checker: disallow deferencing a nil pointer (#18038)

Author: Delta456

vlib/v/checker/checker.v

@@ -3876,6 +3876,15 @@ fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) ast.Type {
 		if right_type.is_voidptr() {
 			c.error('cannot dereference to void', node.pos)
 		}
+		if mut node.right is ast.Ident {
+			if var := node.right.scope.find_var('${node.right.name}') {
+				if var.expr is ast.UnsafeExpr {
+					if var.expr.expr is ast.Nil {
+						c.error('cannot deference a `nil` pointer', node.right.pos)
+					}
+				}
+			}
+		}
 	}
 	if node.op == .bit_not && !c.unwrap_generic(right_type).is_int() && !c.pref.translated
 		&& !c.file.is_translated {

vlib/v/checker/tests/deference_nil_ptr_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/deference_nil_ptr_err.vv:3:10: error: cannot deference a `nil` pointer
+    1 | foo_ptr := unsafe { nil }
+    2 |
+    3 | println(*foo_ptr)
+      |          ~~~~~~~

vlib/v/checker/tests/deference_nil_ptr_err.vv

@@ -0,0 +1,3 @@
+foo_ptr := unsafe { nil }
+
+println(*foo_ptr)