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

3 files changed, 24 insertions, 0 deletions

diff --git a/src/libstd/sys/cloudabi/shims/fs.rs b/src/libstd/sys/cloudabi/shims/fs.rs
index e6160d1457d..e34483447e9 100644
--- a/src/libstd/sys/cloudabi/shims/fs.rs
+++ b/src/libstd/sys/cloudabi/shims/fs.rs
@@ -202,6 +202,10 @@ impl File {
         match self.0 {}
     }
 
+    pub fn can_read_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn write(&self, _buf: &[u8]) -> io::Result<usize> {
         match self.0 {}
     }
@@ -210,6 +214,10 @@ impl File {
         match self.0 {}
     }
 
+    pub fn can_write_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn flush(&self) -> io::Result<()> {
         match self.0 {}
     }
diff --git a/src/libstd/sys/cloudabi/shims/net.rs b/src/libstd/sys/cloudabi/shims/net.rs
index 67c436fa795..22195661a2e 100644
--- a/src/libstd/sys/cloudabi/shims/net.rs
+++ b/src/libstd/sys/cloudabi/shims/net.rs
@@ -47,6 +47,10 @@ impl TcpStream {
         match self.0 {}
     }
 
+    pub fn can_read_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn write(&self, _: &[u8]) -> io::Result<usize> {
         match self.0 {}
     }
@@ -55,6 +59,10 @@ impl TcpStream {
         match self.0 {}
     }
 
+    pub fn can_write_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn peer_addr(&self) -> io::Result<SocketAddr> {
         match self.0 {}
     }
diff --git a/src/libstd/sys/cloudabi/shims/pipe.rs b/src/libstd/sys/cloudabi/shims/pipe.rs
index fb14dc59101..eed8d1fdd56 100644
--- a/src/libstd/sys/cloudabi/shims/pipe.rs
+++ b/src/libstd/sys/cloudabi/shims/pipe.rs
@@ -12,6 +12,10 @@ impl AnonPipe {
         match self.0 {}
     }
 
+    pub fn can_read_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn write(&self, _buf: &[u8]) -> io::Result<usize> {
         match self.0 {}
     }
@@ -20,6 +24,10 @@ impl AnonPipe {
         match self.0 {}
     }
 
+    pub fn can_write_vectored(&self) -> bool {
+        match self.0 {}
+    }
+
     pub fn diverge(&self) -> ! {
         match self.0 {}
     }