Add a lazily calculated hash field to PyString

vm/src/builtins.rs

@@ -250,12 +250,9 @@ fn builtin_format(
     format_spec: OptionalArg<PyStringRef>,
     vm: &VirtualMachine,
 ) -> PyResult<PyStringRef> {
-    let format_spec = format_spec.into_option().unwrap_or_else(|| {
-        PyString {
-            value: "".to_string(),
-        }
-        .into_ref(vm)
-    });
+    let format_spec = format_spec
+        .into_option()
+        .unwrap_or_else(|| PyString::from("").into_ref(vm));
 
     vm.call_method(&value, "__format__", vec![format_spec.into_object()])?
         .downcast()

vm/src/obj/objstr.rs

@@ -54,6 +54,7 @@ use unicode_categories::UnicodeCategories;
 pub struct PyString {
     // TODO: shouldn't be public
     pub value: String,
+    hash: Cell<Option<pyhash::PyHash>>,
 }
 
 impl PyString {
@@ -64,8 +65,15 @@ impl PyString {
 
 impl From<&str> for PyString {
     fn from(s: &str) -> PyString {
+        s.to_string().into()
+    }
+}
+
+impl From<String> for PyString {
+    fn from(s: String) -> PyString {
         PyString {
-            value: s.to_string(),
+            value: s,
+            hash: Cell::default(),
         }
     }
 }
@@ -80,16 +88,13 @@ impl fmt::Display for PyString {
 
 impl TryIntoRef<PyString> for String {
     fn try_into_ref(self, vm: &VirtualMachine) -> PyResult<PyRef<PyString>> {
-        Ok(PyString { value: self }.into_ref(vm))
+        Ok(PyString::from(self).into_ref(vm))
     }
 }
 
 impl TryIntoRef<PyString> for &str {
     fn try_into_ref(self, vm: &VirtualMachine) -> PyResult<PyRef<PyString>> {
-        Ok(PyString {
-            value: self.to_string(),
-        }
-        .into_ref(vm))
+        Ok(PyString::from(self).into_ref(vm))
     }
 }
 
@@ -259,7 +264,14 @@ impl PyString {
 
     #[pymethod(name = "__hash__")]
     fn hash(&self, _vm: &VirtualMachine) -> pyhash::PyHash {
-        pyhash::hash_value(&self.value)
+        match self.hash.get() {
+            Some(hash) => hash,
+            None => {
+                let hash = pyhash::hash_value(&self.value);
+                self.hash.set(Some(hash));
+                hash
+            }
+        }
     }
 
     #[pymethod(name = "__len__")]

vm/src/pyobject.rs

@@ -380,7 +380,7 @@ impl PyContext {
     }
 
     pub fn new_str(&self, s: String) -> PyObjectRef {
-        PyObject::new(objstr::PyString { value: s }, self.str_type(), None)
+        PyObject::new(objstr::PyString::from(s), self.str_type(), None)
     }
 
     pub fn new_bytes(&self, data: Vec<u8>) -> PyObjectRef {