Implement #24353 - Propegate cmd flags in interpolation

Too late for 1.2 for which this was originally marked, but nevertheless,
it's a quick change, so might as well get it in now and have it be available
in 1.3.

Fixes #24353

Author: Keno

NEWS.md

@@ -24,6 +24,8 @@ New library functions
 Standard library changes
 ------------------------
 
+* Cmd interpolation (``` `$(x::Cmd) a b c` ``` where) now propagates `x`'s process flags (environment, flags, working directory, etc) if `x` is the first interpolant and errors otherwise ([#24353]).
+
 #### LinearAlgebra
 
 

base/process.jl

@@ -30,6 +30,12 @@ struct Cmd <: AbstractCmd
     end
 end
 
+has_nondefault_cmd_flags(c::Cmd) =
+    c.ignorestatus ||
+    c.flags != 0x00 ||
+    c.env !== nothing ||
+    c.dir !== ""
+
 """
     Cmd(cmd::Cmd; ignorestatus, detach, windows_verbatim, windows_hide, env, dir)
 
@@ -901,7 +907,12 @@ end
 
 arg_gen() = String[]
 arg_gen(x::AbstractString) = String[cstr(x)]
-arg_gen(cmd::Cmd) = cmd.exec
+function arg_gen(cmd::Cmd)
+    if has_nondefault_cmd_flags(cmd)
+        throw(ArgumentError("Non-default environment behavior is only permitted for the first interpolant."))
+    end
+    cmd.exec
+end
 
 function arg_gen(head)
     if isiterable(typeof(head))
@@ -927,10 +938,20 @@ end
 
 function cmd_gen(parsed)
     args = String[]
-    for arg in parsed
-        append!(args, arg_gen(arg...))
+    if length(parsed) >= 1 && isa(parsed[1], Tuple{Cmd})
+        cmd = parsed[1][1]
+        (ignorestatus, flags, env, dir) = (cmd.ignorestatus, cmd.flags, cmd.env, cmd.dir)
+        append!(args, cmd.exec)
+        for arg in tail(parsed)
+            append!(args, arg_gen(arg...))
+        end
+        return Cmd(Cmd(args), ignorestatus, flags, env, dir)
+    else
+        for arg in parsed
+            append!(args, arg_gen(arg...))
+        end
+        return Cmd(args)
     end
-    return Cmd(args)
 end
 
 """

test/spawn.jl

@@ -456,6 +456,15 @@ end
 @test Set([``, echocmd]) != Set([``, ``])
 @test Set([echocmd, ``, ``, echocmd]) == Set([echocmd, ``])
 
+# test for interpolation of Cmd
+let c = setenv(`x`, "A"=>true)
+    @test (`$c a`).env == String["A=true"]
+    @test (`"$c" a`).env == String["A=true"]
+    @test_throws ArgumentError `a $c`
+    @test (`$(c.exec) a`).env === nothing
+    @test_throws ArgumentError `"$c "`
+end
+
 # equality tests for AndCmds
 @test Base.AndCmds(`$echocmd abc`, `$echocmd def`) == Base.AndCmds(`$echocmd abc`, `$echocmd def`)
 @test Base.AndCmds(`$echocmd abc`, `$echocmd def`) != Base.AndCmds(`$echocmd abc`, `$echocmd xyz`)