fix UB + use weaker ordering

transmuting the Vec is not safe as Rust could make layout optimisations for `Vec<T>`` that it can't for `Vec<UnsafeCell<MaybeUninit<V>>>``

concurrent-map-builder/src/builder.rs

@@ -2,7 +2,7 @@ use std::{
     cell::UnsafeCell,
     cmp::min,
     hash::Hash,
-    mem::{needs_drop, MaybeUninit},
+    mem::{needs_drop, ManuallyDrop, MaybeUninit},
     ops::Range,
     sync::{
         atomic::{AtomicUsize, Ordering},
@@ -25,7 +25,7 @@ pub(crate) struct SharedConcurrentMapBuilder<K, V> {
     /// Values that we are initialising, will be the length of `index_set`.
     ///
     /// The index for a keys value is given by the keys index in `index_set`.
-    values: Option<Vec<UnsafeCell<MaybeUninit<V>>>>,
+    values: Option<ManuallyDrop<Vec<UnsafeCell<MaybeUninit<V>>>>>,
     /// A marker for if a value in `values` is initialised.
     initialised_values: Vec<UnsafeCell<bool>>,
 
@@ -40,11 +40,11 @@ unsafe impl<K: Sync, V: Sync> Sync for SharedConcurrentMapBuilder<K, V> {}
 impl<K, V> SharedConcurrentMapBuilder<K, V> {
     /// Returns a new [`SharedConcurrentMapBuilder`], with the keys needed in an [`IndexSet`].
     pub fn new(keys_needed: IndexSet<K>) -> SharedConcurrentMapBuilder<K, V> {
-        let values = Some(
+        let values = Some(ManuallyDrop::new(
             (0..keys_needed.len())
                 .map(|_| UnsafeCell::new(MaybeUninit::uninit()))
                 .collect(),
-        );
+        ));
         let initialised_values = (0..keys_needed.len())
             .map(|_| UnsafeCell::new(false))
             .collect();
@@ -64,8 +64,9 @@ impl<K, V> Drop for SharedConcurrentMapBuilder<K, V> {
         // Values in a MaybeUninit will not be dropped so we need to drop them manually.
 
         // This will only be ran when all workers have dropped their handles.
+
+        if let Some(values) = &mut self.values {
             if needs_drop::<V>() {
-            if let Some(values) = &self.values {
                 for init_value in self
                     .initialised_values
                     .iter()
@@ -86,6 +87,12 @@ impl<K, V> Drop for SharedConcurrentMapBuilder<K, V> {
                     unsafe { value.assume_init_drop() }
                 }
             }
+
+            // SAFETY:
+            // This is drop code this is the only reference and this will not be used after being dropped.
+            unsafe {
+                ManuallyDrop::drop(values);
+            }
         }
     }
 }
@@ -111,8 +118,8 @@ impl<K, V> ConcurrentMapBuilder<K, V> {
             return Err(*err);
         }
 
-        // TODO: can we use a weaker Ordering?
-        let start = self.0.current_index.fetch_add(amt, Ordering::SeqCst);
+        // TODO: We can use Relaxed, explain why.
+        let start = self.0.current_index.fetch_add(amt, Ordering::Relaxed);
 
         if start >= values.len() {
             // No work to do, all given out.
@@ -142,7 +149,7 @@ impl<K, V> ConcurrentMapBuilder<K, V> {
             return Err(*err);
         }
 
-        let values = inner.values.take().unwrap();
+        let mut values = inner.values.take().unwrap();
 
         if inner.current_index.load(Ordering::Relaxed) < values.len() {
             return Err(ConcurrentMapBuilderError::WorkWasNotFinishedBeforeInit);
@@ -152,8 +159,15 @@ impl<K, V> ConcurrentMapBuilder<K, V> {
         // - UnsafeCell<MaybeUninit<T>> has the same bit pattern as T.
         // - If any value is unitised that means work wasn't handed out which we just
         //   checked for, or work handed out was not completed which is checked for in
-        //   the Drop impl of MapBuilderWork.
-        let values: Vec<V> = unsafe { std::mem::transmute(values) };
+        //   the Drop impl of MapBuilderWork and if the Drop impl is not ran then there
+        //   will be a reference on the Arc so this code will not be reached.
+        // - The `values` Vec is wrapped in ManuallyDrop so the inner vec will not be dropped.
+        let values: Vec<V> = unsafe {
+            let capacity = values.capacity();
+            let len = values.len();
+
+            Vec::from_raw_parts(values.as_mut_ptr() as *mut V, len, capacity)
+        };
 
         Ok(Some(BuiltMap {
             index_set: inner.index_set.take().unwrap(),

concurrent-map-builder/src/lib.rs

@@ -47,9 +47,9 @@ fn build() {
     use std::time::Duration;
 
     let mut keys = IndexSet::new();
-    keys.extend(0..100_u8);
+    keys.extend(0..1000_u16);
 
-    let map_builder = BuiltMap::<u8, u8>::builder(keys);
+    let map_builder = BuiltMap::<u16, u16>::builder(keys);
 
     let map_builder2 = map_builder.clone();
 
@@ -63,7 +63,7 @@ fn build() {
         for key in keys_needed {
             println!("Thread1: {}", key);
             work.insert_next_value(*key).unwrap();
-            std::thread::sleep(Duration::from_millis(100));
+            std::thread::sleep(Duration::from_millis(10));
         }
     });
 
@@ -79,7 +79,7 @@ fn build() {
         for key in keys_needed {
             println!("Thread2: {}", key);
             work.insert_next_value(*key).unwrap();
-            std::thread::sleep(Duration::from_millis(100));
+            std::thread::sleep(Duration::from_millis(10));
         }
     });
 
@@ -95,7 +95,7 @@ fn build() {
         for key in keys_needed {
             println!("Thread3: {}", key);
             work.insert_next_value(*key).unwrap();
-            std::thread::sleep(Duration::from_millis(100));
+            std::thread::sleep(Duration::from_millis(10));
         }
     });