Revert to older error string.

Author: adetaylor

syntax/check.rs

@@ -211,9 +211,7 @@ fn check_api_type(cx: &mut Check, ety: &ExternType) {
 
     if let Some(reason) = cx.types.required_trivial.get(&ety.ident.rust) {
         let what = match reason {
-            TrivialReason::StructField(strct) => {
-                format!("a field of `{}`", strct.ident.cxx.to_fully_qualified())
-            }
+            TrivialReason::StructField(strct) => format!("a field of `{}`", strct.ident.rust),
             TrivialReason::FunctionArgument(efn) => format!("an argument of `{}`", efn.ident.rust),
             TrivialReason::FunctionReturn(efn) => format!("a return value of `{}`", efn.ident.rust),
         };

tests/ui/by_value_not_supported.stderr

@@ -16,7 +16,7 @@ error: using C++ string by value is not supported
 6 |         s: CxxString,
   |         ^^^^^^^^^^^^
 
-error: needs a cxx::ExternType impl in order to be used as a field of `::S`
+error: needs a cxx::ExternType impl in order to be used as a field of `S`
   --> $DIR/by_value_not_supported.rs:10:9
    |
 10 |         type C;