Use the Pin API for the internal strings

Unfortunately this prevents IntoIter implementation from reusing the Strings inplace. Note that even though this moves the Strings from the StringInterner out it should not invalidate any references stored outside since String itself manages its data through an indirection. However, this currently isn't modelled correctly in this PR.

src/lib.rs

@@ -2,6 +2,8 @@
 #![doc(html_root_url = "https://docs.rs/crate/string-interner/0.7.0")]
 #![deny(missing_docs)]
 
+#![feature(pin)]
+
 //! Caches strings efficiently, with minimal memory footprint and associates them with unique symbols.
 //! These symbols allow constant time comparisons and look-ups to the underlying interned strings.
 //!
@@ -76,8 +78,9 @@ mod benches;
 #[cfg(feature = "serde_support")]
 mod serde_impl;
 
-use std::iter::FromIterator;
 use std::{
+	pin::Pin,
+	iter::FromIterator,
 	collections::{hash_map::RandomState, HashMap},
 	hash::{BuildHasher, Hash, Hasher},
 	iter, marker,
@@ -199,7 +202,7 @@ where
 	H: BuildHasher,
 {
 	map: HashMap<InternalStrRef, S, H>,
-	values: Vec<Box<str>>,
+	values: Vec<Pin<Box<str>>>,
 }
 
 impl<S, H> PartialEq for StringInterner<S, H>
@@ -331,8 +334,8 @@ where
 		T: Into<String> + AsRef<str>,
 	{
 		let new_id: S = self.make_symbol();
-		let new_boxed_val = new_val.into().into_boxed_str();
-		let new_ref: InternalStrRef = new_boxed_val.as_ref().into();
+		let new_boxed_val = Pin::new(new_val.into().into_boxed_str());
+		let new_ref: InternalStrRef = Pin::get_ref(new_boxed_val.as_ref()).into();
 		self.values.push(new_boxed_val);
 		self.map.insert(new_ref, new_id);
 		new_id
@@ -349,7 +352,7 @@ where
 	pub fn resolve(&self, symbol: S) -> Option<&str> {
 		self.values
 			.get(symbol.to_usize())
-			.map(|boxed_str| boxed_str.as_ref())
+			.map(|boxed_str| Pin::get_ref(boxed_str.as_ref()))
 	}
 
 	/// Returns the string associated with the given symbol.
@@ -365,7 +368,7 @@ where
 	/// had no associated string for this interner instance.
 	#[inline]
 	pub unsafe fn resolve_unchecked(&self, symbol: S) -> &str {
-		self.values.get_unchecked(symbol.to_usize()).as_ref()
+		Pin::get_ref(self.values.get_unchecked(symbol.to_usize()).as_ref())
 	}
 
 	/// Returns the symbol associated with the given string for this interner
@@ -428,7 +431,7 @@ where
 
 /// Iterator over the pairs of associated symbols and interned strings for a `StringInterner`.
 pub struct Iter<'a, S> {
-	iter: iter::Enumerate<slice::Iter<'a, Box<str>>>,
+	iter: iter::Enumerate<slice::Iter<'a, Pin<Box<str>>>>,
 	mark: marker::PhantomData<S>,
 }
 
@@ -460,7 +463,9 @@ where
 	fn next(&mut self) -> Option<Self::Item> {
 		self.iter
 			.next()
-			.map(|(num, boxed_str)| (S::from_usize(num), boxed_str.as_ref()))
+			.map(|(num, boxed_str)| {
+				(S::from_usize(num), Pin::get_ref(boxed_str.as_ref()))
+			})
 	}
 
 	#[inline]
@@ -474,7 +479,7 @@ pub struct Values<'a, S>
 where
 	S: Symbol + 'a,
 {
-	iter: slice::Iter<'a, Box<str>>,
+	iter: slice::Iter<'a, Pin<Box<str>>>,
 	mark: marker::PhantomData<S>,
 }
 
@@ -503,7 +508,7 @@ where
 
 	#[inline]
 	fn next(&mut self) -> Option<Self::Item> {
-		self.iter.next().map(|boxed_str| boxed_str.as_ref())
+		self.iter.next().map(|boxed_str| Pin::get_ref(boxed_str.as_ref()).into())
 	}
 
 	#[inline]
@@ -535,7 +540,7 @@ pub struct IntoIter<S>
 where
 	S: Symbol,
 {
-	iter: iter::Enumerate<vec::IntoIter<Box<str>>>,
+	iter: iter::Enumerate<vec::IntoIter<Pin<Box<str>>>>,
 	mark: marker::PhantomData<S>,
 }
 
@@ -548,7 +553,9 @@ where
 	fn next(&mut self) -> Option<Self::Item> {
 		self.iter
 			.next()
-			.map(|(num, boxed_str)| (S::from_usize(num), boxed_str.into_string()))
+			.map(|(num, boxed_str)| {
+				(S::from_usize(num), Pin::get_ref(boxed_str.as_ref()).to_string())
+			})
 	}
 
 	#[inline]