Rust: Handle question mark operator in CFG

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

@@ -36,7 +36,13 @@ class SimpleCompletion extends NormalCompletion, TSimpleCompletion {
 
   // `SimpleCompletion` is the "default" completion type, thus it is valid for
   // any node where there isn't another more specific completion type.
-  override predicate isValidFor(AstNode e) { not any(Completion c).isValidForSpecific(e) }
+  override predicate isValidFor(AstNode e) {
+    not any(Completion c).isValidForSpecific(e)
+    or
+    // A `?` expression can both proceed normally or cause an early return, so
+    // we explicitly allow the former here.
+    e instanceof TryExpr
+  }
 
   override string toString() { result = "simple" }
 }
@@ -204,7 +210,9 @@ class ContinueCompletion extends TContinueCompletion, Completion {
 class ReturnCompletion extends TReturnCompletion, Completion {
   override ReturnSuccessor getAMatchingSuccessorType() { any() }
 
-  override predicate isValidForSpecific(AstNode e) { e instanceof ReturnExpr }
+  override predicate isValidForSpecific(AstNode e) {
+    e instanceof ReturnExpr or e instanceof TryExpr
+  }
 
   override string toString() { result = "return" }
 }

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

@@ -586,6 +586,10 @@ class ReturnExprTree extends PostOrderTree instanceof ReturnExpr {
   }
 }
 
+class TryExprTree extends StandardPostOrderTree instanceof TryExpr {
+  override AstNode getChildNode(int i) { i = 0 and result = super.getExpr() }
+}
+
 class TupleExprTree extends StandardPostOrderTree instanceof TupleExpr {
   override AstNode getChildNode(int i) { result = super.getField(i) }
 }

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

@@ -555,9 +555,34 @@ edges
 | test.rs:245:16:247:9 | BlockExpr | test.rs:243:9:247:9 | IfExpr |  |
 | test.rs:246:13:246:17 | false | test.rs:245:16:247:9 | BlockExpr |  |
 | test.rs:253:5:255:5 | enter test_question_mark_operator_1 | test.rs:253:38:253:38 | s |  |
+| test.rs:253:5:255:5 | exit test_question_mark_operator_1 (normal) | test.rs:253:5:255:5 | exit test_question_mark_operator_1 |  |
 | test.rs:253:38:253:38 | s | test.rs:253:38:253:44 | Param | match |
+| test.rs:253:38:253:44 | Param | test.rs:254:9:254:11 | PathExpr |  |
+| test.rs:253:62:255:5 | BlockExpr | test.rs:253:5:255:5 | exit test_question_mark_operator_1 (normal) |  |
+| test.rs:254:9:254:11 | PathExpr | test.rs:254:9:254:26 | MethodCallExpr |  |
+| test.rs:254:9:254:26 | MethodCallExpr | test.rs:254:9:254:27 | TryExpr |  |
+| test.rs:254:9:254:27 | TryExpr | test.rs:253:5:255:5 | exit test_question_mark_operator_1 (normal) | return |
+| test.rs:254:9:254:27 | TryExpr | test.rs:254:31:254:31 | 4 |  |
+| test.rs:254:9:254:31 | ... + ... | test.rs:253:62:255:5 | BlockExpr |  |
+| test.rs:254:31:254:31 | 4 | test.rs:254:9:254:31 | ... + ... |  |
 | test.rs:257:5:262:5 | enter test_question_mark_operator_2 | test.rs:257:38:257:38 | b |  |
+| test.rs:257:5:262:5 | exit test_question_mark_operator_2 (normal) | test.rs:257:5:262:5 | exit test_question_mark_operator_2 |  |
 | test.rs:257:38:257:38 | b | test.rs:257:38:257:52 | Param | match |
+| test.rs:257:38:257:52 | Param | test.rs:258:15:258:15 | b |  |
+| test.rs:257:71:262:5 | BlockExpr | test.rs:257:5:262:5 | exit test_question_mark_operator_2 (normal) |  |
+| test.rs:258:9:261:9 | MatchExpr | test.rs:257:71:262:5 | BlockExpr |  |
+| test.rs:258:15:258:15 | b | test.rs:258:15:258:16 | TryExpr |  |
+| test.rs:258:15:258:16 | TryExpr | test.rs:257:5:262:5 | exit test_question_mark_operator_2 (normal) | return |
+| test.rs:258:15:258:16 | TryExpr | test.rs:259:13:259:16 | LiteralPat |  |
+| test.rs:259:13:259:16 | LiteralPat | test.rs:259:21:259:24 | PathExpr | match |
+| test.rs:259:13:259:16 | LiteralPat | test.rs:260:13:260:17 | LiteralPat | no-match |
+| test.rs:259:21:259:24 | PathExpr | test.rs:259:26:259:30 | false |  |
+| test.rs:259:21:259:31 | CallExpr | test.rs:258:9:261:9 | MatchExpr |  |
+| test.rs:259:26:259:30 | false | test.rs:259:21:259:31 | CallExpr |  |
+| test.rs:260:13:260:17 | LiteralPat | test.rs:260:22:260:25 | PathExpr | match |
+| test.rs:260:22:260:25 | PathExpr | test.rs:260:27:260:30 | true |  |
+| test.rs:260:22:260:31 | CallExpr | test.rs:258:9:261:9 | MatchExpr |  |
+| test.rs:260:27:260:30 | true | test.rs:260:22:260:31 | CallExpr |  |
 | test.rs:267:5:273:5 | enter test_match | test.rs:267:19:267:29 | maybe_digit |  |
 | test.rs:267:5:273:5 | exit test_match (normal) | test.rs:267:5:273:5 | exit test_match |  |
 | test.rs:267:19:267:29 | maybe_digit | test.rs:267:19:267:42 | Param | match |