From c5221e509495397b9c0d02939b37f98db0b5c96b Mon Sep 17 00:00:00 2001
From: matt rice <ratmice@gmail.com>
Date: Wed, 4 Mar 2026 02:18:12 -0800
Subject: [PATCH] Add ability to get a `Span` from a `PIdx`, improve errors

---
 cfgrammar/src/lib/yacc/grammar.rs | 14 ++++++++++++++
 lrpar/src/lib/ctbuilder.rs        | 14 ++++++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/cfgrammar/src/lib/yacc/grammar.rs b/cfgrammar/src/lib/yacc/grammar.rs
index ce323295e..242b59097 100644
--- a/cfgrammar/src/lib/yacc/grammar.rs
+++ b/cfgrammar/src/lib/yacc/grammar.rs
@@ -76,6 +76,10 @@ pub struct YaccGrammar<StorageT = u32> {
     prods_rules: Box<[RIdx<StorageT>]>,
     /// The precedence of each production.
     prod_precs: Box<[Option<Precedence>]>,
+    /// The span for each production.
+    ///
+    /// In the case of an empty span this may be a zero length span.
+    prod_spans: Box<[Span]>,
     /// The index of the rule added for implicit tokens, if they were specified; otherwise
     /// `None`.
     implicit_rule: Option<RIdx<StorageT>>,
@@ -131,6 +135,7 @@ where
             rules_prods: Decode::decode(decoder)?,
             prods_rules: Decode::decode(decoder)?,
             prod_precs: Decode::decode(decoder)?,
+            prod_spans: Decode::decode(decoder)?,
             implicit_rule: Decode::decode(decoder)?,
             actions: Decode::decode(decoder)?,
             action_spans: Decode::decode(decoder)?,
@@ -171,6 +176,7 @@ where
             rules_prods: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
             prods_rules: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
             prod_precs: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
+            prod_spans: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
             implicit_rule: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
             actions: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
             action_spans: ::bincode::BorrowDecode::<'_, __Context>::borrow_decode(decoder)?,
@@ -463,6 +469,7 @@ where
                 .map(|x| x.unwrap().into_boxed_slice())
                 .collect(),
             prod_precs: prod_precs.into_iter().map(Option::unwrap).collect(),
+            prod_spans: ast.prods.iter().map(|prod| prod.prod_span).collect(),
             implicit_rule: implicit_rule.map(|x| rule_map[&x]),
             actions: actions.into_boxed_slice(),
             action_spans: action_spans.into_boxed_slice(),
@@ -513,6 +520,13 @@ where
         self.prod_precs[usize::from(pidx)]
     }
 
+    /// Return the span for a production `pidx`
+    ///
+    /// May return a zero length span such as when there is an empty production.
+    pub fn prod_span(&self, pidx: PIdx<StorageT>) -> Span {
+        self.prod_spans[usize::from(pidx)]
+    }
+
     /// Return the production index of the start rule's sole production (for Yacc grammars the
     /// start rule is defined to have precisely one production).
     pub fn start_prod(&self) -> PIdx<StorageT> {
diff --git a/lrpar/src/lib/ctbuilder.rs b/lrpar/src/lib/ctbuilder.rs
index d078e2e48..a54cd1909 100644
--- a/lrpar/src/lib/ctbuilder.rs
+++ b/lrpar/src/lib/ctbuilder.rs
@@ -1511,10 +1511,16 @@ where
             // Iterate over all $-arguments and replace them with their respective
             // element from the argument vector (e.g. $1 is replaced by args[0]).
             let pre_action = grm.action(pidx).as_ref().ok_or_else(|| {
-                format!(
-                    "Rule {} has a production which is missing action code",
-                    grm.rule_name_str(grm.prod_to_rule(pidx))
-                )
+                let mut s = String::from("\n");
+                let span = grm.prod_span(pidx);
+                s.push_str(&diag.file_location_msg("Error", Some(span)));
+                s.push_str("\n");
+                s.push_str(&diag.underline_span_with_text(
+                    span,
+                    "Production is missing action code".to_string(),
+                    '^',
+                ));
+                ErrorString(s)
             })?;
             let mut last = 0;
             let mut outs = String::new();