Replace UnstableIO with IOContext{IO}

[mkitti]

While trying to send UnstableIO into Julia base, two suggestions arose:

1. Use Base.inferencebarrier to prevent specialization:
   Fix Pkg.BinaryPlatforms invalidations by moving module to Base julia#52249 (comment)
2. Use IOContext{IO} rather than introducing a new type:
   Fix Pkg.BinaryPlatforms invalidations by moving module to Base julia#52249 (comment)

Fix #3722 (IOContext has closewrite)
Addresses part of #3702 by removing UnstableIO and its associated invalidations.

[KristofferC]

Why is this needed?

[mkitti]

Do you have another suggestion as how one might construct a IOContext{IO}?

julia> IOContext{IO}(stdout)
ERROR: MethodError: no method matching IOContext{IO}(::Base.TTY)

Closest candidates are:
  IOContext{IO_t}(::IO_t, ::Base.ImmutableDict{Symbol, Any}) where IO_t<:IO
   @ Base show.jl:291

[KristofferC]

Okay, I understand. You need to call the inner constructor and the default for the dict is what is given here:

https://github.com/JuliaLang/julia/blob/b57f8d168dc88be8570e1c9ff7cd352fc5e11c98/base/show.jl#L303

[mkitti]

An alternative is that I use Base.unwrapcontext instead of copying the code. I'm not sure if that complicates the inference barrier or not.

[mkitti]

This does cause compilation of get for the IO though.

[KristofferC]

This can be removed I think? Or is it used somewhere else?

[mkitti]

There are a few other references to UnstableIO in Pkg.jl. I could get rid of the alias as its uses. Is that what you mean?

[mkitti]

I removed references to UnstableIO in 9d57162

[KristofferC]

Looks like this has to be rebased to update for another use of UnstableIO (looking at the CI):

┌ Pkg
│  WARNING: could not import Pkg.UnstableIO into Operations
└  
     Testing Running tests...
ERROR: LoadError: UndefVarError: `UnstableIO` not defined in `Pkg.Operations`
Stacktrace:
  [1] subprocess_handler(cmd::Cmd, io::IOContext{IO}, error_msg::String)
    @ Pkg.Operations ~/work/Pkg.jl/Pkg.jl/src/Operations.jl:2223
  [2] (::Pkg.Operations.var"#138#143"{Pkg.Types.Context})()
    @ Pkg.Operations ~/work/Pkg.jl/Pkg.jl/src/Operations.jl:2129

[mkitti]

I will take a look, thanks.

[mkitti]

@KristofferC CI is now passing here.

[KristofferC]

👍