Borrow the newtype, not the representation, on primitive newtypes

[emilk]

Related

• Column<T>::as_slice() now yields &[T] for newtypes #41
• Follow-ups from the 0.6 post-merge reviews #55

What

A primitive newtype reported two element types for the same column: as_slice() yielded &[ChunkId] (since #41) while column[i] yielded &[u8; 16], and #43's Deref then put first() (the newtype) next to get() (the representation).

For a Column<ChunkId>, where newtype_data_type!(ChunkId, FixedSizeBinary<16>, primitive):

Call	Before	After
column[i]	&[u8; 16]	&ChunkId
column.first()	&ChunkId	&ChunkId
column.as_slice()	&[ChunkId]	&[ChunkId]
column.value_owned(i)	ChunkId	ChunkId
column.value(i)	&[u8; 16]	&[u8; 16]
column.get(i)	Option<&[u8; 16]>	Option<&[u8; 16]>

So the change is RefType::Ref: indexing (column[i], and Index-based &column[i]) hands back the newtype, agreeing with as_slice, first(), and to_vec(). The primitive arm already requires Pod and layout compatibility, so this is sound the same way values() is: bytemuck::must_cast_ref on the representation's value_ref, size and alignment checked at compile time. Both macros get it — newtype_data_type! and try_newtype_data_type!.

value() / get() still hand out the representation's view. That is LogicalType::Value, which is load-bearing for the fallible arm, so it stays a separate question.

Testing

New assertions on the ChunkId fixture: array[1], as_slice()[1], and value_owned(1) agree on the newtype; value()/get() still give the representation; a plain (non-primitive) newtype still indexes as the representation. cargo test --all-features, cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --check pass.

Compatibility

⚠️ Breaking, for primitive newtypes only, and only at column[i]:

let id: &[u8; 16] = &column[0]; // before
let id: &ChunkId  = &column[0]; // after — `&column[0].0` for the raw bytes

Code that already went through as_slice(), first(), to_vec(), or value_owned() is unaffected. Nothing new is rejected: a newtype that is not layout-compatible cannot be primitive in the first place.

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