First draft of a sound ShallowCopy

This is trying to address the unsoundness that arises from the current
version of `ShallowCopy` (see #74). In the process, it also deals with
the fact that casting between `Inner<.., T, ..>` and `Inner<..,
ManuallyDrop<T>, ..>` is likely not sound
(rust-lang/unsafe-code-guidelines#35 (comment)).
It does not yet work.

Cargo.toml

@@ -1,2 +1,3 @@
 [workspace]
-members = ["evmap", "evmap-derive", "left-right"]
+members = ["evmap", "left-right"]
+exclude = ["evmap-derive"]

evmap/src/inner.rs

@@ -7,10 +7,12 @@ pub(crate) use indexmap::IndexMap as MapImpl;
 pub(crate) use std::collections::HashMap as MapImpl;
 
 use crate::values::Values;
+use crate::ShallowCopy;
 
 pub(crate) struct Inner<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     pub(crate) data: MapImpl<K, Values<V, S>, S>,
@@ -22,7 +24,8 @@ impl<K, V, M, S> fmt::Debug for Inner<K, V, M, S>
 where
     K: Eq + Hash + fmt::Debug,
     S: BuildHasher,
-    V: fmt::Debug,
+    V: ShallowCopy,
+    V::Target: fmt::Debug,
     M: fmt::Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -38,6 +41,7 @@ impl<K, V, M, S> Clone for Inner<K, V, M, S>
 where
     K: Eq + Hash + Clone,
     S: BuildHasher + Clone,
+    V: ShallowCopy,
     M: Clone,
 {
     fn clone(&self) -> Self {
@@ -56,6 +60,7 @@ where
 impl<K, V, M, S> Inner<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     pub fn with_hasher(m: M, hash_builder: S) -> Self {

evmap/src/lib.rs

@@ -216,6 +216,7 @@ pub use crate::read::{MapReadRef, ReadGuardIter, ReadHandle, ReadHandleFactory};
 
 pub mod shallow_copy;
 pub use crate::shallow_copy::ShallowCopy;
+use shallow_copy::MaybeShallowCopied;
 
 // Expose `ReadGuard` since it has useful methods the user will likely care about.
 #[doc(inline)]
@@ -225,27 +226,27 @@ pub use left_right::ReadGuard;
 ///
 /// The predicate function is called once for each distinct value, and `true` if this is the
 /// _first_ call to the predicate on the _second_ application of the operation.
-pub struct Predicate<V>(pub(crate) Box<dyn FnMut(&V, bool) -> bool + Send>);
+pub struct Predicate<V: ?Sized>(pub(crate) Box<dyn FnMut(&V, bool) -> bool + Send>);
 
-impl<V> Predicate<V> {
+impl<V: ?Sized> Predicate<V> {
     /// Evaluate the predicate for the given element
     #[inline]
     pub fn eval(&mut self, value: &V, reset: bool) -> bool {
         (*self.0)(value, reset)
     }
 }
 
-impl<V> PartialEq for Predicate<V> {
+impl<V: ?Sized> PartialEq for Predicate<V> {
     #[inline]
     fn eq(&self, other: &Self) -> bool {
         // only compare data, not vtable: https://stackoverflow.com/q/47489449/472927
         &*self.0 as *const _ as *const () == &*other.0 as *const _ as *const ()
     }
 }
 
-impl<V> Eq for Predicate<V> {}
+impl<V: ?Sized> Eq for Predicate<V> {}
 
-impl<V> fmt::Debug for Predicate<V> {
+impl<V: ?Sized> fmt::Debug for Predicate<V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple("Predicate")
             .field(&format_args!("{:p}", &*self.0 as *const _))
@@ -255,12 +256,14 @@ impl<V> fmt::Debug for Predicate<V> {
 
 /// A pending map operation.
 #[non_exhaustive]
-#[derive(PartialEq, Eq, Debug)]
-pub(crate) enum Operation<K, V, M> {
+pub(crate) enum Operation<K, V, M>
+where
+    V: ShallowCopy,
+{
     /// Replace the set of entries for this key with this value.
-    Replace(K, V),
+    Replace(K, MaybeShallowCopied<V>),
     /// Add this value to the set of entries for this key.
-    Add(K, V),
+    Add(K, MaybeShallowCopied<V>),
     /// Remove this value from the set of entries for this key.
     RemoveValue(K, V),
     /// Remove the value set for this key.
@@ -277,24 +280,48 @@ pub(crate) enum Operation<K, V, M> {
     /// Note that this will iterate once over all the keys internally.
     Purge,
     /// Retains all values matching the given predicate.
-    Retain(K, Predicate<V>),
-    /// Shrinks [`Values`] to their minimum necessary size, freeing memory
+    Retain(K, Predicate<V::Target>),
+    /// Shrinks [`MaybeShallowCopied<V>alues`] to their minimum necessary size, freeing memory
     /// and potentially improving cache locality.
     ///
-    /// If no key is given, all `Values` will shrink to fit.
+    /// If no key is given, all `MaybeShallowCopied<V>alues` will shrink to fit.
     Fit(Option<K>),
-    /// Reserves capacity for some number of additional elements in [`Values`]
+    /// Reserves capacity for some number of additional elements in [`MaybeShallowCopied<V>alues`]
     /// for the given key. If the given key does not exist, allocate an empty
-    /// `Values` with the given capacity.
+    /// `MaybeShallowCopied<V>alues` with the given capacity.
     ///
     /// This can improve performance by pre-allocating space for large bags of values.
     Reserve(K, usize),
     /// Mark the map as ready to be consumed for readers.
     MarkReady,
     /// Set the value of the map meta.
     SetMeta(M),
-    /// Copy over the contents of the read map wholesale as the write map is empty.
-    JustCloneRHandle,
+}
+
+impl<K, V, M> fmt::Debug for Operation<K, V, M>
+where
+    K: fmt::Debug,
+    V: ShallowCopy + fmt::Debug,
+    V::Target: fmt::Debug,
+    M: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            Operation::Replace(ref a, ref b) => f.debug_tuple("Replace").field(a).field(b).finish(),
+            Operation::Add(ref a, ref b) => f.debug_tuple("Add").field(a).field(b).finish(),
+            Operation::RemoveValue(ref a, ref b) => {
+                f.debug_tuple("RemoveValue").field(a).field(b).finish()
+            }
+            Operation::RemoveEntry(ref a) => f.debug_tuple("RemoveEntry").field(a).finish(),
+            Operation::Clear(ref a) => f.debug_tuple("Clear").field(a).finish(),
+            Operation::Purge => f.debug_tuple("Purge").finish(),
+            Operation::Retain(ref a, ref b) => f.debug_tuple("Retain").field(a).field(b).finish(),
+            Operation::Fit(ref a) => f.debug_tuple("Fit").field(a).finish(),
+            Operation::Reserve(ref a, ref b) => f.debug_tuple("Reserve").field(a).field(b).finish(),
+            Operation::MarkReady => f.debug_tuple("MarkReady").finish(),
+            Operation::SetMeta(ref a) => f.debug_tuple("SetMeta").field(a).finish(),
+        }
+    }
 }
 
 /// Options for how to initialize the map.
@@ -373,7 +400,8 @@ where
     where
         K: Eq + Hash + Clone,
         S: BuildHasher + Clone,
-        V: Eq + Hash + ShallowCopy,
+        V: ShallowCopy,
+        V::Target: Eq + Hash,
         M: 'static + Clone,
     {
         let inner = if let Some(cap) = self.capacity {
@@ -399,7 +427,8 @@ pub fn new<K, V>() -> (
 )
 where
     K: Eq + Hash + Clone,
-    V: Eq + Hash + ShallowCopy,
+    V: ShallowCopy,
+    V::Target: Eq + Hash,
 {
     Options::default().construct()
 }
@@ -416,7 +445,8 @@ pub fn with_meta<K, V, M>(
 )
 where
     K: Eq + Hash + Clone,
-    V: Eq + Hash + ShallowCopy,
+    V: ShallowCopy,
+    V::Target: Eq + Hash,
     M: 'static + Clone,
 {
     Options::default().with_meta(meta).construct()
@@ -432,7 +462,8 @@ pub fn with_hasher<K, V, M, S>(
 ) -> (WriteHandle<K, V, M, S>, ReadHandle<K, V, M, S>)
 where
     K: Eq + Hash + Clone,
-    V: Eq + Hash + ShallowCopy,
+    V: ShallowCopy,
+    V::Target: Eq + Hash,
     M: 'static + Clone,
     S: BuildHasher + Clone,
 {

evmap/src/read.rs

@@ -1,5 +1,6 @@
 use crate::inner::Inner;
 use crate::values::Values;
+use crate::ShallowCopy;
 use left_right::ReadGuard;
 
 use std::borrow::Borrow;
@@ -22,6 +23,7 @@ pub use factory::ReadHandleFactory;
 pub struct ReadHandle<K, V, M = (), S = RandomState>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     pub(crate) handle: left_right::ReadHandle<Inner<K, V, M, S>>,
@@ -30,6 +32,7 @@ where
 impl<K, V, M, S> fmt::Debug for ReadHandle<K, V, M, S>
 where
     K: Eq + Hash + fmt::Debug,
+    V: ShallowCopy,
     S: BuildHasher,
     M: fmt::Debug,
 {
@@ -43,6 +46,7 @@ where
 impl<K, V, M, S> Clone for ReadHandle<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     fn clone(&self) -> Self {
@@ -55,6 +59,7 @@ where
 impl<K, V, M, S> ReadHandle<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     pub(crate) fn new(handle: left_right::ReadHandle<Inner<K, V, M, S>>) -> Self {
@@ -65,7 +70,8 @@ where
 impl<K, V, M, S> ReadHandle<K, V, M, S>
 where
     K: Eq + Hash,
-    V: Eq + Hash,
+    V: ShallowCopy,
+    V::Target: Eq + Hash,
     S: BuildHasher,
     M: Clone,
 {
@@ -150,7 +156,7 @@ where
     /// refreshed by the writer. If no refresh has happened, or the map has been destroyed, this
     /// function returns `None`.
     #[inline]
-    pub fn get_one<'rh, Q: ?Sized>(&'rh self, key: &'_ Q) -> Option<ReadGuard<'rh, V>>
+    pub fn get_one<'rh, Q: ?Sized>(&'rh self, key: &'_ Q) -> Option<ReadGuard<'rh, V::Target>>
     where
         K: Borrow<Q>,
         Q: Hash + Eq,
@@ -204,14 +210,12 @@ where
 
     /// Returns true if the map contains the specified value for the specified key.
     ///
-    /// The key and value may be any borrowed form of the map's respective types, but `Hash` and
+    /// The key may be any borrowed form of the map's key type, but `Hash` and
     /// `Eq` on the borrowed form *must* match.
-    pub fn contains_value<Q: ?Sized, W: ?Sized>(&self, key: &Q, value: &W) -> bool
+    pub fn contains_value<Q: ?Sized>(&self, key: &Q, value: &V::Target) -> bool
     where
         K: Borrow<Q>,
-        V: Borrow<W>,
         Q: Hash + Eq,
-        W: Hash + Eq,
     {
         self.get_raw(key.borrow())
             .map(|x| x.contains(value))

evmap/src/read/factory.rs

@@ -1,5 +1,6 @@
 use super::ReadHandle;
 use crate::inner::Inner;
+use crate::ShallowCopy;
 use std::collections::hash_map::RandomState;
 use std::fmt;
 use std::hash::{BuildHasher, Hash};
@@ -13,6 +14,7 @@ use std::hash::{BuildHasher, Hash};
 pub struct ReadHandleFactory<K, V, M, S = RandomState>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     pub(super) factory: left_right::ReadHandleFactory<Inner<K, V, M, S>>,
@@ -21,6 +23,7 @@ where
 impl<K, V, M, S> fmt::Debug for ReadHandleFactory<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -33,6 +36,7 @@ where
 impl<K, V, M, S> Clone for ReadHandleFactory<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     fn clone(&self) -> Self {
@@ -45,6 +49,7 @@ where
 impl<K, V, M, S> ReadHandleFactory<K, V, M, S>
 where
     K: Eq + Hash,
+    V: ShallowCopy,
     S: BuildHasher,
 {
     /// Produce a new [`ReadHandle`] to the same left-right data structure as this factory was