Add note about possible allocation-sharing to Arc/Rc<str/[T]/CStr>::default.

Author: zachs18

alloc/src/ffi/c_str.rs

@@ -914,6 +914,8 @@ impl From<&CStr> for Rc<CStr> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl Default for Arc<CStr> {
     /// Creates an empty CStr inside an Arc
+    ///
+    /// This may or may not share an allocation with other Arcs.
     #[inline]
     fn default() -> Self {
         let c_str: &CStr = Default::default();
@@ -925,6 +927,8 @@ impl Default for Arc<CStr> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl Default for Rc<CStr> {
     /// Creates an empty CStr inside an Rc
+    ///
+    /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
     fn default() -> Self {
         let c_str: &CStr = Default::default();

alloc/src/rc.rs

@@ -2228,6 +2228,8 @@ impl<T: Default> Default for Rc<T> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl Default for Rc<str> {
     /// Creates an empty str inside an Rc
+    ///
+    /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
     fn default() -> Self {
         Rc::from("")
@@ -2238,6 +2240,8 @@ impl Default for Rc<str> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl<T> Default for Rc<[T]> {
     /// Creates an empty `[T]` inside an Rc
+    ///
+    /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
     fn default() -> Self {
         let arr: [T; 0] = [];

alloc/src/sync.rs

@@ -3304,6 +3304,8 @@ impl<T: Default> Default for Arc<T> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl Default for Arc<str> {
     /// Creates an empty str inside an Arc
+    ///
+    /// This may or may not share an allocation with other Arcs.
     #[inline]
     fn default() -> Self {
         Arc::from("")
@@ -3314,6 +3316,8 @@ impl Default for Arc<str> {
 #[stable(feature = "more_rc_default_impls", since = "CURRENT_RUSTC_VERSION")]
 impl<T> Default for Arc<[T]> {
     /// Creates an empty `[T]` inside an Arc
+    ///
+    /// This may or may not share an allocation with other Arcs.
     #[inline]
     fn default() -> Self {
         let arr: [T; 0] = [];