Add support for arbitrary arrays

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Support PyPy 3.7. [#1538](https://github.com/PyO3/pyo3/pull/1538)
 
 ### Added
+- Add conversions for `[T; N]` for all `N` on Rust 1.51 and up. [#1128](https://github.com/PyO3/pyo3/pull/1128)
 - Add conversions between `OsStr`/`OsString`/`Path`/`PathBuf` and Python strings. [#1379](https://github.com/PyO3/pyo3/pull/1379)
 - Add `#[pyo3(from_py_with = "...")]` attribute for function arguments and struct fields to override the default from-Python conversion. [#1411](https://github.com/PyO3/pyo3/pull/1411)
 - Add FFI definition `PyCFunction_CheckExact` for Python 3.9 and later. [#1425](https://github.com/PyO3/pyo3/pull/1425)

build.rs

@@ -746,6 +746,17 @@ fn ensure_python_version(interpreter_config: &InterpreterConfig) -> Result<()> {
     Ok(())
 }
 
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = core::str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    pieces.next()?.parse().ok()
+}
+
 fn emit_cargo_configuration(interpreter_config: &InterpreterConfig) -> Result<()> {
     let target_os = cargo_env_var("CARGO_CFG_TARGET_OS").unwrap();
     let is_extension_module = cargo_env_var("CARGO_FEATURE_EXTENSION_MODULE").is_some();
@@ -850,6 +861,12 @@ fn emit_cargo_configuration(interpreter_config: &InterpreterConfig) -> Result<()
         println!("cargo:rustc-cfg=py_sys_config=\"{}\"", flag)
     }
 
+    // Enable use of const generics on Rust 1.51 and greater
+
+    if rustc_minor_version().unwrap_or(0) >= 51 {
+        println!("cargo:rustc-cfg=min_const_generics");
+    }
+
     Ok(())
 }
 

src/conversions/array.rs

@@ -0,0 +1,282 @@
+use crate::{
+    exceptions, FromPyObject, IntoPy, PyAny, PyErr, PyObject, PyResult, PyTryFrom, Python,
+    ToPyObject,
+};
+
+#[cfg(min_const_generics)]
+impl<T, const N: usize> IntoPy<PyObject> for [T; N]
+where
+    T: ToPyObject,
+{
+    fn into_py(self, py: Python) -> PyObject {
+        self.as_ref().to_object(py)
+    }
+}
+
+#[cfg(min_const_generics)]
+impl<'a, T, const N: usize> FromPyObject<'a> for [T; N]
+where
+    T: FromPyObject<'a>,
+{
+    #[cfg(not(feature = "nightly"))]
+    fn extract(obj: &'a PyAny) -> PyResult<Self> {
+        create_array_from_obj(obj)
+    }
+
+    #[cfg(feature = "nightly")]
+    default fn extract(obj: &'a PyAny) -> PyResult<Self> {
+        create_array_from_obj(obj)
+    }
+}
+
+#[cfg(all(min_const_generics, feature = "nightly"))]
+impl<'source, T, const N: usize> FromPyObject<'source> for [T; N]
+where
+    for<'a> T: Default + FromPyObject<'a> + crate::buffer::Element,
+{
+    fn extract(obj: &'source PyAny) -> PyResult<Self> {
+        use crate::{AsPyPointer, PyNativeType};
+        // first try buffer protocol
+        if unsafe { crate::ffi::PyObject_CheckBuffer(obj.as_ptr()) } == 1 {
+            if let Ok(buf) = crate::buffer::PyBuffer::get(obj) {
+                let mut array = [T::default(); N];
+                if buf.dimensions() == 1 && buf.copy_to_slice(obj.py(), &mut array).is_ok() {
+                    buf.release(obj.py());
+                    return Ok(array);
+                }
+                buf.release(obj.py());
+            }
+        }
+        create_array_from_obj(obj)
+    }
+}
+
+#[cfg(min_const_generics)]
+fn create_array_from_obj<'s, T, const N: usize>(obj: &'s PyAny) -> PyResult<[T; N]>
+where
+    T: FromPyObject<'s>,
+{
+    let seq = <crate::types::PySequence as PyTryFrom>::try_from(obj)?;
+    let seq_len = seq.len()? as usize;
+    if seq_len != N {
+        return Err(invalid_sequence_length(N, seq_len));
+    }
+    array_try_from_fn(|idx| seq.get_item(idx as isize).and_then(PyAny::extract))
+}
+
+// TODO use std::array::try_from_fn, if that stabilises:
+// (https://github.com/rust-lang/rust/pull/75644)
+#[cfg(min_const_generics)]
+fn array_try_from_fn<E, F, T, const N: usize>(mut cb: F) -> Result<[T; N], E>
+where
+    F: FnMut(usize) -> Result<T, E>,
+{
+    // Helper to safely create arrays since the standard library doesn't
+    // provide one yet. Shouldn't be necessary in the future.
+    struct ArrayGuard<T, const N: usize> {
+        dst: *mut T,
+        initialized: usize,
+    }
+
+    impl<T, const N: usize> Drop for ArrayGuard<T, N> {
+        fn drop(&mut self) {
+            debug_assert!(self.initialized <= N);
+            let initialized_part = core::ptr::slice_from_raw_parts_mut(self.dst, self.initialized);
+            unsafe {
+                core::ptr::drop_in_place(initialized_part);
+            }
+        }
+    }
+
+    // [MaybeUninit<T>; N] would be "nicer" but is actually difficult to create - there are nightly
+    // APIs which would make this easier.
+    let mut array: core::mem::MaybeUninit<[T; N]> = core::mem::MaybeUninit::uninit();
+    let mut guard: ArrayGuard<T, N> = ArrayGuard {
+        dst: array.as_mut_ptr() as _,
+        initialized: 0,
+    };
+    unsafe {
+        let mut value_ptr = array.as_mut_ptr() as *mut T;
+        for i in 0..N {
+            core::ptr::write(value_ptr, cb(i)?);
+            value_ptr = value_ptr.offset(1);
+            guard.initialized += 1;
+        }
+        core::mem::forget(guard);
+        Ok(array.assume_init())
+    }
+}
+
+#[cfg(not(min_const_generics))]
+macro_rules! array_impls {
+    ($($N:expr),+) => {
+        $(
+            impl<T> IntoPy<PyObject> for [T; $N]
+            where
+                T: ToPyObject
+            {
+                fn into_py(self, py: Python) -> PyObject {
+                    self.as_ref().to_object(py)
+                }
+            }
+
+            impl<'a, T> FromPyObject<'a> for [T; $N]
+            where
+                T: Copy + Default + FromPyObject<'a>,
+            {
+                #[cfg(not(feature = "nightly"))]
+                fn extract(obj: &'a PyAny) -> PyResult<Self> {
+                    let mut array = [T::default(); $N];
+                    extract_sequence_into_slice(obj, &mut array)?;
+                    Ok(array)
+                }
+
+                #[cfg(feature = "nightly")]
+                default fn extract(obj: &'a PyAny) -> PyResult<Self> {
+                    let mut array = [T::default(); $N];
+                    extract_sequence_into_slice(obj, &mut array)?;
+                    Ok(array)
+                }
+            }
+
+            #[cfg(feature = "nightly")]
+            impl<'source, T> FromPyObject<'source> for [T; $N]
+            where
+                for<'a> T: Default + FromPyObject<'a> + crate::buffer::Element,
+            {
+                fn extract(obj: &'source PyAny) -> PyResult<Self> {
+                    let mut array = [T::default(); $N];
+                    // first try buffer protocol
+                    if unsafe { crate::ffi::PyObject_CheckBuffer(obj.as_ptr()) } == 1 {
+                        if let Ok(buf) = crate::buffer::PyBuffer::get(obj) {
+                            if buf.dimensions() == 1 && buf.copy_to_slice(obj.py(), &mut array).is_ok() {
+                                buf.release(obj.py());
+                                return Ok(array);
+                            }
+                            buf.release(obj.py());
+                        }
+                    }
+                    // fall back to sequence protocol
+                    extract_sequence_into_slice(obj, &mut array)?;
+                    Ok(array)
+                }
+            }
+        )+
+    }
+}
+
+#[cfg(not(min_const_generics))]
+array_impls!(
+    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
+    26, 27, 28, 29, 30, 31, 32
+);
+
+#[cfg(not(min_const_generics))]
+fn extract_sequence_into_slice<'s, T>(obj: &'s PyAny, slice: &mut [T]) -> PyResult<()>
+where
+    T: FromPyObject<'s>,
+{
+    let seq = <crate::types::PySequence as PyTryFrom>::try_from(obj)?;
+    let seq_len = seq.len()? as usize;
+    if seq_len != slice.len() {
+        return Err(invalid_sequence_length(slice.len(), seq_len));
+    }
+    for (value, item) in slice.iter_mut().zip(seq.iter()?) {
+        *value = item?.extract::<T>()?;
+    }
+    Ok(())
+}
+
+fn invalid_sequence_length(expected: usize, actual: usize) -> PyErr {
+    exceptions::PyValueError::new_err(format!(
+        "expected a sequence of length {} (got {})",
+        expected, actual
+    ))
+}
+
+#[cfg(test)]
+mod test {
+    use crate::{PyResult, Python};
+    #[cfg(min_const_generics)]
+    use std::{
+        panic,
+        sync::atomic::{AtomicUsize, Ordering},
+    };
+
+    #[cfg(min_const_generics)]
+    #[test]
+    fn array_try_from_fn() {
+        static DROP_COUNTER: AtomicUsize = AtomicUsize::new(0);
+        struct CountDrop;
+        impl Drop for CountDrop {
+            fn drop(&mut self) {
+                DROP_COUNTER.fetch_add(1, Ordering::SeqCst);
+            }
+        }
+        let _ = catch_unwind_silent(move || {
+            let _: Result<[CountDrop; 4], ()> = super::array_try_from_fn(|idx| {
+                if idx == 2 {
+                    panic!("peek a boo");
+                }
+                Ok(CountDrop)
+            });
+        });
+        assert_eq!(DROP_COUNTER.load(Ordering::SeqCst), 2);
+    }
+
+    #[test]
+    fn test_extract_small_bytearray_to_array() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let v: [u8; 3] = py
+            .eval("bytearray(b'abc')", None, None)
+            .unwrap()
+            .extract()
+            .unwrap();
+        assert!(&v == b"abc");
+    }
+
+    #[test]
+    fn test_extract_invalid_sequence_length() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let v: PyResult<[u8; 3]> = py
+            .eval("bytearray(b'abcdefg')", None, None)
+            .unwrap()
+            .extract();
+        assert_eq!(
+            v.unwrap_err().to_string(),
+            "ValueError: expected a sequence of length 3 (got 7)"
+        );
+    }
+
+    #[cfg(min_const_generics)]
+    #[test]
+    fn test_extract_bytearray_to_array() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let v: [u8; 33] = py
+            .eval(
+                "bytearray(b'abcabcabcabcabcabcabcabcabcabcabc')",
+                None,
+                None,
+            )
+            .unwrap()
+            .extract()
+            .unwrap();
+        assert!(&v == b"abcabcabcabcabcabcabcabcabcabcabc");
+    }
+
+    // https://stackoverflow.com/a/59211505
+    #[cfg(min_const_generics)]
+    fn catch_unwind_silent<F, R>(f: F) -> std::thread::Result<R>
+    where
+        F: FnOnce() -> R + panic::UnwindSafe,
+    {
+        let prev_hook = panic::take_hook();
+        panic::set_hook(Box::new(|_| {}));
+        let result = panic::catch_unwind(f);
+        panic::set_hook(prev_hook);
+        result
+    }
+}

src/conversions/mod.rs

@@ -1,5 +1,5 @@
-//! This module contains conversions between non-String Rust object and their string representation
-//! in Python
+//! This module contains conversions between various Rust object and their representation in Python.
 
+mod array;
 mod osstr;
 mod path;

src/types/list.rs

@@ -178,26 +178,6 @@ where
     }
 }
 
-macro_rules! array_impls {
-    ($($N:expr),+) => {
-        $(
-            impl<T> IntoPy<PyObject> for [T; $N]
-            where
-                T: ToPyObject
-            {
-                fn into_py(self, py: Python) -> PyObject {
-                    self.as_ref().to_object(py)
-                }
-            }
-        )+
-    }
-}
-
-array_impls!(
-    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
-    26, 27, 28, 29, 30, 31, 32
-);
-
 impl<T> ToPyObject for Vec<T>
 where
     T: ToPyObject,