fix subtle bug in is_preferable

and add a test

Author: nikomatsakis

crates/formality-core/src/parse/parser.rs

@@ -266,7 +266,8 @@ where
             l1.len() > l2.len() && (0..l2.len()).all(|i| l1[i] == l2[i])
         }
 
-        prec_i > prec_j || has_prefix(&parse_i.reductions, &parse_j.reductions)
+        prec_i > prec_j
+            || (prec_i == prec_j && has_prefix(&parse_i.reductions, &parse_j.reductions))
     }
 }
 

tests/parser-torture-tests/precedence.rs

@@ -24,3 +24,71 @@ fn precedence() {
     "#]]
     .assert_debug_eq(&term);
 }
+
+#[test]
+#[should_panic(expected = "extra tokens")]
+fn higher_precedence_less_reductions_1() {
+    // Subtle: In this case, A has higher precedence,
+    // so even though we COULD parse the entire string,
+    // we prefer to just parse the first identifier,
+    // which results in a panic.
+    //
+    // In the past, we had a bug in our preference function
+    // that caused it to be order dependent, sometimes
+    // panicking and sometimes not. Hence we have a similar
+    // test with opposite order.
+
+    #[term]
+    pub enum Root {
+        #[cast]
+        #[precedence(1)]
+        A(A),
+
+        #[cast]
+        B(B),
+    }
+
+    #[term($v0)]
+    pub struct A(Id);
+
+    #[term($v0 $v1)]
+    pub struct B(A, Id);
+
+    formality_core::id!(Id);
+
+    let term: Root = crate::ptt::term("my String");
+    expect_test::expect![[r#"
+    "#]]
+    .assert_debug_eq(&term);
+}
+
+#[test]
+#[should_panic(expected = "extra tokens")]
+fn higher_precedence_less_reductions_2() {
+    // Same as `higher_precedence_less_reductions_1` but with
+    // opposite term order. See explanation in that function.
+
+    #[term]
+    pub enum Root {
+        #[cast]
+        B(B),
+
+        #[cast]
+        #[precedence(1)]
+        A(A),
+    }
+
+    #[term($v0)]
+    pub struct A(Id);
+
+    #[term($v0 $v1)]
+    pub struct B(A, Id);
+
+    formality_core::id!(Id);
+
+    let term: Root = crate::ptt::term("my String");
+    expect_test::expect![[r#"
+        my String
+    "#]]
+    .assert_debug_eq(&term);
+}