fix(generate): honor right associativity despite a lower-precedence shift

A SHIFT/REDUCE conflict can bundle several shift interpretations with
different precedences against a single reduce. `handle_conflict` weighed them
with only `shift_is_less` and `shift_is_more`, so a lone lower-precedence
shift set `shift_is_less` and the REDUCE won outright, even when another
interpretation tied the REDUCE in precedence and the REDUCE was declared
right-associative. That tie's associativity should have won by shifting, so a
right-associative rule silently became left-associative as soon as an
unrelated lower-precedence rule was added to the grammar.

Track the equal-precedence case explicitly, and in the reduce-wins branch
shift instead when a tying interpretation exists and the reduce actions are
purely right-associative.

Co-authored-by: Will Lillis <will.lillis24@gmail.com>

crates/generate/src/build_tables/build_parse_table.rs

@@ -770,6 +770,7 @@ impl<'a> ParseTableBuilder<'a> {
 
             // If the SHIFT action has higher precedence, remove all the REDUCE actions.
             let mut shift_is_less = false;
+            let mut shift_is_equal = false;
             let mut shift_is_more = false;
             for p in shift_precedence {
                 match Self::compare_precedence(
@@ -781,7 +782,7 @@ impl<'a> ParseTableBuilder<'a> {
                 ) {
                     Ordering::Greater => shift_is_more = true,
                     Ordering::Less => shift_is_less = true,
-                    Ordering::Equal => {}
+                    Ordering::Equal => shift_is_equal = true,
                 }
             }
 
@@ -790,8 +791,28 @@ impl<'a> ParseTableBuilder<'a> {
             }
             // If the REDUCE actions have higher precedence, remove the SHIFT action.
             else if shift_is_less && !shift_is_more {
-                entry.actions.pop();
-                conflicting_items.retain(|item| item.is_done());
+                // Exception: if one SHIFT interpretation ties the REDUCE actions in
+                // precedence while another has lower precedence, and the REDUCE
+                // actions are purely right associative, honor that right
+                // associativity by shifting rather than reducing. The
+                // lower-precedence interpretation coexists with the tying one, so on
+                // its own it must not force a REDUCE that would flip the tie to left
+                // associative.
+                if shift_is_equal
+                    && matches!(
+                        (
+                            reduction_info.has_left_assoc,
+                            reduction_info.has_non_assoc,
+                            reduction_info.has_right_assoc,
+                        ),
+                        (false, false, true)
+                    )
+                {
+                    entry.actions.drain(0..entry.actions.len() - 1);
+                } else {
+                    entry.actions.pop();
+                    conflicting_items.retain(|item| item.is_done());
+                }
             }
             // If the SHIFT and REDUCE actions have the same precedence, consider
             // the REDUCE actions' associativity.

test/fixtures/test_grammars/associativity_left_with_lower_precedence_shift/corpus.txt

@@ -0,0 +1,37 @@
+================================================================================
+Addition is left associative
+================================================================================
+
+1 + 1 + 1 + 1
+
+--------------------------------------------------------------------------------
+
+(expression
+  (addition
+    (expression
+      (addition
+        (expression
+          (addition
+            (expression
+              (number))
+            (expression
+              (number))))
+        (expression
+          (number))))
+    (expression
+      (number))))
+
+================================================================================
+Superaddition still parses
+================================================================================
+
+1 + + 1
+
+--------------------------------------------------------------------------------
+
+(expression
+  (superaddition
+    (expression
+      (number))
+    (expression
+      (number))))

test/fixtures/test_grammars/associativity_left_with_lower_precedence_shift/grammar.js

@@ -0,0 +1,16 @@
+export default grammar({
+    name: 'associativity_left_with_lower_precedence_shift',
+
+    rules: {
+        expression: $ => choice(
+            $.addition,
+            $.superaddition,
+            $.number,
+        ),
+
+        addition: $ => prec.left(1, seq($.expression, '+', $.expression)),
+        superaddition: $ => prec.right(seq($.expression, '+', '+', $.expression)),
+
+        number: _ => '1',
+    }
+});

test/fixtures/test_grammars/associativity_right_with_lower_precedence_shift/corpus.txt

@@ -0,0 +1,22 @@
+================================================================================
+Addition is right associative
+================================================================================
+
+1 + 1 + 1 + 1
+
+--------------------------------------------------------------------------------
+
+(expression
+  (addition
+    (expression
+      (number))
+    (expression
+      (addition
+        (expression
+          (number))
+        (expression
+          (addition
+            (expression
+              (number))
+            (expression
+              (number))))))))

test/fixtures/test_grammars/associativity_right_with_lower_precedence_shift/grammar.js

@@ -0,0 +1,15 @@
+module.exports = grammar({
+    name: 'associativity_right_with_lower_precedence_shift',
+  
+    rules: {
+      expression: $ => choice(
+        $.addition,
+        $.superaddition,
+        $.number
+      ),
+      addition: $ => prec.right(1, seq($.expression, "+", $.expression)),
+      superaddition: $ => prec.right(seq($.expression, "+", "+", $.expression)),
+      number: _ => "1"
+    }
+  });
+