add peek(io, UInt8) and peek(io, Char) (#16025)

[bicycle1885]

This pull request adds and exports peek from Base, and will close #16025. This works on AbstractIOBuffer and IOStream; more generic definition will be possible if the IO type supports seek.

[nalimilan]

Could you also test with a non-ASCII char?

[nalimilan]

Thanks, looks good! You should run make julia-genstdlib to update the docs. You should also add peek to doc/stdlib/io-network.rst (and maybe mention it in the manual too).

[nalimilan]

The tests failure is due to priorityqueue.jl having using Base.Collections, and Base.Collections having a peek function. Not sure how to best handle that. Note that it also has an eval function; so that's not a new issue.

[laborg]

This has been implemented. See #28811

On Julia 1.7.1:

julia> using Test

julia> let n = tempname()
           w = open(n, "a")
           io = open(n)
           write(w, "A"); flush(w)
           @test peek(io, UInt8) === 0x41
           @test peek(io, Char) === 'A'
           read(io, UInt8)
           @test_throws EOFError peek(io, UInt8)
           @test_throws EOFError peek(io, Char)
           close(io); close(w)
           rm(n)
       end

succeeds.