fix: Buffer::into_mutable return error instead of panic for converting owned sliced when not start at 0 and fix returned Mutable length

[rluvaton]

Which issue does this PR close?

• Closes Buffer::into_mutable is not consistent regarding sliced data and can lead to panics #10117

Rationale for this change

This include 2 fixes:

1. Calling Buffer::into_mutable on owned sliced that Buffer::ptr_offset() > 0 panicked before, now returning error
2. Calling Buffer::into_mutable on owned slice that Buffer::ptr_offset() == 0 returned MutableBuffer with length greater than the original buffer length

What changes are included in this PR?

1. Updating Buffer::into_mutable to return error when ptr_offset > 0
2. Set the returned MutableBuffer length to the original buffer length so length is the same

Are these changes tested?

yes

Are there any user-facing changes?

not really, except error instead of panic and correct mutable length

[alamb]

Thanks @rluvaton

I agree we should avoid panics and intead return error. However, this PR seems to be trying to allow some sort of mutable buffer conversion even when the buffer is sliced.

I am not sure about the usecase for this API change -- it seems like the API would get more complicated with this PR.

If you want to allow reusing an allocation from a sliced buffer, how about something like

let unsliced_buffer = buffer.unslice(); <-- new API, clear the offset
let mutable =  unsliced_buffer.int_mutable();
..

Then we could add a doc example showing how to unslice a buffer and how to reuse a sliced buffer

[rluvaton]

I like the idea of unslice will split to 2 PRs

[alamb]

Marking as draft as I think this PR is no longer waiting on feedback and I am trying to make it easier to find PRs in need of review. Please mark it as ready for review when it is ready for another look

[rluvaton]

@alamb I fixed the code, it fix 2 bugs, the panic and different length for sliced

[alamb]

Thank you @rluvaton

[rluvaton]

@Jefffrey / @alamb can you please merge this?

[alamb]

✅ thank you @rluvaton