From 5a927088355a275cad054c0505b8407d75fad2cc Mon Sep 17 00:00:00 2001
From: Caio <c410.f3r@gmail.com>
Date: Tue, 1 Sep 2020 10:38:37 -0300
Subject: [PATCH] Support for arbitrary arrays

---
 Cargo.toml            |  4 ++++
 src/lib.rs            |  2 ++
 src/types/list.rs     | 12 ++++++++++
 src/types/mod.rs      | 37 ++++++++++++++++++++++++++++++
 src/types/sequence.rs | 52 +++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 107 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index e9e14fa6270..6383bab158d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -35,6 +35,10 @@ rustversion = "1.0"
 [features]
 default = ["macros"]
 macros = ["ctor", "indoc", "inventory", "paste", "pyo3cls", "unindent"]
+
+# Supports arrays of arbitrary size
+const-generics = []
+
 # Optimizes PyObject to Vec conversion and so on.
 nightly = []
 
diff --git a/src/lib.rs b/src/lib.rs
index 10f3e768f8e..5bd29ae0169 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(feature = "const-generics", feature(min_const_generics))]
+#![cfg_attr(feature = "nightly", allow(incomplete_features))]
 #![cfg_attr(feature = "nightly", feature(specialization))]
 #![allow(clippy::missing_safety_doc)] // FIXME (#698)
 
diff --git a/src/types/list.rs b/src/types/list.rs
index 4a0586e4fcd..a23940936b6 100644
--- a/src/types/list.rs
+++ b/src/types/list.rs
@@ -178,6 +178,17 @@ where
     }
 }
 
+#[cfg(feature = "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(not(feature = "const-generics"))]
 macro_rules! array_impls {
     ($($N:expr),+) => {
         $(
@@ -193,6 +204,7 @@ macro_rules! array_impls {
     }
 }
 
+#[cfg(not(feature = "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
diff --git a/src/types/mod.rs b/src/types/mod.rs
index 32bbfe8e77e..72f2b782515 100644
--- a/src/types/mod.rs
+++ b/src/types/mod.rs
@@ -235,3 +235,40 @@ mod slice;
 mod string;
 mod tuple;
 mod typeobject;
+
+#[cfg(feature = "const-generics")]
+struct ArrayGuard<T, const N: usize> {
+    dst: *mut T,
+    initialized: usize,
+}
+
+#[cfg(feature = "const-generics")]
+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);
+        }
+    }
+}
+
+#[cfg(feature = "const-generics")]
+fn try_create_array<E, F, T, const N: usize>(mut cb: F) -> Result<[T; N], E>
+where
+    F: FnMut(usize) -> Result<T, E>,
+{
+    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 {
+        for (idx, value_ptr) in (&mut *array.as_mut_ptr()).iter_mut().enumerate() {
+            core::ptr::write(value_ptr, cb(idx)?);
+            guard.initialized += 1;
+        }
+        core::mem::forget(guard);
+        Ok(array.assume_init())
+    }
+}
diff --git a/src/types/sequence.rs b/src/types/sequence.rs
index 1b84352a5b0..f4dc1d78062 100644
--- a/src/types/sequence.rs
+++ b/src/types/sequence.rs
@@ -257,6 +257,41 @@ impl PySequence {
     }
 }
 
+#[cfg(feature = "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(feature = "const-generics", feature = "nightly"))]
+impl<'source, T, const N: usize> FromPyObject<'source> for [T; N]
+where
+    for<'a> T: FromPyObject<'a> + crate::buffer::Element,
+{
+    fn extract(obj: &'source PyAny) -> PyResult<Self> {
+        let mut array = create_array_from_obj(obj)?;
+        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());
+        }
+        Ok(array)
+    }
+}
+
+#[cfg(not(feature = "const-generics"))]
 macro_rules! array_impls {
     ($($N:expr),+) => {
         $(
@@ -303,6 +338,7 @@ macro_rules! array_impls {
     }
 }
 
+#[cfg(not(feature = "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
@@ -343,6 +379,21 @@ where
     }
 }
 
+#[cfg(feature = "const-generics")]
+fn create_array_from_obj<'s, T, const N: usize>(obj: &'s PyAny) -> PyResult<[T; N]>
+where
+    T: FromPyObject<'s>,
+{
+    let seq = <PySequence as PyTryFrom>::try_from(obj)?;
+    crate::types::try_create_array(|idx| {
+        seq.get_item(idx as isize)
+            .map_err(|_| {
+                exceptions::PyBufferError::py_err("Slice length does not match buffer length.")
+            })?
+            .extract::<T>()
+    })
+}
+
 fn extract_sequence<'s, T>(obj: &'s PyAny) -> PyResult<Vec<T>>
 where
     T: FromPyObject<'s>,
@@ -355,6 +406,7 @@ where
     Ok(v)
 }
 
+#[cfg(not(feature = "const-generics"))]
 fn extract_sequence_into_slice<'s, T>(obj: &'s PyAny, slice: &mut [T]) -> PyResult<()>
 where
     T: FromPyObject<'s>,