Use correct budget when rewriting generics of trait

Author: topecongiro

src/items.rs

@@ -938,7 +938,7 @@ pub fn format_trait(context: &RewriteContext, item: &ast::Item, offset: Indent)
 
         let body_lo = context.codemap.span_after(item.span, "{");
 
-        let shape = Shape::indented(offset, context.config);
+        let shape = Shape::indented(offset, context.config).offset_left(result.len())?;
         let generics_str =
             rewrite_generics(context, generics, shape, mk_sp(item.span.lo(), body_lo))?;
         result.push_str(&generics_str);