fix(PR-6192): keep keyword choice parser outcome private

Author: matthewevans

crates/engine/src/parser/oracle_effect/subject.rs

@@ -3356,9 +3356,8 @@ fn build_keyword_choice_sub_ability(
     // "gains your choice of ...". `parse_keyword_choice_grant` anchors on the
     // bare "gain ..." form, so deconjugate the remainder here first.
     let normalized = deconjugate_verb(remainder);
-    let (keywords, duration) = match parse_keyword_choice_grant(&normalized) {
-        KeywordChoiceGrant::Parsed { keywords, duration } => (keywords, duration),
-        KeywordChoiceGrant::NoMatch | KeywordChoiceGrant::UnsupportedNestedChoice => return None,
+    let Ok(Some((keywords, duration))) = parse_keyword_choice_grant(&normalized) else {
+        return None;
     };
     let affected = static_affected_for_application(application);
     let choice_duration = duration.clone();
@@ -3378,16 +3377,9 @@ fn build_keyword_choice_sub_ability(
     ))
 }
 
-pub(super) enum KeywordChoiceGrant {
-    NoMatch,
-    Parsed {
-        keywords: Vec<Keyword>,
-        duration: Option<Duration>,
-    },
-    UnsupportedNestedChoice,
-}
-
-pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant {
+fn parse_keyword_choice_grant(
+    predicate: &str,
+) -> Result<Option<(Vec<Keyword>, Option<Duration>)>, ()> {
     let lower = predicate.to_lowercase();
 
     // CR 608.2d: "gain your choice of X, Y, or Z" offers a resolution-time choice.
@@ -3401,12 +3393,12 @@ pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant
     {
         let (keyword_text, duration) = super::strip_trailing_duration(choice_text);
         let Some(items) = super::split_choice_list_items(keyword_text.trim()) else {
-            return KeywordChoiceGrant::NoMatch;
+            return Ok(None);
         };
         // `separated_list1` succeeds on a single item when there is no separator
         // at all; require ≥2 so a lone keyword is not mistaken for a "choice".
         if items.len() < 2 {
-            return KeywordChoiceGrant::NoMatch;
+            return Ok(None);
         }
         // An item such as "landwalk of your choice" or "protection from a color
         // of your choice" needs an additional typed choice after the outer menu.
@@ -3417,19 +3409,19 @@ pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant
             .iter()
             .any(|item| nom_primitives::scan_contains(item, "of your choice"))
         {
-            return KeywordChoiceGrant::UnsupportedNestedChoice;
+            return Err(());
         }
         let Some(keywords) = items
             .iter()
             .map(|item| parse_granted_keyword_fragment(item.trim()))
             .collect::<Option<Vec<Keyword>>>()
         else {
-            return KeywordChoiceGrant::NoMatch;
+            return Ok(None);
         };
-        return KeywordChoiceGrant::Parsed {
+        return Ok(Some((
             keywords,
-            duration: duration.or(Some(Duration::UntilEndOfTurn)),
-        };
+            duration.or(Some(Duration::UntilEndOfTurn)),
+        )));
     }
 
     // Shape 2: "gain/have protection from X or from the color of your choice"
@@ -3445,7 +3437,7 @@ pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant
     ))
     .parse(lower.as_str())
     .ok() else {
-        return KeywordChoiceGrant::NoMatch;
+        return Ok(None);
     };
     let (quality_text, duration) = super::strip_trailing_duration(remainder);
     // GUARDRAIL: split on the literal " or from " (NOT " or "). Splitting on
@@ -3457,7 +3449,7 @@ pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant
     let Some((_, (left, right))) =
         nom_primitives::split_once_on(quality_text.trim(), " or from ").ok()
     else {
-        return KeywordChoiceGrant::NoMatch;
+        return Ok(None);
     };
     // The halves are bare qualities (the "protection from " prefix is already
     // stripped), so map each with parse_protection_target — NOT
@@ -3466,10 +3458,10 @@ pub(super) fn parse_keyword_choice_grant(predicate: &str) -> KeywordChoiceGrant
     let second = Keyword::Protection(crate::types::keywords::parse_protection_target(
         right.trim(),
     ));
-    KeywordChoiceGrant::Parsed {
-        keywords: vec![first, second],
-        duration: duration.or(Some(Duration::UntilEndOfTurn)),
-    }
+    Ok(Some((
+        vec![first, second],
+        duration.or(Some(Duration::UntilEndOfTurn)),
+    )))
 }
 
 fn keyword_choice_branch(
@@ -3499,10 +3491,8 @@ fn build_keyword_choice_clause(
     application: &SubjectApplication,
     predicate: &str,
 ) -> Result<Option<ParsedEffectClause>, ()> {
-    let (keywords, duration) = match parse_keyword_choice_grant(predicate) {
-        KeywordChoiceGrant::NoMatch => return Ok(None),
-        KeywordChoiceGrant::Parsed { keywords, duration } => (keywords, duration),
-        KeywordChoiceGrant::UnsupportedNestedChoice => return Err(()),
+    let Some((keywords, duration)) = parse_keyword_choice_grant(predicate)? else {
+        return Ok(None);
     };
     let affected = static_affected_for_application(application);
     let branches = keywords

crates/engine/src/parser/oracle_effect/tests.rs

@@ -13525,16 +13525,6 @@ fn assassin_initiate_three_way_keyword_choice_grant_is_generic() {
 /// must decline rather than advertise the card as supported.
 #[test]
 fn nary_keyword_choice_grant_defers_nested_choice_keywords() {
-    assert!(
-        matches!(
-            super::parse_keyword_choice_grant(
-            "gain your choice of banding, landwalk of your choice, or protection from a color of your choice until end of turn"
-        ),
-            super::KeywordChoiceGrant::UnsupportedNestedChoice
-        ),
-        "nested keyword choices require their own typed resolution prompts"
-    );
-
     let def = parse_effect_chain(
         "This creature gains your choice of banding, landwalk of your choice, or protection from a color of your choice until end of turn",
         AbilityKind::Spell,