checker: check that the module `init` fn, should have no params, and no return type (#8988)

vlib/v/checker/checker.v

@@ -5797,6 +5797,15 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.ensure_type_exists(arg.typ, node.pos) or { return }
 		}
 	}
+	if node.language == .v && node.name.after_char(`.`) == 'init' && !node.is_method
+		&& node.params.len == 0 {
+		if node.is_pub {
+			c.error('fn `init` must not be public', node.pos)
+		}
+		if node.return_type != table.void_type {
+			c.error('fn `init` cannot have a return type', node.pos)
+		}
+	}
 	if node.return_type != table.Type(0) {
 		c.ensure_type_exists(node.return_type, node.pos) or { return }
 		if node.language == .v && node.is_method && node.name == 'str' {

vlib/v/checker/tests/fn_init_sig.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/fn_init_sig.vv:1:1: error: fn `init` cannot have a return type
+    1 | fn init() int {
+      | ~~~~~~~~~~~~~
+    2 |     return 1
+    3 | }
+vlib/v/checker/tests/fn_init_sig.vv:4:1: error: fn `init` must not be public
+    2 |     return 1
+    3 | }
+    4 | pub fn init() int {
+      | ~~~~~~~~~~~~~~~~~
+    5 |     return 1
+    6 | }

vlib/v/checker/tests/fn_init_sig.vv

@@ -0,0 +1,6 @@
+fn init() int {
+    return 1
+}
+pub fn init() int {
+    return 1
+}