prevent unneeded offset check

[ritchie46]

Calling ListArray::new does O(n) work where n = offset_length.

However, this work can be prevented in a few cases because we convert from a struct that has the same invariants.

The same logic applies to a Utf8 conversion, where we just determined the offsets and converted &str to bytes, so a utf8 check is redundant.

[ritchie46]

Hmm.. it fails on forbid_unsafe in avro/json. I can remove it there. I can also add an extra debug check on the invariants.

[ritchie46]

We could actually abstract these unsafe calls by having two newtype structs

#[repr(transparent)]
struct OffsetBuffer<O: Offset>(Vec<O>)

impl OffsetBuffer {

  fn add_lenght(&mut self, len: usize);
  
  unsafe fn from_vec();
}

// this could also be just `String`
// but maybe this is easier with delaying Utf8 validation
#[repr(transparent)]
struct StringBuffer(Vec<u8>)

impl StringBuffer {

   fn push_str(&mut self, val: &str);
   
   unsafe fn push_bytes(&mut self, bytes: &[u8]);
   
   unsafe fn from_vec();
}

A MutableList can then call new and pass on its internal state with the given invariants. That way you can keep the io module unsafe free without doing redundant checks.

[codecov]

Codecov Report

Merging #1059 (49e5c82) into main (86e533f) will increase coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1059      +/-   ##
==========================================
+ Coverage   81.25%   81.32%   +0.06%     
==========================================
  Files         363      363              
  Lines       34673    34685      +12     
==========================================
+ Hits        28172    28206      +34     
+ Misses       6501     6479      -22     

Impacted Files	Coverage Δ
src/array/list/mutable.rs	75.44% <66.66%> (-0.68%)	⬇️
src/io/ipc/read/stream_async.rs	74.38% <0.00%> (+0.82%)	⬆️
src/array/list/mod.rs	84.81% <0.00%> (+13.08%)	⬆️

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[jorgecarleitao]

Great idea - we can offer a safe API around those!