Embedded Interface - go

Question

I'm still quite new to Go and I was surprised to not be able to use the subtype of an embedded interface.
Here is a small example to explain what I mean:
func test(sl bufio.ReadWriter){
// cannot use sl(type bufio.ReadWriter) as type bufio.Reader in function argument
readStuff(sl)
[...]
writeStuff(sl) // same kind of error
}
func readStuff(sl bufio.Reader){
[...]
}
As every interface have the same memory layout and ReadWriter is a Reader and a Writer, I was expecting this code to work.
I did try to convert the interface type with:
readStuff(sl.(buffio.Reader))
But it doesn't work either. So I've got two questions:
Why doesn't it work?
What's the go philosophy about that problem?

Answer 1

They're different types. However, a bufio.ReadWriter contains a pointer to both a bufio.Reader type and a bufio.Writer type as elements of its struct. So passing the correct one should be easy enough. Try this:
func test(sl bufio.ReadWriter){
readStuff(sl.Reader)
[...]
writeStuff(sl.Writer)
}
// Changed this bufio.Reader to a pointer receiver
func readStuff(sl *bufio.Reader) {
[...]
}

Answer 2

bufio.ReadWriter is a concrete type, not an interface. However, it does satisfy an interface (io.ReadWriter) so it can be assigned to a variable/function argument of an appropriate interface type. Then it works the way you may have anticipated (your code actually doesn't use any interfaces):
package main
import (
"bufio"
"bytes"
"fmt"
"io"
"log"
)
func readStuff(r io.Reader) {
b := make([]byte, 10)
n, err := r.Read(b)
if err != nil && err != io.EOF {
log.Fatal(err)
}
fmt.Printf("readStuff: %q\n", b[:n])
}
func writeStuff(w io.Writer) {
b := []byte("written")
n, err := w.Write(b)
if n != len(b) {
log.Fatal("Short write")
}
if err != nil {
log.Fatal(err)
}
}
func test(rw io.ReadWriter) {
readStuff(rw)
writeStuff(rw)
}
func main() {
r := io.Reader(bytes.NewBufferString("source"))
var uw bytes.Buffer
w := io.Writer(&uw)
rw := bufio.NewReadWriter(bufio.NewReader(r), bufio.NewWriter(w))
test(rw)
rw.Flush()
fmt.Printf("The underlying bytes.Buffer writer contains %q\n", uw.Bytes())
}
(Also here)
Output:
readStuff: "source"
The underlying bytes.Buffer writer contains "written"
This way test can consume any io.ReadWriter, not only a specific one. Which is a hint towards your question about go "philosophy".