std::process (windows): Implement From<io::Stdout> etc. for imp::Stdio

This involves a new variant `imp;::Stdio::InheritSpecific`.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/sys/windows/process.rs

@@ -170,6 +170,7 @@ pub struct Command {
 
 pub enum Stdio {
     Inherit,
+    InheritSpecific { from_stdio_id: c::DWORD },
     Null,
     MakePipe,
     Pipe(AnonPipe),
@@ -533,6 +534,7 @@ impl Stdio {
         };
         match *self {
             Stdio::Inherit => use_stdio_id(stdio_id),
+            Stdio::InheritSpecific { from_stdio_id } => use_stdio_id(from_stdio_id),
 
             Stdio::MakePipe => {
                 let ours_readable = stdio_id != c::STD_INPUT_HANDLE;
@@ -580,6 +582,18 @@ impl From<File> for Stdio {
     }
 }
 
+impl From<io::Stdout> for Stdio {
+    fn from(_: io::Stdout) -> Stdio {
+        Stdio::InheritSpecific { from_stdio_id: c::STD_OUTPUT_HANDLE }
+    }
+}
+
+impl From<io::Stderr> for Stdio {
+    fn from(_: io::Stderr) -> Stdio {
+        Stdio::InheritSpecific { from_stdio_id: c::STD_ERROR_HANDLE }
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Processes
 ////////////////////////////////////////////////////////////////////////////////