Further seal the OffsetSizeTrait

[HaoYang670]

Signed-off-by: remzi 13716567376yh@gmail.com

This is a follow-up PR of #1819.

Rationale for this change

OffsetSizeTrait should only be implemented for i32 and i64

What changes are included in this PR?

1. seal the OffsetSizeTrait
2. update docs

Are there any user-facing changes?

Yes.

verify

impl OffsetSizeTrait for u32 {
    const IS_LARGE: bool = false;
}

gives compile error

error[E0277]: the trait bound `u32: array_list::private::Sealed` is not satisfied
  --> arrow/src/array/array_list.rs:49:6
   |
49 | impl OffsetSizeTrait for u32 {
   |      ^^^^^^^^^^^^^^^ the trait `array_list::private::Sealed` is not implemented for `u32`
   |
   = help: the following implementations were found:
             <i32 as array_list::private::Sealed>
             <i64 as array_list::private::Sealed>
note: required by a bound in `array_list::OffsetSizeTrait`
  --> arrow/src/array/array_list.rs:36:55
   |
35 | pub trait OffsetSizeTrait:
   |           --------------- required by a bound in this
36 |     ArrowNativeType + std::ops::AddAssign + Integer + private::Sealed
   |                                                       ^^^^^^^^^^^^^^^ required by this bound in `array_list::OffsetSizeTrait`

, which is good!

[codecov-commenter]

Codecov Report

Merging #1823 (93677f9) into master (ba38ebe) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1823      +/-   ##
==========================================
- Coverage   83.44%   83.44%   -0.01%     
==========================================
  Files         199      199              
  Lines       56651    56651              
==========================================
- Hits        47272    47270       -2     
- Misses       9379     9381       +2     

Impacted Files	Coverage Δ
arrow/src/array/array_list.rs	96.16% <ø> (ø)
parquet_derive/src/parquet_field.rs	65.53% <0.00%> (-0.46%)	⬇️

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[tustvold]

I don't follow why this is necessary, the orphan rules prevent defining OffsetSize for foreign types outside of arrow-rs. As ArrowNativeType is sealed, by definition there can be no non-foreign types that implement it?

To put it another way, impl OffsetSizeTrait for u32 is illegal outside of arrow-rs.

Further, the rationale for sealing is avoiding transmute funky. There are no such semantics attached to OffsetSizeTrait, only indirectly via its relationship with ArrowNativeType. Even if you could impl OffsetSizeTrait for u32, it wouldn't be potentially unsound.

[HaoYang670]

Thank you @tustvold for your explanation, which makes perfect sense to me.
This is my first time learning Rust’s orphan rules. 😁

[HaoYang670]

I will close this PR as OffsetSizeTrait has been sealed.
Thank you @tustvold for your review!