@ pub struct Condvar {
     cnt: AtomicUsize,
 }
 
+pub type MovableCondvar = Condvar;
+
 // Condition variables are implemented with a simple counter internally that is
 // likely to cause spurious wakeups. Blocking on a condition variable will first
 // read the value of the internal counter, unlock the given mutex, and then
diff --git a/library/std/src/sys/wasm/mutex_atomics.rs b/library/std/src/sys/wasm/mutex_atomics.rs
index 4b1a7c9b481..2970fcf806c 100644
--- a/library/std/src/sys/wasm/mutex_atomics.rs
+++ b/library/std/src/sys/wasm/mutex_atomics.rs
@@ -8,6 +8,8 @@ pub struct Mutex {
     locked: AtomicUsize,
 }
 
+pub type MovableMutex = Mutex;
+
 // Mutexes have a pretty simple implementation where they contain an `i32`
 // internally that is 0 when unlocked and 1 when the mutex is locked.
 // Acquisition has a fast path where it attempts to cmpxchg the 0 to a 1, and
diff --git a/library/std/src/sys/windows/condvar.rs b/library/std/src/sys/windows/condvar.rs
index 8f7f6854cc2..44547a5c51a 100644
--- a/library/std/src/sys/windows/condvar.rs
+++ b/library/std/src/sys/windows/condvar.rs
@@ -8,6 +8,8 @@ pub struct Condvar {
     inner: UnsafeCell<c::CONDITION_VARIABLE>,
 }
 
+pub type MovableCondvar = Condvar;
+
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
diff --git a/library/std/src/sys/windows/mutex.rs b/library/std/src/sys/windows/mutex.rs
index e2aaca59fe2..fa51b006c34 100644
--- a/library/std/src/sys/windows/mutex.rs
+++ b/library/std/src/sys/windows/mutex.rs
@@ -29,6 +29,11 @@ pub struct Mutex {
     lock: AtomicUsize,
 }
 
+// Windows SRW Locks are movable (while not borrowed).
+// ReentrantMutexes (in Inner) are not, but those are stored indirectly through
+// a Box, so do not move when the Mutex it self is moved.
+pub type MovableMutex = Mutex;
+
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}