From c7eb0024c72197fcbaa47a6500d12c03f4fec106 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Tue, 21 Jan 2025 00:38:24 +0100
Subject: [PATCH] Error on switch case fallthough if there is more than one
 newline #1849.

---
 releasenotes.md                                |  1 +
 src/compiler/parse_stmt.c                      | 18 ++++++++++++++----
 .../statements/switch_error_fallthrough.c3     | 11 +++++++++++
 3 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 test/test_suite/statements/switch_error_fallthrough.c3

diff --git a/releasenotes.md b/releasenotes.md
index 7e76c5a33..d21daf396 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -5,6 +5,7 @@
 ### Changes / improvements
 - Contracts @require/@ensure are no longer treated as conditionals, but must be explicitly bool.
 - Add `win-debug` setting to be able to pick dwarf for output #1855.
+- Error on switch case fallthough if there is more than one newline #1849.
 
 ### Fixes
 - Fix issue requiring prefix on a generic interface declaration.
diff --git a/src/compiler/parse_stmt.c b/src/compiler/parse_stmt.c
index e55e7be4c..7628edfb5 100644
--- a/src/compiler/parse_stmt.c
+++ b/src/compiler/parse_stmt.c
@@ -522,9 +522,17 @@ static inline bool token_type_ends_case(TokenType type, TokenType case_type, Tok
 	return type == case_type || type == default_type || type == TOKEN_RBRACE || type == TOKEN_CT_ENDSWITCH;
 }
 
-static inline Ast *parse_case_stmts(ParseContext *c, TokenType case_type, TokenType default_type)
+static inline Ast *parse_case_stmts(ParseContext *c, TokenType case_type, TokenType default_type, uint32_t row)
 {
-	if (token_type_ends_case(c->tok, case_type, default_type)) return NULL;
+	if (token_type_ends_case(c->tok, case_type, default_type))
+	{
+		if (c->span.row > row + 1 && (c->tok == TOKEN_CASE || c->tok == TOKEN_DEFAULT))
+		{
+			PRINT_ERROR_LAST("Fallthrough cases with empty rows or comments have unclear meaning, an explicit 'break' or 'nextcase' is needed (or remove the spacing!).");
+			return poisoned_ast;
+		}
+		return NULL;
+	}
 	Ast *compound = new_ast(AST_COMPOUND_STMT, c->span);
 	AstId *next = &compound->compound_stmt.first_stmt;
 	while (!token_type_ends_case(c->tok, case_type, default_type))
@@ -678,13 +686,14 @@ static inline Ast *parse_case_stmt(ParseContext *c, TokenType case_type, TokenTy
 	{
 		ASSIGN_EXPRID_OR_RET(ast->case_stmt.to_expr, parse_expr(c), poisoned_ast);
 	}
+	uint32_t row = c->span.row;
 	if (!try_consume(c, TOKEN_COLON))
 	{
 		print_error_at(c->prev_span, "Missing ':' after case");
 		return poisoned_ast;
 	}
 	RANGE_EXTEND_PREV(ast);
-	ASSIGN_AST_OR_RET(ast->case_stmt.body, parse_case_stmts(c, case_type, default_type), poisoned_ast);
+	ASSIGN_AST_OR_RET(ast->case_stmt.body, parse_case_stmts(c, case_type, default_type, row), poisoned_ast);
 	return ast;
 }
 
@@ -697,8 +706,9 @@ static inline Ast *parse_default_stmt(ParseContext *c, TokenType case_type, Toke
 	Ast *ast = new_ast(AST_DEFAULT_STMT, c->span);
 	advance(c);
 	TRY_CONSUME_OR_RET(TOKEN_COLON, "Expected ':' after 'default'.", poisoned_ast);
+	uint32_t row = c->span.row;
 	RANGE_EXTEND_PREV(ast);
-	ASSIGN_AST_OR_RET(ast->case_stmt.body, parse_case_stmts(c, case_type, default_type), poisoned_ast);
+	ASSIGN_AST_OR_RET(ast->case_stmt.body, parse_case_stmts(c, case_type, default_type, row), poisoned_ast);
 	ast->case_stmt.expr = 0;
 	return ast;
 }
diff --git a/test/test_suite/statements/switch_error_fallthrough.c3 b/test/test_suite/statements/switch_error_fallthrough.c3
new file mode 100644
index 000000000..991d1fd1d
--- /dev/null
+++ b/test/test_suite/statements/switch_error_fallthrough.c3
@@ -0,0 +1,11 @@
+fn int main()
+{
+	int a;
+	switch (a)
+	{
+		case 1: // #error: Fallthrough cases
+			// ...
+		case 2:
+	}
+    return 0;
+}
\ No newline at end of file