Add Read/Write::can_read/write_vectored

When working with an arbitrary reader or writer, code that uses vectored
operations may end up being slower than code that copies into a single
buffer when the underlying reader or writer doesn't actually support
vectored operations. These new methods allow you to ask the reader or
witer up front if vectored operations are efficiently supported.

Currently, you have to use some heuristics to guess by e.g. checking if
the read or write only accessed the first buffer. Hyper is one concrete
example of a library that has to do this dynamically:
https://github.com/hyperium/hyper/blob/0eaf304644a396895a4ce1f0146e596640bb666a/src/proto/h1/io.rs#L582-L594

1 files changed, 10 insertions, 0 deletions

diff --git a/src/libstd/io/util.rs b/src/libstd/io/util.rs
index b09161b97aa..01947cd8b89 100644
--- a/src/libstd/io/util.rs
+++ b/src/libstd/io/util.rs
@@ -180,6 +180,11 @@ impl Read for Repeat {
     }
 
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -236,6 +241,11 @@ impl Write for Sink {
     }
 
     #[inline]
+    fn can_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }