Replace the cubic_spline_interpolation macro with a generic function (

#11605)

# Objective

- Address a `TODO` item in `bevy_animation`.

## Solution

- Replace the `cubic_spline_interpolation` macro with a function.

The function isn't marked as `#[inline(always)]` but from what I checked
with `cargo asm` it gets inlined (even in debug, unless I explicitly add
`#[inline(never)]`), so this should be identical to the macro. If needed
I can add the attribute.

crates/bevy_animation/src/lib.rs

@@ -2,7 +2,7 @@
 
 #![warn(missing_docs)]
 
-use std::ops::Deref;
+use std::ops::{Add, Deref, Mul};
 use std::time::Duration;
 
 use bevy_app::{App, Plugin, PostUpdate};
@@ -672,16 +672,22 @@ fn get_keyframe(target_count: usize, keyframes: &[f32], key_index: usize) -> &[f
     &keyframes[start..end]
 }
 
-// Helper macro for cubic spline interpolation
-// it needs to work on `f32`, `Vec3` and `Quat`
-// TODO: replace by a function if the proper trait bounds can be figured out
-macro_rules! cubic_spline_interpolation {
-    ($value_start: expr, $tangent_out_start: expr, $tangent_in_end: expr, $value_end: expr, $lerp: expr, $step_duration: expr,) => {
-        $value_start * (2.0 * $lerp.powi(3) - 3.0 * $lerp.powi(2) + 1.0)
-            + $tangent_out_start * ($step_duration) * ($lerp.powi(3) - 2.0 * $lerp.powi(2) + $lerp)
-            + $value_end * (-2.0 * $lerp.powi(3) + 3.0 * $lerp.powi(2))
-            + $tangent_in_end * ($step_duration) * ($lerp.powi(3) - $lerp.powi(2))
-    };
+/// Helper function for cubic spline interpolation.
+fn cubic_spline_interpolation<T>(
+    value_start: T,
+    tangent_out_start: T,
+    tangent_in_end: T,
+    value_end: T,
+    lerp: f32,
+    step_duration: f32,
+) -> T
+where
+    T: Mul<f32, Output = T> + Add<Output = T>,
+{
+    value_start * (2.0 * lerp.powi(3) - 3.0 * lerp.powi(2) + 1.0)
+        + tangent_out_start * (step_duration) * (lerp.powi(3) - 2.0 * lerp.powi(2) + lerp)
+        + value_end * (-2.0 * lerp.powi(3) + 3.0 * lerp.powi(2))
+        + tangent_in_end * step_duration * (lerp.powi(3) - lerp.powi(2))
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -828,7 +834,7 @@ fn apply_keyframe(
             let tangent_out_start = keyframes[step_start * 3 + 2];
             let tangent_in_end = keyframes[(step_start + 1) * 3];
             let value_end = keyframes[(step_start + 1) * 3 + 1];
-            let result = cubic_spline_interpolation!(
+            let result = cubic_spline_interpolation(
                 value_start,
                 tangent_out_start,
                 tangent_in_end,
@@ -852,7 +858,7 @@ fn apply_keyframe(
             let tangent_out_start = keyframes[step_start * 3 + 2];
             let tangent_in_end = keyframes[(step_start + 1) * 3];
             let value_end = keyframes[(step_start + 1) * 3 + 1];
-            let result = cubic_spline_interpolation!(
+            let result = cubic_spline_interpolation(
                 value_start,
                 tangent_out_start,
                 tangent_in_end,
@@ -876,7 +882,7 @@ fn apply_keyframe(
             let tangent_out_start = keyframes[step_start * 3 + 2];
             let tangent_in_end = keyframes[(step_start + 1) * 3];
             let value_end = keyframes[(step_start + 1) * 3 + 1];
-            let result = cubic_spline_interpolation!(
+            let result = cubic_spline_interpolation(
                 value_start,
                 tangent_out_start,
                 tangent_in_end,
@@ -918,8 +924,8 @@ fn apply_keyframe(
                     .zip(tangents_in_end)
                     .zip(morph_end)
                     .map(
-                        |(((value_start, tangent_out_start), tangent_in_end), value_end)| {
-                            cubic_spline_interpolation!(
+                        |(((&value_start, &tangent_out_start), &tangent_in_end), &value_end)| {
+                            cubic_spline_interpolation(
                                 value_start,
                                 tangent_out_start,
                                 tangent_in_end,