bugfix(semantic): writeln!(f) with no format string writes a newline.

`writeln!(f)` (a formatter with no format string) errored with "Macro expected format string argument", though in Rust it writes just a newline. Accept the no-format-string case for `writeln!` and emit a newline; `write!(f)` still requires a format string. This also makes `println!()` valid — it expands to `writeln!(f, )` — matching its documented "prints an empty line" example, while `print!()` → `write!(f, )` still errors.
orizi · Jul 3, 2026 · Jul 1, 2026 · Jul 2, 2026 · 030ea75ff92f0c273151a5088123ec55ba7d9f17
commit 030ea75ff92f0c273151a5088123ec55ba7d9f17
diff --git a/corelib/src/test/fmt_test.cairo b/corelib/src/test/fmt_test.cairo
@@ -29,6 +29,16 @@ fn test_format() {
     );
 }
 
+#[test]
+fn test_writeln() {
+    let mut f: core::fmt::Formatter = Default::default();
+    // `writeln!` with no format string writes just a newline.
+    let _ = writeln!(f);
+    assert(f.buffer == "\n", 'empty writeln bad formatting');
+    let _ = writeln!(f, "hello");
+    assert(f.buffer == "\nhello\n", 'writeln bad formatting');
+}
+
 #[derive(Debug, Drop)]
 struct StructExample {
     felt_value: felt252,

diff --git a/crates/cairo-lang-semantic/src/expr/test_data/inline_macros b/crates/cairo-lang-semantic/src/expr/test_data/inline_macros
@@ -575,11 +575,6 @@ error[E2200]: Plugin diagnostic: Macro expected formatter argument.
     writeln!();
             ^
 
-error[E2200]: Plugin diagnostic: Macro expected format string argument.
- --> lib.cairo:12:13
-    writeln!(f);
-            ^
-
 error[E2200]: Plugin diagnostic: Formatter argument must not be a string literal.
  --> lib.cairo:15:14
     writeln!("{}", ba);
@@ -943,11 +938,6 @@ error[E2200]: Plugin diagnostic: Macro `writeln` does not support this bracket t
     println!["{}", ba];
             ^
 
-error[E2200]: Plugin diagnostic: Macro expected format string argument.
- --> lib.cairo:8:13
-    println!();
-            ^
-
 error[E2200]: Plugin diagnostic: Format string argument must be a string literal.
  --> lib.cairo:11:14
     println!(ba);

diff --git a/crates/cairo-lang-semantic/src/inline_macros/write.rs b/crates/cairo-lang-semantic/src/inline_macros/write.rs
@@ -113,7 +113,7 @@ fn generate_code_inner<'db>(
     db: &'db dyn Database,
     with_newline: bool,
 ) -> InlinePluginResult<'db> {
-    let info = match FormattingInfo::extract(db, syntax) {
+    let info = match FormattingInfo::extract(db, syntax, with_newline) {
         Ok(info) => info,
         Err(diagnostics) => return InlinePluginResult { code: None, diagnostics },
     };
@@ -163,6 +163,7 @@ impl<'db> FormattingInfo<'db> {
     fn extract(
         db: &'db dyn Database,
         syntax: &ast::ExprInlineMacro<'db>,
+        with_newline: bool,
     ) -> Result<FormattingInfo<'db>, Vec<PluginDiagnostic<'db>>> {
         let Some(legacy_inline_macro) = syntax.as_legacy_inline_macro(db) else {
             return Err(vec![not_legacy_macro_diagnostic(syntax.as_syntax_node().stable_ptr(db))]);
@@ -206,10 +207,23 @@ impl<'db> FormattingInfo<'db> {
             )]);
         }
         let Some(format_string_arg) = args_iter.next() else {
-            return Err(vec![error_with_inner_span(
-                arguments.lparen(db).as_syntax_node(),
-                "Macro expected format string argument.",
-            )]);
+            // `writeln!(f)` (no format string) writes just a newline; `write!(f)` still errors.
+            // There is no format string, so the arg node is unused here — reuse the formatter's.
+            return if with_newline {
+                Ok(FormattingInfo {
+                    formatter_arg_node: RewriteNode::from_ast_trimmed(&formatter_arg),
+                    format_string_arg: formatter_arg.clone(),
+                    format_string: String::new(),
+                    format_string_source: "",
+                    args: vec![],
+                    macro_ast: syntax.clone(),
+                })
+            } else {
+                Err(vec![error_with_inner_span(
+                    arguments.lparen(db).as_syntax_node(),
+                    "Macro expected format string argument.",
+                )])
+            };
         };
         let Some(format_string_expr) = try_extract_unnamed_arg(db, &format_string_arg) else {
             return Err(vec![error_with_inner_span(