error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
  --> $DIR/mut-borrow-needed-by-trait.rs:21:29
   |
LL |     let fp = BufWriter::new(fp);
   |                             ^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
   |
   = note: `std::io::Write` is implemented for `&mut dyn std::io::Write`, but not for `&dyn std::io::Write`
   = note: required by `std::io::BufWriter::<W>::new`

error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
   |
LL |     let fp = BufWriter::new(fp);
   |              ^^^^^^^^^^^^^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
   |
   = note: `std::io::Write` is implemented for `&mut dyn std::io::Write`, but not for `&dyn std::io::Write`
   = note: required by `std::io::BufWriter`

error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
   |
LL |     let fp = BufWriter::new(fp);
   |              ^^^^^^^^^^^^^^^^^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
   |
   = note: `std::io::Write` is implemented for `&mut dyn std::io::Write`, but not for `&dyn std::io::Write`
   = note: required by `std::io::BufWriter`

error[E0599]: no method named `write_fmt` found for struct `std::io::BufWriter<&dyn std::io::Write>` in the current scope
  --> $DIR/mut-borrow-needed-by-trait.rs:26:5
   |
LL |     writeln!(fp, "hello world").unwrap();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `std::io::BufWriter<&dyn std::io::Write>`
   | 
  ::: $SRC_DIR/libstd/io/buffered.rs:LL:COL
   |
LL | pub struct BufWriter<W: Write> {
   | ------------------------------ doesn't satisfy `_: std::io::Write`
   |
   = note: the method `write_fmt` exists but the following trait bounds were not satisfied:
           `&dyn std::io::Write: std::io::Write`
           which is required by `std::io::BufWriter<&dyn std::io::Write>: std::io::Write`
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0277, E0599.
For more information about an error, try `rustc --explain E0277`.