fix(const_eval): don't panic when a vecN constructor's arg. count is less than N (#6508)

CHANGELOG.md

@@ -72,6 +72,7 @@ Bottom level categories:
 #### Naga
 
 - Fix crash when a texture argument is missing. By @aedm in [#6486](https://github.com/gfx-rs/wgpu/pull/6486)
+- Emit an error in constant evaluation, rather than crash, in certain cases where `vecN` constructors have less than N arguments. By @ErichDonGubler in [#6508](https://github.com/gfx-rs/wgpu/pull/6508).
 
 #### General
 

naga/src/proc/constant_evaluator.rs

@@ -137,8 +137,8 @@ macro_rules! gen_component_wise_extractor {
                             for idx in 0..(size as u8).into() {
                                 let group = component_groups
                                     .iter()
-                                    .map(|cs| cs[idx])
-                                    .collect::<ArrayVec<_, N>>()
+                                    .map(|cs| cs.get(idx).cloned().ok_or(err.clone()))
+                                    .collect::<Result<ArrayVec<_, N>, _>>()?
                                     .into_inner()
                                     .unwrap();
                                 new_components.push($ident(