[PATCH v2 1/6] rust: Add sparse_array! helper macro

[Matthew Maurer <mmaurer@google.com>]

An idiom in C code is to have an array of nullable values which is
partially initialized via `{ [0] = x, [7] = y}`. Because Rust expects
everything to be fully initialized, it does not have this idiom by
default.

`sparse_array!` allows declaration of `[Option<T>; _]` constants to
allow Rust code to more easily mimic the safe version of this pattern.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/slice.rs | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/rust/kernel/slice.rs b/rust/kernel/slice.rs
index ca2cde13506196d46c9169aa6e4ab2ac42af6f5b..826b6f77f0d07775bd22837cc1773b59ec96936c 100644
--- a/rust/kernel/slice.rs
+++ b/rust/kernel/slice.rs
@@ -47,3 +47,40 @@ fn as_flattened_mut(&mut self) -> &mut [T] {
         self.flatten_mut()
     }
 }
+
+/// Create a sparse array of `[Option<T>; _]`.
+///
+/// This is intended for use when C code would write `{ [0] = x, [7] = y}` to perform partial
+/// initialization of an array.
+///
+/// # Example
+/// ```
+/// use kernel::sparse_array;
+/// const FOO: &[Option<usize>] = &sparse_array! {
+///   0: 10,
+///   7: 16,
+/// };
+/// assert_eq!(FOO[0], Some(10));
+/// assert_eq!(FOO[1], None);
+/// assert_eq!(FOO[7], Some(16));
+/// ```
+#[macro_export]
+macro_rules! sparse_array {
+    ($(
+        $index:literal: $value:expr
+    ),* $(,)?) => {{
+        const SIZE: usize = {
+            let mut size = 0;
+            $(if $index >= size {
+                size = $index + 1;
+            })*
+            size
+        };
+
+        const {
+            let mut arr = [None; SIZE];
+            $(arr[$index] = Some($value);)*
+            arr
+        }
+    }}
+}

-- 
2.53.0.rc2.204.g2597b5adb4-goog