Mesh::merge to return a Result (#17475)

# Objective

Make `Mesh::merge` more resilient to use.

Currently, it's difficult to make sure `Mesh::merge` will not panic
(we'd have to check if all attributes are compatible).

- I'd appreciate it for utility function to convert different mesh
representations such as:
dimforge/bevy_rapier#628.

## Solution

- Make `Mesh::merge` return a `Result`.

## Testing

- It builds

## Migration Guide

- `Mesh::merge` now returns a `Result<(), MeshMergeError>`.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Greeble <166992735+greeble-dev@users.noreply.github.com>
Co-authored-by: Benjamin Brienen <benjamin.brienen@outlook.com>

crates/bevy_mesh/src/mesh.rs

@@ -14,6 +14,7 @@ use bevy_image::Image;
 use bevy_math::{primitives::Triangle3d, *};
 use bevy_reflect::Reflect;
 use bytemuck::cast_slice;
+use thiserror::Error;
 use tracing::warn;
 use wgpu_types::{VertexAttribute, VertexFormat, VertexStepMode};
 
@@ -280,7 +281,7 @@ impl Mesh {
         self.attributes.contains_key(&id.into())
     }
 
-    /// Retrieves the data currently set to the vertex attribute with the specified `name`.
+    /// Retrieves the data currently set to the vertex attribute with the specified [`MeshVertexAttributeId`].
     #[inline]
     pub fn attribute(
         &self,
@@ -289,6 +290,15 @@ impl Mesh {
         self.attributes.get(&id.into()).map(|data| &data.values)
     }
 
+    /// Retrieves the full data currently set to the vertex attribute with the specified [`MeshVertexAttributeId`].
+    #[inline]
+    pub(crate) fn attribute_data(
+        &self,
+        id: impl Into<MeshVertexAttributeId>,
+    ) -> Option<&MeshAttributeData> {
+        self.attributes.get(&id.into())
+    }
+
     /// Retrieves the data currently set to the vertex attribute with the specified `name` mutably.
     #[inline]
     pub fn attribute_mut(
@@ -788,19 +798,18 @@ impl Mesh {
     ///
     /// `Aabb` of entities with modified mesh are not updated automatically.
     ///
-    /// # Panics
+    /// # Errors
     ///
-    /// Panics if the vertex attribute values of `other` are incompatible with `self`.
+    /// Returns [`Err(MergeMeshError)`](MergeMeshError) if the vertex attribute values of `other` are incompatible with `self`.
     /// For example, [`VertexAttributeValues::Float32`] is incompatible with [`VertexAttributeValues::Float32x3`].
-    pub fn merge(&mut self, other: &Mesh) {
+    pub fn merge(&mut self, other: &Mesh) -> Result<(), MergeMeshError> {
         use VertexAttributeValues::*;
 
         // The indices of `other` should start after the last vertex of `self`.
         let index_offset = self.count_vertices();
 
         // Extend attributes of `self` with attributes of `other`.
         for (attribute, values) in self.attributes_mut() {
-            let enum_variant_name = values.enum_variant_name();
             if let Some(other_values) = other.attribute(attribute.id) {
                 #[expect(
                     clippy::match_same_arms,
@@ -835,11 +844,14 @@ impl Mesh {
                     (Snorm8x4(vec1), Snorm8x4(vec2)) => vec1.extend(vec2),
                     (Uint8x4(vec1), Uint8x4(vec2)) => vec1.extend(vec2),
                     (Unorm8x4(vec1), Unorm8x4(vec2)) => vec1.extend(vec2),
-                    _ => panic!(
-                        "Incompatible vertex attribute types {} and {}",
-                        enum_variant_name,
-                        other_values.enum_variant_name()
-                    ),
+                    _ => {
+                        return Err(MergeMeshError {
+                            self_attribute: *attribute,
+                            other_attribute: other
+                                .attribute_data(attribute.id)
+                                .map(|data| data.attribute),
+                        })
+                    }
                 }
             }
         }
@@ -848,6 +860,7 @@ impl Mesh {
         if let (Some(indices), Some(other_indices)) = (self.indices_mut(), other.indices()) {
             indices.extend(other_indices.iter().map(|i| (i + index_offset) as u32));
         }
+        Ok(())
     }
 
     /// Transforms the vertex positions, normals, and tangents of the mesh by the given [`Transform`].
@@ -1213,6 +1226,14 @@ impl core::ops::Mul<Mesh> for Transform {
     }
 }
 
+/// Error that can occur when calling [`Mesh::merge`].
+#[derive(Error, Debug, Clone)]
+#[error("Incompatible vertex attribute types {} and {}", self_attribute.name, other_attribute.map(|a| a.name).unwrap_or("None"))]
+pub struct MergeMeshError {
+    pub self_attribute: MeshVertexAttribute,
+    pub other_attribute: Option<MeshVertexAttribute>,
+}
+
 #[cfg(test)]
 mod tests {
     use super::Mesh;

crates/bevy_mesh/src/primitives/extrusion.rs

@@ -216,7 +216,7 @@ where
 
         // An extrusion of depth 0 does not need a mantel
         if self.half_depth == 0. {
-            front_face.merge(&back_face);
+            front_face.merge(&back_face).unwrap();
             return front_face;
         }
 
@@ -408,8 +408,8 @@ where
                 .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
         };
 
-        front_face.merge(&back_face);
-        front_face.merge(&mantel);
+        front_face.merge(&back_face).unwrap();
+        front_face.merge(&mantel).unwrap();
         front_face
     }
 }