Improve error message when image write without format used (#8294)

Author: cwfitzgerald

naga/src/front/spv/error.rs

@@ -107,6 +107,8 @@ pub enum Error {
     InvalidGlobalVar(crate::Expression),
     #[error("invalid image/sampler expression {0:?}")]
     InvalidImageExpression(crate::Expression),
+    #[error("image write without format is not currently supported. See https://github.com/gfx-rs/wgpu/issues/6797")]
+    InvalidImageWriteType,
     #[error("invalid image base type {0:?}")]
     InvalidImageBaseType(Handle<crate::Type>),
     #[error("invalid image {0:?}")]

naga/src/front/spv/mod.rs

@@ -5635,7 +5635,7 @@ impl<I: Iterator<Item = u32>> Frontend<I> {
         let is_depth = self.next()?;
         let is_array = self.next()? != 0;
         let is_msaa = self.next()? != 0;
-        let _is_sampled = self.next()?;
+        let is_sampled = self.next()?;
         let format = self.next()?;
 
         let dim = map_image_dim(dim)?;
@@ -5670,6 +5670,8 @@ impl<I: Iterator<Item = u32>> Frontend<I> {
                     format: map_image_format(format)?,
                     access: crate::StorageAccess::default(),
                 }
+            } else if is_sampled == 2 {
+                return Err(Error::InvalidImageWriteType);
             } else {
                 crate::ImageClass::Sampled {
                     kind,