checker: add infix checks for nil (#22045)

vlib/v/checker/infix.v

@@ -197,6 +197,37 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 					}
 				}
 			}
+
+			// Do not allow comparing nil to non-pointers
+			if node.left.is_nil() {
+				mut final_type := right_type
+				if mut right_sym.info is ast.Alias
+					&& right_sym.info.parent_type.is_any_kind_of_pointer() {
+					final_type = right_sym.info.parent_type
+				}
+				if !final_type.is_any_kind_of_pointer() && (right_final_sym.kind != .function
+					|| (right_final_sym.language != .c && right_final_sym.kind == .placeholder))
+					&& !right_final_sym.is_heap() {
+					rt := c.table.sym(right_type).name
+					c.error('cannot compare with `nil` because `${rt}` is not a pointer',
+						node.pos)
+				}
+			}
+
+			if node.right.is_nil() {
+				mut final_type := left_type
+				if mut left_sym.info is ast.Alias
+					&& left_sym.info.parent_type.is_any_kind_of_pointer() {
+					final_type = left_sym.info.parent_type
+				}
+				if !final_type.is_any_kind_of_pointer() && (left_final_sym.kind != .function
+					|| (left_final_sym.language != .c && left_final_sym.kind == .placeholder))
+					&& !left_final_sym.is_heap() {
+					lt := c.table.sym(left_type).name
+					c.error('cannot compare with `nil` because `${lt}` is not a pointer',
+						node.pos)
+				}
+			}
 		}
 		.key_in, .not_in {
 			match right_final_sym.kind {
@@ -542,6 +573,9 @@ fn (mut c Checker) infix_expr(mut node ast.InfixExpr) ast.Type {
 				c.error('unwrapped Option cannot be compared in an infix expression',
 					opt_comp_pos)
 			}
+			if node.left.is_nil() || node.right.is_nil() {
+				c.error('cannot use `${node.op.str()}` with `nil`', node.pos)
+			}
 		}
 		.key_like {
 			node.promoted_type = ast.bool_type

vlib/v/checker/tests/nil_compare_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/nil_compare_err.vv:3:6: error: cannot compare with `nil` because `[]int` is not a pointer
+    1 | a := [12, 34, 56]
+    2 | 
+    3 | if a != unsafe { nil } {
+      |      ~~
+    4 |     println(a)
+    5 | }
+vlib/v/checker/tests/nil_compare_err.vv:7:7: error: cannot use `>` with `nil`
+    5 | }
+    6 | 
+    7 | if 23 > unsafe { nil } {
+      |       ^
+    8 |     println('hey')
+    9 | }

vlib/v/checker/tests/nil_compare_err.vv

@@ -0,0 +1,9 @@
+a := [12, 34, 56]
+
+if a != unsafe { nil } {
+	println(a)
+}
+
+if 23 > unsafe { nil } {
+	println('hey')
+}