diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index 119bdfcb0f442..6f63dec91d865 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -1370,6 +1370,10 @@ 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 {
diff --git a/src/libstd/path.rs b/src/libstd/path.rs
index b8361d3e82599..53d3ad62f5ec6 100644
--- a/src/libstd/path.rs
+++ b/src/libstd/path.rs
@@ -846,6 +846,10 @@ 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")]
diff --git a/src/libstd/sys/cloudabi/shims/fs.rs b/src/libstd/sys/cloudabi/shims/fs.rs
index e6160d1457d26..f2a960b16d3ba 100644
--- a/src/libstd/sys/cloudabi/shims/fs.rs
+++ b/src/libstd/sys/cloudabi/shims/fs.rs
@@ -140,6 +140,7 @@ impl Iterator for ReadDir {
     fn next(&mut self) -> Option<io::Result<DirEntry>> {
         match self.0 {}
     }
+
 }
 
 impl DirEntry {
diff --git a/src/libstd/sys/cloudabi/shims/net.rs b/src/libstd/sys/cloudabi/shims/net.rs
index 67c436fa7955d..a5f0407f414e9 100644
--- a/src/libstd/sys/cloudabi/shims/net.rs
+++ b/src/libstd/sys/cloudabi/shims/net.rs
@@ -299,6 +299,9 @@ impl Iterator for LookupHost {
     fn next(&mut self) -> Option<SocketAddr> {
         match self.0 {}
     }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 impl TryFrom<&str> for LookupHost {
diff --git a/src/libstd/sys/cloudabi/shims/os.rs b/src/libstd/sys/cloudabi/shims/os.rs
index 779e6d54b7c9f..25eb80aaf189d 100644
--- a/src/libstd/sys/cloudabi/shims/os.rs
+++ b/src/libstd/sys/cloudabi/shims/os.rs
@@ -43,6 +43,9 @@ 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)]
diff --git a/src/libstd/sys/wasm/fs.rs b/src/libstd/sys/wasm/fs.rs
index e6160d1457d26..25d3c6f10f3dc 100644
--- a/src/libstd/sys/wasm/fs.rs
+++ b/src/libstd/sys/wasm/fs.rs
@@ -140,6 +140,10 @@ 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 {
diff --git a/src/libstd/sys/wasm/net.rs b/src/libstd/sys/wasm/net.rs
index b7c3108f172f6..bd2e6b0eb9140 100644
--- a/src/libstd/sys/wasm/net.rs
+++ b/src/libstd/sys/wasm/net.rs
@@ -296,6 +296,9 @@ impl Iterator for LookupHost {
     fn next(&mut self) -> Option<SocketAddr> {
         match self.0 {}
     }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
 impl TryFrom<&str> for LookupHost {
diff --git a/src/libstd/sys/wasm/os.rs b/src/libstd/sys/wasm/os.rs
index 91afdc8a5a0cc..fd8f9cc8e1496 100644
--- a/src/libstd/sys/wasm/os.rs
+++ b/src/libstd/sys/wasm/os.rs
@@ -33,6 +33,9 @@ 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)]
@@ -70,6 +73,9 @@ 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 {
diff --git a/src/libstd/sys/windows/os.rs b/src/libstd/sys/windows/os.rs
index a0da2498bb7e0..68fcec0ff8d1d 100644
--- a/src/libstd/sys/windows/os.rs
+++ b/src/libstd/sys/windows/os.rs
@@ -185,6 +185,15 @@ 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)]