feat(const_eval): impl. trunc

CHANGELOG.md

@@ -83,6 +83,7 @@ Bottom level categories:
         - `radians`
         - `reverseBits`
         - `sign`
+        - `trunc`
 - Eager release of GPU resources comes from device.trackers. By @bradwerth in [#5075](https://github.com/gfx-rs/wgpu/pull/5075)
 - `wgpu-types`'s `trace` and `replay` features have been replaced by the `serde` feature. By @KirmesBude in [#5149](https://github.com/gfx-rs/wgpu/pull/5149)
 - `wgpu-core`'s `serial-pass` feature has been removed. Use `serde` instead. By @KirmesBude in [#5149](https://github.com/gfx-rs/wgpu/pull/5149)

naga/src/proc/constant_evaluator.rs

@@ -1014,6 +1014,9 @@ impl<'a> ConstantEvaluator<'a> {
                     Ok([if edge <= x { 1.0 } else { 0.0 }])
                 })
             }
+            crate::MathFunction::Trunc => {
+                component_wise_float!(self, span, [arg], |e| { Ok([e.trunc()]) })
+            }
             fun => Err(ConstantEvaluatorError::NotImplemented(format!(
                 "{fun:?} built-in function"
             ))),