add deprecation mechanism for (io,p) = open(cmd)

base/deprecated.jl

@@ -1307,6 +1307,15 @@ end
      end
 end
 
+# 12807
+start(::Union{Process, ProcessChain}) = 1
+done(::Union{Process, ProcessChain}, i::Int) = (i == 3)
+next(p::Union{Process, ProcessChain}, i::Int) = (getindex(p, i), i + 1)
+@noinline function getindex(p::Union{Process, ProcessChain}, i::Int)
+    depwarn("open(cmd) now returns only a Process<:IO object", :getindex)
+    return i == 1 ? getfield(p, p.openstream) : p
+end
+
 # PR #16984
 @deprecate MersenneTwister() MersenneTwister(0)
 

base/process.jl

@@ -315,6 +315,7 @@ mutable struct Process <: AbstractPipe
     termsignal::Int32
     exitnotify::Condition
     closenotify::Condition
+    openstream::Symbol # for open(cmd) deprecation
     function Process(cmd::Cmd, handle::Ptr{Void},
                      in::Union{Redirectable, Ptr{Void}},
                      out::Union{Redirectable, Ptr{Void}},
@@ -344,7 +345,9 @@ struct ProcessChain <: AbstractPipe
     in::Redirectable
     out::Redirectable
     err::Redirectable
+    openstream::Symbol # for open(cmd) deprecation
     ProcessChain(stdios::StdIOSet) = new(Process[], stdios[1], stdios[2], stdios[3])
+    ProcessChain(chain::ProcessChain, openstream::Symbol) = new(chain.processes, chain.in, chain.out, chain.err, openstream) # for open(cmd) deprecation
 end
 pipe_reader(p::ProcessChain) = p.out
 pipe_writer(p::ProcessChain) = p.in
@@ -589,11 +592,21 @@ function open(cmds::AbstractCmd, mode::AbstractString="r", other::Redirectable=D
         out = Pipe()
         processes = spawn(cmds, (in,out,STDERR))
         close(out.in)
+        if isa(processes, ProcessChain) # for open(cmd) deprecation
+            processes = ProcessChain(processes, :out)
+        else
+            processes.openstream = :out
+        end
     elseif mode == "w"
         in = Pipe()
         out = other
         processes = spawn(cmds, (in,out,STDERR))
         close(in.out)
+        if isa(processes, ProcessChain) # for open(cmd) deprecation
+            processes = ProcessChain(processes, :in)
+        else
+            processes.openstream = :in
+        end
     else
         throw(ArgumentError("mode must be \"r\" or \"w\", not \"$mode\""))
     end