Use opaque blobs for unknown template definition types

When we don't know how to generate a Rust type from a template definition (eg
because it uses non-type template parameters), then we should fall back to using
the instantiation's layout to generate the opaque blob.

Author: fitzgen

src/codegen/mod.rs

@@ -2118,6 +2118,15 @@ impl ToRustTy for Type {
                 let decl = inst.template_definition();
                 let mut ty = decl.to_rust_ty(ctx).unwrap();
 
+                // If we gave up when making a type for the template definition,
+                // check if maybe we can make a better opaque blob for the
+                // instantiation.
+                if ty == aster::AstBuilder::new().ty().unit().unwrap() {
+                    if let Some(layout) = self.layout(ctx) {
+                        ty = BlobTyBuilder::new(layout).build().unwrap()
+                    }
+                }
+
                 // TODO: If the decl type is a template class/struct
                 // declaration's member template declaration, it could rely on
                 // generic template parameters from its outer template