Rust: Step correctly over method arguments

Previously the math was wrong and the first argument (with `i` being `0`) had two edges out of it.

Author: paldepind

rust/ql/lib/codeql/rust/controlflow/internal/ControlFlowGraphImpl.qll

@@ -524,10 +524,7 @@ class MatchExprTree extends PostOrderTree instanceof MatchExpr {
 
 class MethodCallExprTree extends StandardPostOrderTree, MethodCallExpr {
   override AstNode getChildNode(int i) {
-    i = 0 and
-    result = this.getReceiver()
-    or
-    result = this.getArgList().getArg(i + 1)
+    if i = 0 then result = this.getReceiver() else result = this.getArgList().getArg(i - 1)
   }
 }
 

rust/ql/test/library-tests/controlflow/Cfg.expected

@@ -19,10 +19,9 @@ edges
 | test.rs:10:13:10:19 | map | test.rs:11:9:11:28 | ExprStmt | match |
 | test.rs:10:23:10:34 | PathExpr | test.rs:10:23:10:36 | CallExpr |  |
 | test.rs:10:23:10:36 | CallExpr | test.rs:10:13:10:19 | map |  |
-| test.rs:11:9:11:11 | map | test.rs:11:9:11:27 | MethodCallExpr |  |
+| test.rs:11:9:11:11 | map | test.rs:11:20:11:21 | 37 |  |
 | test.rs:11:9:11:27 | MethodCallExpr | test.rs:9:22:12:5 | BlockExpr |  |
-| test.rs:11:9:11:28 | ExprStmt | test.rs:11:20:11:21 | 37 |  |
-| test.rs:11:20:11:21 | 37 | test.rs:11:9:11:11 | map |  |
+| test.rs:11:9:11:28 | ExprStmt | test.rs:11:9:11:11 | map |  |
 | test.rs:11:20:11:21 | 37 | test.rs:11:24:11:26 | "a" |  |
 | test.rs:11:24:11:26 | "a" | test.rs:11:9:11:27 | MethodCallExpr |  |
 | test.rs:17:5:33:5 | enter test_break_and_continue | test.rs:17:32:17:32 | n |  |

rust/ql/test/library-tests/variables/Cfg.expected

@@ -707,10 +707,10 @@ edges
 | variables.rs:332:5:332:17 | ExprStmt | variables.rs:332:5:332:13 | PathExpr |  |
 | variables.rs:332:15:332:15 | a | variables.rs:332:5:332:16 | CallExpr |  |
 | variables.rs:333:5:333:27 | MethodCallExpr | variables.rs:334:5:334:17 | ExprStmt |  |
-| variables.rs:333:5:333:28 | ExprStmt | variables.rs:333:25:333:26 | 10 |  |
-| variables.rs:333:6:333:11 | RefExpr | variables.rs:333:5:333:27 | MethodCallExpr |  |
+| variables.rs:333:5:333:28 | ExprStmt | variables.rs:333:11:333:11 | a |  |
+| variables.rs:333:6:333:11 | RefExpr | variables.rs:333:25:333:26 | 10 |  |
 | variables.rs:333:11:333:11 | a | variables.rs:333:6:333:11 | RefExpr |  |
-| variables.rs:333:25:333:26 | 10 | variables.rs:333:11:333:11 | a |  |
+| variables.rs:333:25:333:26 | 10 | variables.rs:333:5:333:27 | MethodCallExpr |  |
 | variables.rs:334:5:334:13 | PathExpr | variables.rs:334:15:334:15 | a |  |
 | variables.rs:334:5:334:16 | CallExpr | variables.rs:329:17:335:1 | BlockExpr |  |
 | variables.rs:334:5:334:17 | ExprStmt | variables.rs:334:5:334:13 | PathExpr |  |