`String` constructor on a `view` can mutate parent

[nhz2]

This has happened since #53896

Here is a MWE.

julia> a = Memory{UInt8}(undef, 10);

julia> fill!(a, 0x01);

julia> String(view(a, 1:2))
"\x01\x01"

julia> a
0-element Memory{UInt8}

I expected a to not be changed.

This is causing issues in https://github.com/JuliaWeb/HTTP.jl/issues/1166 And I am also using String(view( in https://github.com/JuliaIO/ZipArchives.jl/blob/5bacade2f71e0c28e2aeba09d21a665ecad131cd/src/reader.jl#L106

[adienes]

I believe this is intended

see also: https://github.com/JuliaLang/julia/issues/32528

[nhz2]

The issue is that view protects the parent from being truncated by String in 1.10, but in 1.11.0-beta1 this is broken. If this is intended what is the correct way to protect the parent from being truncated?

[vtjnash]

In older versions, the view function also enforced a copy, which seems awkward to expect there. We should possibly check though that the truncation of the Memory object only applies if it is backed by a String (and doesn't make a copy)?

[nhz2]

Maybe the view method that converts a Memory to a Vector could be called something like make_vector to avoid confusion with normal view? The difference would be that the result of view would have a fixed size, and create a copy when passed to String.

[KristofferC]

Maybe https://github.com/JuliaLang/julia/pull/53896 should be reverted / rethought in light of this.

[vtjnash]

We also possibly should not unsafely mutate this Memory in this case. That is somewhat more of an Array feature anyways that you should possibly lose when accessing Memory directly

[nhz2]

If I understand: https://github.com/JuliaLang/julia/blob/c38e7cd76ad5576a52e4726f78229e8c6e5cc212/src/genericmemory.c#L196-L207 correctly, there are two possible mutations, setting length to 0, and setting the byte after the end to null. Are String required to always be null terminated?

[nhz2]

Fixed by #54927