Rename NoCell to Immutable

We selected `Immutable` on @rcoh's suggestion that our name for this be a
single, evocative word, like `Send` and `Sync`. As with `Send` and `Sync`,
we do not hope that `Immutable`, on its own, captures the entire semantics
of this trait; the burden of entirely capturing its semantics rests on its
documentation.

We are not overly concerned with the risk of future conflict with Rust. If
and when Rust stabilizes its own version of this trait, we will adjust
zerocopy to use Rust's trait instead.

Closes #994

README.md

@@ -44,7 +44,7 @@ functionality themselves, but are required to call certain methods provided
 by the conversion traits:
 - `KnownLayout` indicates that zerocopy can reason about certain layout
   qualities of a type
-- `NoCell` indicates that a type does not contain any `UnsafeCell`s
+- `Immutable` indicates that a type does not contain any `UnsafeCell`s
 - `Unaligned` indicates that a type's alignment requirement is 1
 
 You should generally derive these marker traits whenever possible.

src/byteorder.rs

@@ -35,10 +35,10 @@
 //!
 //! ```rust,edition2021
 //! # #[cfg(feature = "derive")] { // This example uses derives, and won't compile without them
-//! use zerocopy::{IntoBytes, FromBytes, KnownLayout, NoCell, Ref, SplitByteSlice, Unaligned};
+//! use zerocopy::{IntoBytes, FromBytes, KnownLayout, Immutable, Ref, SplitByteSlice, Unaligned};
 //! use zerocopy::byteorder::network_endian::U16;
 //!
-//! #[derive(FromBytes, IntoBytes, KnownLayout, NoCell, Unaligned)]
+//! #[derive(FromBytes, IntoBytes, KnownLayout, Immutable, Unaligned)]
 //! #[repr(C)]
 //! struct UdpHeader {
 //!     src_port: U16,
@@ -378,7 +378,7 @@ example of how it can be used for parsing UDP packets.
 [`IntoBytes`]: crate::IntoBytes
 [`Unaligned`]: crate::Unaligned"),
             #[derive(Copy, Clone, Eq, PartialEq, Hash)]
-            #[cfg_attr(any(feature = "derive", test), derive(KnownLayout, NoCell, FromBytes, IntoBytes, Unaligned))]
+            #[cfg_attr(any(feature = "derive", test), derive(KnownLayout, Immutable, FromBytes, IntoBytes, Unaligned))]
             #[repr(transparent)]
             pub struct $name<O>([u8; $bytes], PhantomData<O>);
         }
@@ -390,9 +390,9 @@ example of how it can be used for parsing UDP packets.
             /// SAFETY:
             /// `$name<O>` is `repr(transparent)`, and so it has the same layout
             /// as its only non-zero field, which is a `u8` array. `u8` arrays
-            /// are `NoCell`, `TryFromBytes`, `FromZeros`, `FromBytes`,
+            /// are `Immutable`, `TryFromBytes`, `FromZeros`, `FromBytes`,
             /// `IntoBytes`, and `Unaligned`.
-            impl_or_verify!(O => NoCell for $name<O>);
+            impl_or_verify!(O => Immutable for $name<O>);
             impl_or_verify!(O => TryFromBytes for $name<O>);
             impl_or_verify!(O => FromZeros for $name<O>);
             impl_or_verify!(O => FromBytes for $name<O>);
@@ -903,7 +903,7 @@ mod tests {
     use compatibility::*;
 
     // A native integer type (u16, i32, etc).
-    trait Native: Arbitrary + FromBytes + IntoBytes + NoCell + Copy + PartialEq + Debug {
+    trait Native: Arbitrary + FromBytes + IntoBytes + Immutable + Copy + PartialEq + Debug {
         const ZERO: Self;
         const MAX_VALUE: Self;
 
@@ -948,7 +948,7 @@ mod tests {
     }
 
     trait ByteArray:
-        FromBytes + IntoBytes + NoCell + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
+        FromBytes + IntoBytes + Immutable + Copy + AsRef<[u8]> + AsMut<[u8]> + Debug + Default + Eq
     {
         /// Invert the order of the bytes in the array.
         fn invert(self) -> Self;

src/deprecated.rs

@@ -15,7 +15,7 @@ use super::*;
 impl<B, T> Ref<B, [T]>
 where
     B: ByteSlice,
-    T: NoCell,
+    T: Immutable,
 {
     #[deprecated(since = "0.8.0", note = "`Ref::new` now supports slices")]
     #[doc(hidden)]
@@ -28,7 +28,7 @@ where
 impl<B, T> Ref<B, [T]>
 where
     B: ByteSlice,
-    T: Unaligned + NoCell,
+    T: Unaligned + Immutable,
 {
     #[deprecated(since = "0.8.0", note = "`Ref::new_unaligned` now supports slices")]
     #[doc(hidden)]
@@ -41,7 +41,7 @@ where
 impl<'a, B, T> Ref<B, [T]>
 where
     B: 'a + IntoByteSlice<'a>,
-    T: FromBytes + NoCell,
+    T: FromBytes + Immutable,
 {
     #[deprecated(since = "0.8.0", note = "`Ref::into_ref` now supports slices")]
     #[doc(hidden)]
@@ -54,7 +54,7 @@ where
 impl<'a, B, T> Ref<B, [T]>
 where
     B: 'a + IntoByteSliceMut<'a>,
-    T: FromBytes + IntoBytes + NoCell,
+    T: FromBytes + IntoBytes + Immutable,
 {
     #[deprecated(since = "0.8.0", note = "`Ref::into_mut` now supports slices")]
     #[doc(hidden)]