Backport pyo3::intern! from pyo3 0.16 (#7355)

This should speed up all of these code-paths when called repeatedly.

Author: alex

src/rust/src/intern.rs

@@ -0,0 +1,44 @@
+// This file is dual licensed under the terms of the Apache License, Version
+// 2.0, and the BSD License. See the LICENSE file in the root of this repository
+// for complete details.
+
+// This file is a backport of `pyo3::intern!` from pyo3 0.16.
+
+#[macro_export]
+macro_rules! intern {
+    ($py: expr, $text: expr) => {{
+        static INTERNED: $crate::intern::Interned = $crate::intern::Interned::new($text);
+        INTERNED.get($py)
+    }};
+}
+
+#[doc(hidden)]
+pub struct Interned(
+    &'static str,
+    pyo3::once_cell::GILOnceCell<pyo3::Py<pyo3::types::PyString>>,
+);
+
+impl Interned {
+    pub const fn new(value: &'static str) -> Self {
+        Interned(value, pyo3::once_cell::GILOnceCell::new())
+    }
+
+    #[inline]
+    pub fn get<'py>(&'py self, py: pyo3::Python<'py>) -> &'py pyo3::types::PyString {
+        self.1
+            .get_or_init(py, || pyo3::types::PyString::new(py, self.0).into())
+            .as_ref(py)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Interned;
+
+    #[test]
+    fn test_interned_new() {
+        for s in ["abc", "123"] {
+            Interned::new(s);
+        }
+    }
+}

src/rust/src/lib.rs

@@ -5,6 +5,7 @@
 #![deny(rust_2018_idioms)]
 
 mod asn1;
+mod intern;
 pub(crate) mod oid;
 mod x509;
 

src/rust/src/oid.rs

@@ -32,7 +32,7 @@ impl ObjectIdentifier {
     ) -> pyo3::PyResult<&'p pyo3::PyAny> {
         let oid_names = py
             .import("cryptography.hazmat._oid")?
-            .getattr("_OID_NAMES")?;
+            .getattr(crate::intern!(py, "_OID_NAMES"))?;
         oid_names.call_method1("get", (slf, "Unknown OID"))
     }
 }