use reduction prefix to decide between variants

We used to also consider precedence, but we are moving away from that
approach. Remove some tests that no longer test a relevant edge case.

Author: nikomatsakis

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

@@ -334,9 +334,7 @@ where
             l1.len() > l2.len() && (0..l2.len()).all(|i| l1[i] == l2[i])
         }
 
-        s_i.precedence.level > s_j.precedence.level
-            || (s_i.precedence.level == s_j.precedence.level
-                && has_prefix(&s_i.reductions, &s_j.reductions))
+        has_prefix(&s_i.reductions, &s_j.reductions)
     }
 }
 

tests/parser-torture-tests/precedence.rs

@@ -59,71 +59,3 @@ fn equal_precedence_panics() {
     "#]]
     .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);
-}