Implement size_hint on various sys iterators

src/libstd/fs.rs

@@ -1260,6 +1260,10 @@ impl Iterator for ReadDir {
     fn next(&mut self) -> Option<io::Result<DirEntry>> {
         self.0.next().map(|entry| entry.map(DirEntry))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 impl DirEntry {

src/libstd/path.rs

@@ -897,6 +897,10 @@ impl<'a> Iterator for Iter<'a> {
     fn next(&mut self) -> Option<&'a OsStr> {
         self.inner.next().map(Component::as_os_str)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libstd/sys/cloudabi/shims/fs.rs

@@ -150,6 +150,10 @@ impl Iterator for ReadDir {
     fn next(&mut self) -> Option<io::Result<DirEntry>> {
         match self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 impl DirEntry {

src/libstd/sys/cloudabi/shims/net.rs

@@ -289,6 +289,10 @@ impl Iterator for LookupHost {
     fn next(&mut self) -> Option<SocketAddr> {
         match self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 pub fn lookup_host(_: &str) -> io::Result<LookupHost> {

src/libstd/sys/cloudabi/shims/os.rs

@@ -53,6 +53,10 @@ impl<'a> Iterator for SplitPaths<'a> {
     fn next(&mut self) -> Option<PathBuf> {
         match *self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 #[derive(Debug)]

src/libstd/sys/redox/fs.rs

@@ -167,6 +167,16 @@ impl Iterator for ReadDir {
             }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // There's at most one entry for every newline; and there are at most
+        // two skipped entries.
+        let upper = self.data[(i + 1)..].iter()
+            .filter(|byte| byte == b'\n')
+            .count();
+        let lower = upper.saturating_sub(2);
+        (lower, Some(upper))
+    }
 }
 
 impl DirEntry {

src/libstd/sys/redox/net/mod.rs

@@ -36,6 +36,10 @@ impl Iterator for LookupHost {
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 pub fn lookup_host(host: &str) -> Result<LookupHost> {

src/libstd/sys/unix/l4re.rs

@@ -429,6 +429,10 @@ pub mod net {
         fn next(&mut self) -> Option<SocketAddr> {
             None
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (0, Some(0))
+        }
     }
 
     unsafe impl Sync for LookupHost {}

src/libstd/sys/wasm/fs.rs

@@ -152,6 +152,10 @@ impl Iterator for ReadDir {
     fn next(&mut self) -> Option<io::Result<DirEntry>> {
         match self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 impl DirEntry {

src/libstd/sys/wasm/net.rs

@@ -291,6 +291,10 @@ impl Iterator for LookupHost {
     fn next(&mut self) -> Option<SocketAddr> {
         match self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 pub fn lookup_host(_: &str) -> io::Result<LookupHost> {

src/libstd/sys/wasm/os.rs

@@ -43,6 +43,10 @@ impl<'a> Iterator for SplitPaths<'a> {
     fn next(&mut self) -> Option<PathBuf> {
         match *self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 #[derive(Debug)]
@@ -77,6 +81,10 @@ impl Iterator for Env {
     fn next(&mut self) -> Option<(OsString, OsString)> {
         match self.0 {}
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 pub fn env() -> Env {

src/libstd/sys/windows/os.rs

@@ -192,6 +192,15 @@ impl<'a> Iterator for SplitPaths<'a> {
             Some(super::os2path(&in_progress))
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // There will be at most N + 1 entries, where N is the number of
+        // remaining semicolons.
+        let data = self.data.clone();
+        let semicolons = data.filter(|b| b == (';' as u16)).count();
+
+        (0, Some(semicolons + 1))
+    }
 }
 
 #[derive(Debug)]