src/test/ui/methods/method-mut-self-modifies-mut-slice-lvalue.rs

   1 // run-pass
   2 // Test that an `&mut self` method, when invoked on a place whose
   3 // type is `&mut [u8]`, passes in a pointer to the place and not a
   4 // temporary. Issue #19147.
   5
   6 use std::slice;
   7 use std::cmp;
   8
   9 trait MyWriter {
  10     fn my_write(&mut self, buf: &[u8]) -> Result<(), ()>;
  11 }
  12
  13 impl<'a> MyWriter for &'a mut [u8] {
  14     fn my_write(&mut self, buf: &[u8]) -> Result<(), ()> {
  15         let amt = cmp::min(self.len(), buf.len());
  16         self[..amt].clone_from_slice(&buf[..amt]);
  17
  18         let write_len = buf.len();
  19         unsafe {
  20             *self = slice::from_raw_parts_mut(
  21                 self.as_mut_ptr().add(write_len),
  22                 self.len() - write_len
  23             );
  24         }
  25
  26         Ok(())
  27     }
  28 }
  29
  30 fn main() {
  31     let mut buf = [0; 6];
  32
  33     {
  34         let mut writer: &mut [_] = &mut buf;
  35         writer.my_write(&[0, 1, 2]).unwrap();
  36         writer.my_write(&[3, 4, 5]).unwrap();
  37     }
  38
  39     // If `my_write` is not modifying `buf` in place, then we will
  40     // wind up with `[3, 4, 5, 0, 0, 0]` because the first call to
  41     // `my_write()` doesn't update the starting point for the write.
  42
  43     assert_eq!(buf, [0, 1, 2, 3, 4, 5]);
  44 }