Check for left recursion inside precedence!()

Fixes #238
kevinmehall · Feb 21, 2021 · a8510d9 · a8510d9
1 parent a8d0473
commit a8510d9
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diff --git a/peg-macros/analysis.rs b/peg-macros/analysis.rs
@@ -139,7 +139,25 @@ impl<'a> RecursionVisitor<'a> {
 
             MatchStrExpr(ref expr) | QuietExpr(ref expr) => self.walk_expr(expr),
 
-            PrecedenceExpr { .. } => RuleInfo { nullable: false },
+            PrecedenceExpr { ref levels } => {
+                let mut nullable = false;
+
+                for level in levels {
+                    for operator in &level.operators {
+                        let mut operator_nullable = true;
+                        for element in &operator.elements {
+                            let res = self.walk_expr(&element.expr);
+                            if !res.nullable {
+                                operator_nullable = false;
+                                break;
+                            }
+                        }
+                        nullable |= operator_nullable;
+                    }
+                }
+
+                RuleInfo { nullable }
+            },
 
             LiteralExpr(_) | PatternExpr(_) | MethodExpr(_, _) | FailExpr(_) | MarkerExpr(_) => {
                 RuleInfo { nullable: false }

diff --git a/tests/compile-fail/left_recursion.rs b/tests/compile-fail/left_recursion.rs
@@ -10,6 +10,10 @@ peg::parser!(grammar foo() for str {
     rule bar()
         = "bar" bar()
         / foo() //~ ERROR left recursive rules create an infinite loop: foo -> bar -> foo
+
+    rule prec() = precedence! {
+        prec() { () } //~ ERROR
+    }
 });
 
 fn main() {}
diff --git a/tests/compile-fail/left_recursion.stderr b/tests/compile-fail/left_recursion.stderr
@@ -15,3 +15,9 @@ error: left recursive rules create an infinite loop: bar -> foo -> bar
   |
 8 |         / bar() //~ ERROR left recursive rules create an infinite loop: bar -> foo -> bar
   |           ^^^
+
+error: left recursive rules create an infinite loop: prec -> prec
+  --> $DIR/left_recursion.rs:15:9
+   |
+15 |         prec() { () } //~ ERROR
+   |         ^^^^