Only read the trailing comma of outermost fn call

src/expr.rs

@@ -2359,11 +2359,24 @@ pub fn wrap_args_with_parens(
     }
 }
 
+/// Return true if a function call or a method call represented by the given span ends with a
+/// trailing comma. This function is used when rewriting macro, as adding or removing a trailing
+/// comma from macro can potentially break the code.
 fn span_ends_with_comma(context: &RewriteContext, span: Span) -> bool {
-    let snippet = context.snippet(span);
-    snippet
-        .trim_right_matches(|c: char| c == ')' || c.is_whitespace())
-        .ends_with(',')
+    let mut encountered_closing_paren = false;
+    for c in context.snippet(span).chars().rev() {
+        match c {
+            ',' => return true,
+            ')' => if encountered_closing_paren {
+                return false;
+            } else {
+                encountered_closing_paren = true;
+            },
+            _ if c.is_whitespace() => continue,
+            _ => return false,
+        }
+    }
+    false
 }
 
 fn rewrite_paren(context: &RewriteContext, subexpr: &ast::Expr, shape: Shape) -> Option<String> {

tests/source/macros.rs

@@ -21,6 +21,12 @@ fn main() {
     kaas!(/* comments */ a /* post macro */, b /* another */);
 
     trailingcomma!( a , b , c , );
+    // Preserve trailing comma only when necessary.
+    ok!(file.seek(
+        SeekFrom::Start(
+            table.map(|table| fixture.offset(table)).unwrap_or(0),
+        )
+    ));
 
     noexpr!( i am not an expression, OK? );
 

tests/target/macros.rs

@@ -33,6 +33,10 @@ fn main() {
     );
 
     trailingcomma!(a, b, c,);
+    // Preserve trailing comma only when necessary.
+    ok!(file.seek(SeekFrom::Start(
+        table.map(|table| fixture.offset(table)).unwrap_or(0),
+    )));
 
     noexpr!( i am not an expression, OK? );