Added size_hint impls for {PyDict,PyList,PySet,PyTuple}Iterators

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Add `serde` feature which provides implementations of `Serialize` and `Deserialize` for `Py<T>`. [#1366](https://github.com/PyO3/pyo3/pull/1366)
 - Add FFI definition `_PyCFunctionFastWithKeywords` on Python 3.7 and up. [#1384](https://github.com/PyO3/pyo3/pull/1384)
 - Add `PyDateTime::new_with_fold()` method. [#1398](https://github.com/PyO3/pyo3/pull/1398)
+- Add `size_hint` impls for `{PyDict,PyList,PySet,PyTuple}Iterator`s. [#1699](https://github.com/PyO3/pyo3/pull/1699)
 
 ### Changed
 - `prepare_freethreaded_python` will no longer register an `atexit` handler to call `Py_Finalize`. This resolves a number of issues with incompatible C extensions causing crashes at finalization. [#1355](https://github.com/PyO3/pyo3/pull/1355)

src/types/dict.rs

@@ -203,6 +203,12 @@ impl<'py> Iterator for PyDictIterator<'py> {
             }
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.dict.len().unwrap_or_default();
+        (len, Some(len))
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PyDict {

src/types/list.rs

@@ -151,6 +151,12 @@ impl<'a> Iterator for PyListIterator<'a> {
             None
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.list.len();
+        (len, Some(len))
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PyList {

src/types/set.rs

@@ -172,6 +172,12 @@ impl<'py> Iterator for PySetIterator<'py> {
             }
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.set.len().unwrap_or_default();
+        (len, Some(len))
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PySet {

src/types/tuple.rs

@@ -121,6 +121,11 @@ impl<'a> Iterator for PyTupleIterator<'a> {
             None
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.length, Some(self.length))
+    }
 }
 
 impl<'a> IntoIterator for &'a PyTuple {