Rollup merge of rust-lang#60656 - petertodd:2019-inline-cursor-over-s…

…lice, r=sfackler

Inline some Cursor calls for slices

(Partially) brings back rust-lang#33921

I've noticed in some serialization code I was writing that writes to slices produce much, much, worse code than you'd expect even with optimizations turned on. For example, you'd expect something like this to be zero cost:

```
use std::io::{self, Cursor, Write};

pub fn serialize((a, b): (u64, u64)) -> [u8;8+8] {
    let mut r = [0u8;16];
    {
        let mut w = Cursor::new(&mut r[..]);

        w.write(&a.to_le_bytes()).unwrap();
        w.write(&b.to_le_bytes()).unwrap();
    }
    r
}
```

...but it compiles down to [dozens of instructions](https://rust.godbolt.org/z/bdwDzb) because the `slice_write()` calls aren't inlined, which in turn means `unwrap()` can't be optimized away, and so on.

To be clear, this pull-req isn't sufficient by itself: if we want to go down that path we also need to add `#[inline]`'s to the default implementations for functions like `write_all()` in the `Write` trait and so on, or implement them separately in the `Cursor` impls. But I figured I'd start a conversation about what tradeoffs we're expecting here.

src/libstd/io/cursor.rs

@@ -265,13 +265,15 @@ impl<T> BufRead for Cursor<T> where T: AsRef<[u8]> {
 }
 
 // Non-resizing write implementation
+#[inline]
 fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
  let pos = cmp::min(*pos_mut, slice.len() as u64);
  let amt = (&mut slice[(pos as usize)..]).write(buf)?;
  *pos_mut += amt as u64;
  Ok(amt)
 }
 
+#[inline]
 fn slice_write_vectored(
  pos_mut: &mut u64,
  slice: &mut [u8],
@@ -341,6 +343,7 @@ impl Write for Cursor<&mut [u8]> {
  slice_write_vectored(&mut self.pos, self.inner, bufs)
  }
 
+ #[inline]
  fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }
 
@@ -354,6 +357,7 @@ impl Write for Cursor<&mut Vec<u8>> {
  vec_write_vectored(&mut self.pos, self.inner, bufs)
  }
 
+ #[inline]
  fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }
 
@@ -367,6 +371,7 @@ impl Write for Cursor<Vec<u8>> {
  vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
  }
 
+ #[inline]
  fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }
 
@@ -382,6 +387,7 @@ impl Write for Cursor<Box<[u8]>> {
  slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
  }
 
+ #[inline]
  fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }