Review: check for cap rather than adding cap, fix error messages.

Author: ElectronicRU

crates/rustc_codegen_spirv/src/codegen_cx/entry.rs

@@ -680,25 +680,28 @@ impl<'tcx> CodegenCx<'tcx> {
             ),
             _ => false,
         };
-        if let Some(attachment_index) = attrs.input_attachment_index.map(|attr| attr.value) {
-            if is_subpass_input {
-                self.emit_global().capability(Capability::InputAttachment);
+        if let Some(attachment_index) = attrs.input_attachment_index {
+            if is_subpass_input && self.builder.has_capability(Capability::InputAttachment) {
                 self.emit_global().decorate(
                     var,
                     Decoration::InputAttachmentIndex,
-                    std::iter::once(Operand::LiteralInt32(attachment_index)),
+                    std::iter::once(Operand::LiteralInt32(attachment_index.value)),
                 )
+            } else if is_subpass_input {
+                self.tcx
+                    .sess
+                    .span_err(hir_param.ty_span, "Missing capability InputAttachment")
             } else {
                 self.tcx.sess.span_err(
-                    attrs.input_attachment_index.unwrap().span,
-                    "#[spirv(attachment_index)] is only valid on Image types with dim = SubpassData"
+                    attachment_index.span,
+                    "#[spirv(input_attachment_index)] is only valid on Image types with dim = SubpassData"
                 );
             }
             decoration_supersedes_location = true;
         } else if is_subpass_input {
             self.tcx.sess.span_err(
                 hir_param.ty_span,
-                "Image types with dim = SubpassData require #[spirv(attachment_index)] decoration",
+                "Image types with dim = SubpassData require #[spirv(input_attachment_index)] decoration",
             )
         }
 

tests/ui/image/read_subpass.rs

@@ -1,4 +1,5 @@
 // build-pass
+// compile-flags: -C target-feature=+InputAttachment
 
 use spirv_std::{arch, Image};