Default to empty string in the case of missing doc comment

martian-derive/src/lib.rs

@@ -563,13 +563,7 @@ pub fn martian_struct(item: proc_macro::TokenStream) -> proc_macro::TokenStream
         };
 
         if mro_filename.is_some() && doc_comment.is_none() {
-            return syn::Error::new_spanned(field,
-            "ERROR: mro_filename attribute was specified for field but a doc comment was not defined for the field. \
-            mro_filename can only be specified when a doc comment is specified for a field, \
-            because doc comments are converted into Martian struct field descriptions which precede \
-            the Martian filename column (optional 3rd and 4th columns in Martian struct def)")
-                .to_compile_error()
-                .into();
+            doc_comment = Some("".to_string());
         }
 
         let doc_comment_code = match doc_comment {

martian-derive/tests/mro/test_struct.mro

@@ -21,7 +21,7 @@ struct Config(
 struct ChemistryDef(
     string name           "The chemistry name",
     string barcode_read,
-    int    barcode_length,
+    int    barcode_length ""                   "the_bc_length",
 )
 
 struct RnaChunk(

martian-derive/tests/test_full_mro.rs

@@ -424,6 +424,7 @@ fn test_with_struct() {
         /// The chemistry name
         name: String,
         barcode_read: String,
+        #[mro_filename = "the_bc_length"]
         barcode_length: u8,
     }