How to cast a vect<u8> to a stream of utf-8 characters?

I'm trying to analyse, line-per-line, the ouput of a subprocess:

let mut amulecmd = Command::new("amulecmd")
   .stdin(Stdio::piped())
   .stdout(Stdio::piped())
   .spawn()
   .unwrap();
let amulecmd_stdin = amulecmd.stdin.as_mut().unwrap();
match amulecmd_stdin.write_all( b"show dl\nquit" ) {
   Ok(_) => {
      let status = amulecmd.wait_with_output();
      let reader = BufReader::new(status.unwrap().stdout);
      for line in reader.lines() {
         // TODO: if line is a "data" line
         // split it into fields
         println!("{}", line);
      }
   },
   Err(error) => panic!("Unable to write to amulecmd stdin: {:?}", error),
};

The line let reader = BufReader::new(status.unwrap().stdout); doesn't compile:

error[E0277]: the trait bound `Vec<u8>: std::io::Read` is not satisfied
  --> src/main.rs:64:38
   |
64 |          let reader = BufReader::new(status.unwrap().stdout);
   |                       -------------- ^^^^^^^^^^^^^^^^^^^^^^ the trait `std::io::Read` is not implemented for `Vec<u8>`
   |                       |
   |                       required by a bound introduced by this call
   |
note: required by a bound in `BufReader::<R>::new`
  --> /home/aubin/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/io/buffered/bufreader.rs:55:9
   |
55 | impl<R: Read> BufReader<R> {
   |         ^^^^ required by this bound in `BufReader::<R>::new`

I'm looking for a general purpose solution, I don't want to load all the lines into a single string because I have to deal with processes which runs forever or networks streams never closed.

Have a look at the stdout field. It has type Option<ChildStdout>, and ChildStdout implements Read.

If you want to stream the command's output, don't call wait_with_output, as that accumulates all the remaining output into an Output, which as you said you don't want to do. Instead perhaps use try_wait if you need to check if the command has ended without blocking. Or possibly reaching the end of the stdout stream will indicate that it exited -- I haven't tried it myself and the documentation doesn't say outright.