From e8120a1f979b89564a10c47115ec7dd9f035ddc1 Mon Sep 17 00:00:00 2001
From: Ary Borenszweig <asterite@gmail.com>
Date: Tue, 1 Apr 2025 14:54:48 -0300
Subject: [PATCH] feat(parser): improve error recovery when `fn` is missing
 between modifiers and name

---
 .../noirc_frontend/src/parser/parser/item.rs  | 56 ++++++++++++++++++-
 .../src/parser/parser/modifiers.rs            |  9 +++
 2 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/compiler/noirc_frontend/src/parser/parser/item.rs b/compiler/noirc_frontend/src/parser/parser/item.rs
index 47bee93f6ed..4a53664b182 100644
--- a/compiler/noirc_frontend/src/parser/parser/item.rs
+++ b/compiler/noirc_frontend/src/parser/parser/item.rs
@@ -219,7 +219,21 @@ impl<'a> Parser<'a> {
             )];
         }
 
-        if self.eat_keyword(Keyword::Fn) {
+        let is_function = if self.eat_keyword(Keyword::Fn) {
+            true
+        } else if !modifiers.is_empty()
+            && matches!(self.token.token(), Token::Ident(..))
+            && self.next_is(Token::LeftParen)
+        {
+            // If it's something like `pub foo(` then it's likely the user forgot to put `fn` after `pub`,
+            // so we error but keep parsing what comes next as a function.
+            self.expected_token(Token::Keyword(Keyword::Fn));
+            true
+        } else {
+            false
+        };
+
+        if is_function {
             self.mutable_not_applicable(modifiers);
 
             return vec![ItemKind::Function(self.parse_function(
@@ -250,7 +264,7 @@ mod tests {
     use crate::{
         parse_program_with_dummy_file,
         parser::{
-            ItemKind,
+            ItemKind, Parser,
             parser::tests::{get_single_error, get_source_with_error_span},
         },
     };
@@ -323,4 +337,42 @@ mod tests {
         assert_eq!(attributes[0].to_string(), "#[one]");
         assert_eq!(attributes[1].to_string(), "#[two]");
     }
+
+    #[test]
+    fn error_recovery_for_missing_fn_between_visibility_and_name() {
+        let src = "
+        pub foo() { }
+            ^^^
+        ";
+        let (src, span) = get_source_with_error_span(src);
+        let mut parser = Parser::for_str_with_dummy_file(&src);
+        let module = parser.parse_program();
+        assert_eq!(module.items.len(), 1);
+        let ItemKind::Function(noir_function) = &module.items[0].kind else {
+            panic!("Expected function");
+        };
+        assert_eq!(noir_function.name(), "foo");
+
+        let reason = get_single_error(&parser.errors, span);
+        assert_eq!(reason.to_string(), "Expected a 'fn' but found 'foo'");
+    }
+
+    #[test]
+    fn error_recovery_for_missing_fn_between_unconstrained_and_name() {
+        let src = "
+        unconstrained foo() { }
+                      ^^^
+        ";
+        let (src, span) = get_source_with_error_span(src);
+        let mut parser = Parser::for_str_with_dummy_file(&src);
+        let module = parser.parse_program();
+        assert_eq!(module.items.len(), 1);
+        let ItemKind::Function(noir_function) = &module.items[0].kind else {
+            panic!("Expected function");
+        };
+        assert_eq!(noir_function.name(), "foo");
+
+        let reason = get_single_error(&parser.errors, span);
+        assert_eq!(reason.to_string(), "Expected a 'fn' but found 'foo'");
+    }
 }
diff --git a/compiler/noirc_frontend/src/parser/parser/modifiers.rs b/compiler/noirc_frontend/src/parser/parser/modifiers.rs
index 896a27c0416..cb78a79ed71 100644
--- a/compiler/noirc_frontend/src/parser/parser/modifiers.rs
+++ b/compiler/noirc_frontend/src/parser/parser/modifiers.rs
@@ -13,6 +13,15 @@ pub(crate) struct Modifiers {
     pub(crate) mutable: Option<Location>,
 }
 
+impl Modifiers {
+    pub(crate) fn is_empty(&self) -> bool {
+        self.visibility == ItemVisibility::Private
+            && self.unconstrained.is_none()
+            && self.comptime.is_none()
+            && self.mutable.is_none()
+    }
+}
+
 impl Parser<'_> {
     /// Modifiers = ItemVisibility 'unconstrained'? 'comptime'? 'mut'?
     ///