Detect when a slice on the stack is accidentally returned from a function.

releasenotes.md

@@ -108,6 +108,7 @@
 - Add deprecation for `@param foo "abc"`.
 - Add `--header-output` and `header-output` options for controlling header output folder.
 - Generic faults is disallowed.
+- Detect when a slice on the stack is accidentally returned from a function.
 
 ### Fixes
 - Assert triggered when casting from `int[2]` to `uint[2]` #2115

src/compiler/sema_stmts.c

@@ -600,6 +600,13 @@ INLINE bool sema_check_not_stack_variable_escape(SemaContext *context, Expr *exp
 	Expr *outer = expr;
 	expr = sema_dive_into_expression(expr);
 	bool allow_pointer = false;
+	if (expr_is_const_slice(expr) && expr->const_expr.slice_init)
+	{
+		RETURN_SEMA_ERROR(outer, "A slice literal is backed by a stack allocated array which will be invalid once the function returns. "
+						  "However, you can place the literal in a global or 'static' variable and safely return that value as long "
+		                  "as the caller of the function won't modify the slice.");
+
+	}
 	// We only want && and &
 	if (expr->expr_kind == EXPR_SUBSCRIPT_ADDR)
 	{

test/test_suite/arrays/slice_literal_fn_return.c3

@@ -0,0 +1,9 @@
+import std;
+fn int[] hello()
+{
+	return { 1, 2 }; // #error: will be invalid
+}
+fn void main()
+{
+	hello();
+}