Seal ArrowNativeType and OffsetSizeTrait (#1028) (#1819)

* Seal ArrowNativeType (#1028)

* Add docs

arrow/src/datatypes/native.rs

@@ -19,15 +19,36 @@ use super::DataType;
 use half::f16;
 use serde_json::{Number, Value};
 
+mod private {
+    pub trait Sealed {}
+}
+
 /// Trait declaring any type that is serializable to JSON. This includes all primitive types (bool, i32, etc.).
 pub trait JsonSerializable: 'static {
     fn into_json_value(self) -> Option<Value>;
 }
 
 /// Trait expressing a Rust type that has the same in-memory representation
 /// as Arrow. This includes `i16`, `f32`, but excludes `bool` (which in arrow is represented in bits).
+///
 /// In little endian machines, types that implement [`ArrowNativeType`] can be memcopied to arrow buffers
 /// as is.
+///
+/// # Transmute Safety
+///
+/// A type T implementing this trait means that any arbitrary slice of bytes of length and
+/// alignment `size_of::<T>()` can be safely interpreted as a value of that type without
+/// being unsound, i.e. potentially resulting in undefined behaviour.
+///
+/// Note: in the case of floating point numbers this transmutation can result in a signalling
+/// NaN, which, whilst sound, can be unwieldy. In general, whilst it is perfectly sound to
+/// reinterpret bytes as different types using this trait, it is likely unwise
+///
+/// Note: `bool` is restricted to `0` or `1`, and so `bool: !ArrowNativeType`
+///
+/// # Sealed
+///
+/// Due to the above restrictions, this trait is sealed to prevent accidental misuse
 pub trait ArrowNativeType:
     std::fmt::Debug
     + Send
@@ -37,6 +58,7 @@ pub trait ArrowNativeType:
     + std::str::FromStr
     + Default
     + JsonSerializable
+    + private::Sealed
 {
     /// Convert native type from usize.
     #[inline]
@@ -109,6 +131,7 @@ impl JsonSerializable for i8 {
     }
 }
 
+impl private::Sealed for i8 {}
 impl ArrowNativeType for i8 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -132,6 +155,7 @@ impl JsonSerializable for i16 {
     }
 }
 
+impl private::Sealed for i16 {}
 impl ArrowNativeType for i16 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -155,6 +179,7 @@ impl JsonSerializable for i32 {
     }
 }
 
+impl private::Sealed for i32 {}
 impl ArrowNativeType for i32 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -184,6 +209,7 @@ impl JsonSerializable for i64 {
     }
 }
 
+impl private::Sealed for i64 {}
 impl ArrowNativeType for i64 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -217,6 +243,7 @@ impl JsonSerializable for i128 {
     }
 }
 
+impl private::Sealed for i128 {}
 impl ArrowNativeType for i128 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -246,6 +273,7 @@ impl JsonSerializable for u8 {
     }
 }
 
+impl private::Sealed for u8 {}
 impl ArrowNativeType for u8 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -269,6 +297,7 @@ impl JsonSerializable for u16 {
     }
 }
 
+impl private::Sealed for u16 {}
 impl ArrowNativeType for u16 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -292,6 +321,7 @@ impl JsonSerializable for u32 {
     }
 }
 
+impl private::Sealed for u32 {}
 impl ArrowNativeType for u32 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -315,6 +345,7 @@ impl JsonSerializable for u64 {
     }
 }
 
+impl private::Sealed for u64 {}
 impl ArrowNativeType for u64 {
     #[inline]
     fn from_usize(v: usize) -> Option<Self> {
@@ -351,8 +382,11 @@ impl JsonSerializable for f64 {
 }
 
 impl ArrowNativeType for f16 {}
+impl private::Sealed for f16 {}
 impl ArrowNativeType for f32 {}
+impl private::Sealed for f32 {}
 impl ArrowNativeType for f64 {}
+impl private::Sealed for f64 {}
 
 /// Allows conversion from supported Arrow types to a byte slice.
 pub trait ToByteSlice {