Rust: Fix dead end in CFG for match expressions with no arms

Author: paldepind

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

@@ -494,9 +494,13 @@ class MatchExprTree extends PostOrderTree instanceof MatchExpr {
   override predicate first(AstNode node) { first(super.getExpr(), node) }
 
   override predicate succ(AstNode pred, AstNode succ, Completion c) {
-    // Edge from the scrutinee to the first arm.
+    // Edge from the scrutinee to the first arm or to the match expression if no arms.
     last(super.getExpr(), pred, c) and
-    first(super.getArm(0).getPat(), succ) and
+    (
+      first(super.getArm(0).getPat(), succ)
+      or
+      not exists(super.getArm(0)) and succ = this
+    ) and
     completionIsNormal(c)
     or
     // Edge from a failed pattern or guard in one arm to the beginning of the next arm.

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

@@ -681,6 +681,8 @@ edges
 | test.rs:311:26:311:30 | value | test.rs:310:9:313:9 | MatchExpr |  |
 | test.rs:312:13:312:22 | TupleStructPat | test.rs:312:17:312:21 | never | match |
 | test.rs:312:17:312:21 | never | test.rs:312:33:312:37 | never | match |
+| test.rs:312:27:312:40 | MatchExpr | test.rs:310:9:313:9 | MatchExpr |  |
+| test.rs:312:33:312:37 | never | test.rs:312:27:312:40 | MatchExpr |  |
 | test.rs:319:5:322:5 | enter test_let_match | test.rs:319:23:319:23 | a |  |
 | test.rs:319:5:322:5 | exit test_let_match (normal) | test.rs:319:5:322:5 | exit test_let_match |  |
 | test.rs:319:23:319:23 | a | test.rs:319:23:319:36 | Param | match |