[backports-release-1.11] Make String(::Memory) copy

[nhz2]

Manual backport of #54457 to fix #54369 in 1.11

[giordano]

https://buildkite.com/julialang/julia-release-1-dot-11/builds/325#01931891-b8f7-414d-8bce-0c0908b26d74/875-1652

The global RNG seed was 0xc55e57e524be7f332615c29ea9da899b.
Error in testset operators:
Test Failed at /cache/build/tester-amdci5-9/julialang/julia-release-1-dot-11/julia-114953cb48/share/julia/test/operators.jl:158
  Expression: +()
    Expected: MethodError
      Thrown: StackOverflowError
      StackOverflowError:
      Stacktrace:
       [1] +() (repeats 79984 times)
         @ Main.Test32Main_LinearAlgebra_matmul.TestMatmul /cache/build/tester-amdci5-9/julialang/julia-release-1-dot-11/julia-114953cb48/share/julia/stdlib/v1.11/LinearAlgebra/test/matmul.jl:1119

I seem to have seen this error before, maybe we need to backport some other patches. @jishnub perhaps you remember what was it? I thought it was #56184, which fixed #56136, but that was in a different testset.

[jishnub]

This was caused by #56089, and was fixed subsequently by #56184.

[giordano]

and was fixed subsequently by #56184.

But that's changing stdlib/LinearAlgebra/test/matmul.jl, the failure is in test/operators.jl

[jishnub]

IIUC, the failure happens if the test/operators.jl is run in the same process after the LinearAlgebra matmul tests that introduce the stack-overflowing method. The offending method

julia/stdlib/LinearAlgebra/test/matmul.jl

Line 1142 in 2a6ce58

Base.:+(t::Thing...) = +(getfield.(t, :data)...)

is committing type-piracy by defining +() = +(), so this would impact any subsequent invocation of +() that is run in the same process. This showed up in the test/errorshow.jl failure after the PR was merged, but apparently test/operators.jl also checks the same method.

[giordano]

I see, tests weren't isolated enough. I pushed the backport to #56228, this is failing also there anyway

[KristofferC]

Feels a bit too intrusive to backport to a patch release, or?

[nhz2]

This fixes code like String(str isa Vector{UInt8} ? copy(str) : str) in JuliaData/Parsers.jl#193

Since this is how the docs of String recommend copying an AbstractVector{UInt8} into a String there may be other existing packages that are fixed by this. This shouldn't break anything unless someone is using the internal Base.StringMemory to create a huge string, and there isn't enough memory left to make a copy.
Using String on a vector created with Base.StringVector(len) still avoids copying like before, since this only affects String(::Memory).