StableDeref trait into core

compiler/rustc_hir/src/lang_items.rs

@@ -220,6 +220,7 @@ language_item_table! {
     DerefPure,               sym::deref_pure,          deref_pure_trait,           Target::Trait,          GenericRequirement::Exact(0);
     DerefTarget,             sym::deref_target,        deref_target,               Target::AssocTy,        GenericRequirement::None;
     Receiver,                sym::receiver,            receiver_trait,             Target::Trait,          GenericRequirement::None;
+    StableDeref,             sym::stable_deref,        stable_deref_trait,         Target::Trait,          GenericRequirement::Exact(0);
 
     Fn,                      kw::Fn,                   fn_trait,                   Target::Trait,          GenericRequirement::Exact(1);
     FnMut,                   sym::fn_mut,              fn_mut_trait,               Target::Trait,          GenericRequirement::Exact(1);

compiler/rustc_span/src/symbol.rs

@@ -302,6 +302,7 @@ symbols! {
         SliceIter,
         Some,
         SpanCtxt,
+        StableDeref,
         String,
         StructuralPartialEq,
         SubdiagMessage,
@@ -1767,6 +1768,7 @@ symbols! {
         sse,
         sse4a_target_feature,
         stable,
+        stable_deref,
         staged_api,
         start,
         state,

library/core/src/ops/deref.rs

@@ -309,3 +309,25 @@ impl<T: ?Sized> Receiver for &T {}
 
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for &mut T {}
+
+#[lang = "stable_deref"]
+#[unstable(feature = "stable_deref_trait", issue = "123430")]
+/// # Safety
+///
+/// Any two calls to `deref` must return the same value at the same address unless
+/// `self` has been modified in the meantime. Moves and unsizing coercions of `self`
+/// are not considered modifications.
+///
+/// Here, "same value" means that if `deref` returns a trait object, then the actual
+/// type behind that trait object must not change. Additionally, when you unsize
+/// coerce from `Self` to `Unsized`, then if you call `deref` on `Unsized` and get a
+/// trait object, then the underlying type of that trait object must be `<Self as
+/// Deref>::Target`.
+///
+/// Analogous requirements apply to other unsized types. E.g., if `deref` returns
+/// `[T]`, then the length must not change. In other words, the underlying type
+/// must not change from `[T; N]` to `[T; M]`.
+///
+/// If this type implements `DerefMut`, then the same restrictions apply to calls
+/// to `deref_mut`.
+pub unsafe trait StableDeref: Deref {}