[pointer] Add separate PtrInner

`PtrInner` carries all invariants which are not controlled by type
parameters. Since `PtrInner` does not promise to uphold aliasing,
alignment, or validity, we can move some utility methods to `PtrInner`
which previously were responsible for maintaining invariants orthogonal
to their purpose.

Makes progress on #1892 (still needs to be fixed on v0.8.x)
Closes #1890

Author: joshlf

src/impls.rs

@@ -180,7 +180,7 @@ safety_comment! {
     ///
     /// [4] TODO(#429): Justify this claim.
     unsafe_impl!(char: TryFromBytes; |candidate: MaybeAligned<u32>| {
-        let candidate = candidate.read_unaligned();
+        let candidate = candidate.read_unaligned::<BecauseImmutable>();
         char::from_u32(candidate).is_some()
     });
 }
@@ -310,18 +310,18 @@ safety_comment! {
     ///
     /// [2] `NonZeroXxx` self-evidently does not contain `UnsafeCell`s. This is
     ///     not a proof, but we are accepting this as a known risk per #1358.
-    unsafe_impl!(NonZeroU8: TryFromBytes; |n: MaybeAligned<u8>| NonZeroU8::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroI8: TryFromBytes; |n: MaybeAligned<i8>| NonZeroI8::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroU16: TryFromBytes; |n: MaybeAligned<u16>| NonZeroU16::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroI16: TryFromBytes; |n: MaybeAligned<i16>| NonZeroI16::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroU32: TryFromBytes; |n: MaybeAligned<u32>| NonZeroU32::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroI32: TryFromBytes; |n: MaybeAligned<i32>| NonZeroI32::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroU64: TryFromBytes; |n: MaybeAligned<u64>| NonZeroU64::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroI64: TryFromBytes; |n: MaybeAligned<i64>| NonZeroI64::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroU128: TryFromBytes; |n: MaybeAligned<u128>| NonZeroU128::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroI128: TryFromBytes; |n: MaybeAligned<i128>| NonZeroI128::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroUsize: TryFromBytes; |n: MaybeAligned<usize>| NonZeroUsize::new(n.read_unaligned()).is_some());
-    unsafe_impl!(NonZeroIsize: TryFromBytes; |n: MaybeAligned<isize>| NonZeroIsize::new(n.read_unaligned()).is_some());
+    unsafe_impl!(NonZeroU8: TryFromBytes; |n: MaybeAligned<u8>| NonZeroU8::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroI8: TryFromBytes; |n: MaybeAligned<i8>| NonZeroI8::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroU16: TryFromBytes; |n: MaybeAligned<u16>| NonZeroU16::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroI16: TryFromBytes; |n: MaybeAligned<i16>| NonZeroI16::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroU32: TryFromBytes; |n: MaybeAligned<u32>| NonZeroU32::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroI32: TryFromBytes; |n: MaybeAligned<i32>| NonZeroI32::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroU64: TryFromBytes; |n: MaybeAligned<u64>| NonZeroU64::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroI64: TryFromBytes; |n: MaybeAligned<i64>| NonZeroI64::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroU128: TryFromBytes; |n: MaybeAligned<u128>| NonZeroU128::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroI128: TryFromBytes; |n: MaybeAligned<i128>| NonZeroI128::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroUsize: TryFromBytes; |n: MaybeAligned<usize>| NonZeroUsize::new(n.read_unaligned::<BecauseImmutable>()).is_some());
+    unsafe_impl!(NonZeroIsize: TryFromBytes; |n: MaybeAligned<isize>| NonZeroIsize::new(n.read_unaligned::<BecauseImmutable>()).is_some());
 }
 safety_comment! {
     /// SAFETY: