Fix cloning Symbols not increasing their ref count

Author: Veykril

crates/intern/src/symbol.rs

@@ -5,7 +5,7 @@ use std::{
     borrow::Borrow,
     fmt,
     hash::{BuildHasherDefault, Hash, Hasher},
-    mem,
+    mem::{self, ManuallyDrop},
     ptr::NonNull,
     sync::OnceLock,
 };
@@ -25,6 +25,15 @@ const _: () = assert!(std::mem::align_of::<Box<str>>() == std::mem::align_of::<&
 const _: () = assert!(std::mem::size_of::<Arc<Box<str>>>() == std::mem::size_of::<&&str>());
 const _: () = assert!(std::mem::align_of::<Arc<Box<str>>>() == std::mem::align_of::<&&str>());
 
+const _: () =
+    assert!(std::mem::size_of::<*const *const str>() == std::mem::size_of::<TaggedArcPtr>());
+const _: () =
+    assert!(std::mem::align_of::<*const *const str>() == std::mem::align_of::<TaggedArcPtr>());
+
+const _: () = assert!(std::mem::size_of::<Arc<Box<str>>>() == std::mem::size_of::<TaggedArcPtr>());
+const _: () =
+    assert!(std::mem::align_of::<Arc<Box<str>>>() == std::mem::align_of::<TaggedArcPtr>());
+
 /// A pointer that points to a pointer to a `str`, it may be backed as a `&'static &'static str` or
 /// `Arc<Box<str>>` but its size is that of a thin pointer. The active variant is encoded as a tag
 /// in the LSB of the alignment niche.
@@ -38,21 +47,27 @@ unsafe impl Sync for TaggedArcPtr {}
 
 impl TaggedArcPtr {
     const BOOL_BITS: usize = true as usize;
+    const ALIGNMENT_NICHE: usize = mem::align_of::<TaggedArcPtr>().trailing_zeros() as usize;
 
     const fn non_arc(r: &'static &'static str) -> Self {
-        Self {
-            // SAFETY: The pointer is non-null as it is derived from a reference
-            // Ideally we would call out to `pack_arc` but for a `false` tag, unfortunately the
-            // packing stuff requires reading out the pointer to an integer which is not supported
-            // in const contexts, so here we make use of the fact that for the non-arc version the
-            // tag is false (0) and thus does not need touching the actual pointer value.ext)
-            packed: unsafe {
-                NonNull::new_unchecked((r as *const &str).cast::<*const str>().cast_mut())
-            },
-        }
+        assert!(
+            mem::align_of::<&'static &'static str>().trailing_zeros() as usize > Self::BOOL_BITS
+        );
+        // SAFETY: The pointer is non-null as it is derived from a reference
+        // Ideally we would call out to `pack_arc` but for a `false` tag, unfortunately the
+        // packing stuff requires reading out the pointer to an integer which is not supported
+        // in const contexts, so here we make use of the fact that for the non-arc version the
+        // tag is false (0) and thus does not need touching the actual pointer value.ext)
+
+        let packed =
+            unsafe { NonNull::new_unchecked((r as *const &str).cast::<*const str>().cast_mut()) };
+        Self { packed }
     }
 
     fn arc(arc: Arc<Box<str>>) -> Self {
+        assert!(
+            mem::align_of::<&'static &'static str>().trailing_zeros() as usize > Self::BOOL_BITS
+        );
         Self {
             packed: Self::pack_arc(
                 // Safety: `Arc::into_raw` always returns a non null pointer
@@ -63,12 +78,14 @@ impl TaggedArcPtr {
 
     /// Retrieves the tag.
     #[inline]
-    pub(crate) fn try_as_arc_owned(self) -> Option<Arc<Box<str>>> {
+    pub(crate) fn try_as_arc_owned(self) -> Option<ManuallyDrop<Arc<Box<str>>>> {
         // Unpack the tag from the alignment niche
         let tag = Strict::addr(self.packed.as_ptr()) & Self::BOOL_BITS;
         if tag != 0 {
             // Safety: We checked that the tag is non-zero -> true, so we are pointing to the data offset of an `Arc`
-            Some(unsafe { Arc::from_raw(self.pointer().as_ptr().cast::<Box<str>>()) })
+            Some(ManuallyDrop::new(unsafe {
+                Arc::from_raw(self.pointer().as_ptr().cast::<Box<str>>())
+            }))
         } else {
             None
         }
@@ -122,10 +139,11 @@ impl TaggedArcPtr {
     }
 }
 
-#[derive(PartialEq, Eq, Hash, Clone, Debug)]
+#[derive(PartialEq, Eq, Hash, Debug)]
 pub struct Symbol {
     repr: TaggedArcPtr,
 }
+
 const _: () = assert!(std::mem::size_of::<Symbol>() == std::mem::size_of::<NonNull<()>>());
 const _: () = assert!(std::mem::align_of::<Symbol>() == std::mem::align_of::<NonNull<()>>());
 
@@ -185,19 +203,27 @@ impl Symbol {
     fn drop_slow(arc: &Arc<Box<str>>) {
         let (mut shard, hash) = Self::select_shard(arc);
 
-        if Arc::count(arc) != 2 {
-            // Another thread has interned another copy
-            return;
+        match Arc::count(arc) {
+            0 => unreachable!(),
+            1 => unreachable!(),
+            2 => (),
+            _ => {
+                // Another thread has interned another copy
+                return;
+            }
         }
 
-        match shard.raw_entry_mut().from_key_hashed_nocheck::<str>(hash, arc.as_ref()) {
-            RawEntryMut::Occupied(occ) => occ.remove_entry(),
-            RawEntryMut::Vacant(_) => unreachable!(),
-        }
-        .0
-         .0
-        .try_as_arc_owned()
-        .unwrap();
+        ManuallyDrop::into_inner(
+            match shard.raw_entry_mut().from_key_hashed_nocheck::<str>(hash, arc.as_ref()) {
+                RawEntryMut::Occupied(occ) => occ.remove_entry(),
+                RawEntryMut::Vacant(_) => unreachable!(),
+            }
+            .0
+             .0
+            .try_as_arc_owned()
+            .unwrap(),
+        );
+        debug_assert_eq!(Arc::count(&arc), 1);
 
         // Shrink the backing storage if the shard is less than 50% occupied.
         if shard.len() * 2 < shard.capacity() {
@@ -219,7 +245,13 @@ impl Drop for Symbol {
             Self::drop_slow(&arc);
         }
         // decrement the ref count
-        drop(arc);
+        ManuallyDrop::into_inner(arc);
+    }
+}
+
+impl Clone for Symbol {
+    fn clone(&self) -> Self {
+        Self { repr: increase_arc_refcount(self.repr) }
     }
 }
 
@@ -228,8 +260,7 @@ fn increase_arc_refcount(repr: TaggedArcPtr) -> TaggedArcPtr {
         return repr;
     };
     // increase the ref count
-    mem::forget(arc.clone());
-    mem::forget(arc);
+    mem::forget(Arc::clone(&arc));
     repr
 }
 

crates/intern/src/symbol/symbols.rs

@@ -10,6 +10,7 @@ use crate::{
     symbol::{SymbolProxy, TaggedArcPtr},
     Symbol,
 };
+
 macro_rules! define_symbols {
     (@WITH_NAME: $($alias:ident = $value:literal),* $(,)? @PLAIN: $($name:ident),* $(,)?) => {
         $(