Rust: Fix dead end in CFG for empty tuple and struct patterns

Author: paldepind

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

@@ -612,6 +612,8 @@ module PatternTrees {
       result = rank[i + 1](Pat pat, int j | pat = this.getPat(j) | pat order by j)
     }
 
+    predicate isEmpty() { not any(Pat p) = this.getPat(0) }
+
     override predicate propagatesAbnormal(AstNode child) { child = this.getPatRanked(_) }
 
     override predicate succ(AstNode pred, AstNode succ, Completion c) {
@@ -629,8 +631,11 @@ module PatternTrees {
 
     override predicate last(AstNode node, Completion c) {
       node = this and
-      completionIsValidFor(c, this) and
-      c.(MatchCompletion).failed()
+      (
+        completionIsValidFor(c, this) and c.(MatchCompletion).failed()
+        or
+        this.isEmpty() and node = this and c.(MatchCompletion).succeeded()
+      )
       or
       exists(int i | last(this.getPatRanked(i), node, c) |
         c.(MatchCompletion).failed()

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

@@ -633,14 +633,23 @@ edges
 | test.rs:282:13:282:24 | PathPat | test.rs:282:29:282:29 | 5 | match |
 | test.rs:282:29:282:29 | 5 | test.rs:276:9:283:9 | MatchExpr |  |
 | test.rs:289:5:292:5 | enter empty_tuple_pattern | test.rs:289:28:289:31 | unit |  |
+| test.rs:289:5:292:5 | exit empty_tuple_pattern (normal) | test.rs:289:5:292:5 | exit empty_tuple_pattern |  |
 | test.rs:289:28:289:31 | unit | test.rs:289:28:289:35 | Param | match |
 | test.rs:289:28:289:35 | Param | test.rs:290:9:290:22 | LetStmt |  |
 | test.rs:290:9:290:22 | LetStmt | test.rs:290:18:290:21 | unit |  |
+| test.rs:290:13:290:14 | TuplePat | test.rs:291:9:291:15 | ExprStmt | match |
 | test.rs:290:18:290:21 | unit | test.rs:290:13:290:14 | TuplePat |  |
+| test.rs:291:9:291:14 | ReturnExpr | test.rs:289:5:292:5 | exit empty_tuple_pattern (normal) | return |
+| test.rs:291:9:291:15 | ExprStmt | test.rs:291:9:291:14 | ReturnExpr |  |
 | test.rs:296:5:300:5 | enter empty_struct_pattern | test.rs:296:29:296:30 | st |  |
+| test.rs:296:5:300:5 | exit empty_struct_pattern (normal) | test.rs:296:5:300:5 | exit empty_struct_pattern |  |
 | test.rs:296:29:296:30 | st | test.rs:296:29:296:40 | Param | match |
 | test.rs:296:29:296:40 | Param | test.rs:297:15:297:16 | st |  |
+| test.rs:296:50:300:5 | BlockExpr | test.rs:296:5:300:5 | exit empty_struct_pattern (normal) |  |
+| test.rs:297:9:299:9 | MatchExpr | test.rs:296:50:300:5 | BlockExpr |  |
 | test.rs:297:15:297:16 | st | test.rs:298:13:298:23 | RecordPat |  |
+| test.rs:298:13:298:23 | RecordPat | test.rs:298:28:298:28 | 1 | match |
+| test.rs:298:28:298:28 | 1 | test.rs:297:9:299:9 | MatchExpr |  |
 | test.rs:304:5:309:5 | enter test_infinite_loop | test.rs:305:9:307:9 | ExprStmt |  |
 | test.rs:305:9:307:9 | ExprStmt | test.rs:306:13:306:13 | 1 |  |
 | test.rs:305:14:307:9 | BlockExpr | test.rs:306:13:306:13 | 1 |  |