fix(parser): bind a where-X count's "they" to the scoped player (Citadel of Pain, #6508)

[rykerwilliams]

Closes #6508.

The bug

Citadel of Pain:

At the beginning of each player's end step, this enchantment deals X damage to that player, where X is the number of untapped lands they control.

On an opponent's end step the trigger fired and correctly hit that opponent, but it dealt them the controller's untapped-land count. On a two-player board where P0 controls Citadel with 5 untapped lands and P1 has 2, P1 took 5 instead of 2.

Root cause

The lowered trigger was:

DealDamage { amount: ObjectCount { Land, Untapped, controller: You }, target: ScopedPlayer }

The target was right. The count's controller was not.

• The trigger's own parse context was correct. relative_player_scope_for_condition sets ScopedPlayer for "at the beginning of each player's ".
• The runtime was correct too. build_triggered_ability binds scoped_player to the active player for TriggerMode::Phase, and scoped_player_or_controller reads it.
• The where-X clause never saw that scope. The clause is carried as text (ClauseIr.where_x_expression) and bound during assembly by apply_where_x_ability_expression → parse_where_x_quantity_expression, whose count arm called the context-free parse_cda_quantity. The type-phrase suffix maps "they control" to relative_player_scope.unwrap_or(You), so with no context it became You.

The fix

parse_where_x_quantity_expression now parses its count through parse_cda_quantity_with_context with a context whose third-person referent is ControllerRef::ScopedPlayer, via a private where_x_count_context().

• This is the house default for counts: parse_for_each_clause_with_context already uses .unwrap_or(ControllerRef::ScopedPlayer), and parse_possessive_objects_they_control emits it.
• A printed "you control" stays You.
• Both where-X count leaves share the context. Self-review found a sibling with the identical defect: parse_trailing_where_x_quantity, which binds the count of "create X of those tokens, where X is …" (Adipose Offspring, The Final Days), also called the context-free parse_cda_quantity. It now uses the same where_x_count_context(), pinned by match_create_of_those_tokens_where_x_binds_they_to_scoped_player. No printed card in the census uses that path with a third-person count, so this closes a latent gap rather than changing a card. The other two context-free parse_cda_quantity calls in lower.rs are not where-X clauses: try_parse_distribute_damage and parse_dynamic_counter_suffix_body.
• No new enum variants or fields, and the runtime is untouched.

Alternatives rejected

1. Thread the chain's relative_player_scope through where-X binding, as EffectChainIr already does for actor and in_trigger. Where-X binding is string-driven across 16 functions. bind_where_x_quantity alone has 25 callers, apply_where_x_quantity_expression 18 and apply_where_x_to_filter 14, so this would be roughly 80 edits for the same ScopedPlayer result on every card measured below.
2. Add a " they control" arm to parse_quantity_controller_suffix. That nom path runs before the context-aware folding, so it would pre-empt a real scope. It breaks cda_quantity_uses_relative_player_scope_for_they_control, which expects DefendingPlayer.
3. A post-lowering ScopedPlayer rewrite beside rewrite_player_quantity_refs_to_source_chosen. After lowering, a "they"-derived You can't be told apart from a printed "you control", so the rewrite would corrupt genuine "you" counts.

Blast radius — measured, not estimated

I parsed every card with a third-person where-X count on two builds: lower.rs at upstream/main, and with this change. That's 28 cards whose where-X clause contains "they control", "that player controls", "their …" or "that player's …", with Oracle text fetched verbatim from Scryfall.

outcome	cards
Changed: the where-X count's filter.controller only, You → ScopedPlayer	Citadel of Pain, Curious Herd, Jovial Evil, Pact of the Serpent
Byte-identical parse	the other 24, including the Black Vise and The Rack hand-size families, Keening Stone, Bulwark, Dark Suspicions, Natural Balance
Unimplemented both before and after	Champions of Minas Tirith, Cruel Calculations, Kylox, Power Surge, Psychic Allergy, Witch-king

How the four changes behave at runtime:

• Citadel of Pain: fixed, since the Phase trigger binds scoped_player to the active player.
• Curious Herd, Jovial Evil and Pact of the Serpent: no behaviour change. All three are spells naming a target player, with no player_scope or repeat_for. While a spell resolves no player scope is bound, so ScopedPlayer falls through scoped_player_or_controller to the caster, exactly as You did. Traced from code, not runtime-probed: bind_resolution_scope builds a triggered scope only for StackEntryKind::TriggeredAbility; the batch map's only production writer is push_pending_trigger_to_stack_with_firing_and_duration_events; and resolve_top clears current_trigger_event after every resolution.
  They were already wrong before this PR, since "that player"/"they" is the target. That is a separate target-binding gap, noted under follow-ups.

The CI parse-diff sticky comment is the corpus-wide check of this table.

Tests

crates/engine/tests/integration/issue_6508_citadel_of_pain_each_players_end_step.rs:

• citadel_of_pain_hits_each_player_for_their_own_untapped_lands runs Citadel's verbatim Oracle text on a two-player board with 5 vs 2 untapped lands. At P0's end step P0 takes 5 and P1 takes nothing, which is the positive reach-guard. A trigger-on-stack assertion confirms the P0-controlled trigger fires at P1's step. P1 then takes exactly 2 and P0 nothing more.
• match_create_of_those_tokens_where_x_binds_they_to_scoped_player (unit) covers the count-bound where-X leaf: "they control" lowers to ScopedPlayer and "you control" to You.
• where_x_count_binds_they_to_the_scoped_player_and_keeps_you (SHAPE) checks that "they control" lowers to ScopedPlayer. The same trigger with "you control" must lower to You, and each case is the other's reach-guard.

Revert-proof. With lower.rs reverted to upstream/main, both fail:

• left: 15, right: 18: P1 takes the controller's 5.
• left: Some(You), right: Some(ScopedPlayer).

With the fix restored, both pass.

A test-harness trap worth knowing about

The first version of the runtime test used GameRunner::advance_to_phase to leave P0's end step. It reported Citadel hitting P0 twice. That is not an engine bug: the same board walked with PassPriority fires it exactly once. advance_to_phase begins by calling turns::auto_advance, which dispatches on state.phase, so calling it while priority is already open inside a step re-runs that step's beginning-of-step triggers. The test now walks with PassPriority, and its helper says why.

Verification

• Targeted: both new tests pass, and natural_balance, issue_2360_the_rack and each_player_they_control_scope stay green.
• cargo test -p phase-engine --test integration: 7039 passed, 0 failed (4 ignored) on ddec0cf51. The head after that changes only a #[cfg(test)] import in lower.rs, so no non-test code differs from the tree that ran.
• cargo test -p phase-engine --lib: 21378 passed, 0 failed (8 ignored) on the final head 6c6cf4c42, including both where-X unit tests.
• cargo fmt --all --check is clean. Clippy was not run locally; CI's "Rust lint (fmt, clippy, parser gate)" job owns it.
• Every CR number was checked against docs/MagicCompRules.txt: 107.3c, 608.2c, 603.2, 120.3a.

Follow-ups, not in this PR

• Target-player referent in where-X counts: Pact of the Serpent, Jovial Evil, Curious Herd and Inscription of Abundance count the caster's permanents instead of the target's.
• Psychic Allergy: "…of the chosen color they control" doesn't parse, and stays honestly Unimplemented.
• GameRunner::advance_to_phase re-fires beginning-of-step triggers when called with priority already open.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Fixed Citadel of Pain so its end-step damage is calculated from the untapped lands controlled by the appropriate player.
  • Corrected “they control” wording in quantity-based effects to refer to the scoped player, preventing incorrect damage or count calculations.
  • Preserved the intended behavior of “you control” wording.

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📥 Commits

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📒 Files selected for processing (2)

• crates/engine/tests/integration/issue_6508_create_of_those_tokens_where_x_scope.rs
• crates/engine/tests/integration/main.rs

📝 Walkthrough

Walkthrough

The parser now applies a scoped-player context to where-X quantity expressions. New Citadel of Pain integration tests verify that each player takes damage based on their own untapped lands.

Changes

Where-X scoped-player resolution

Layer / File(s)	Summary
Scoped quantity parsing and parser coverage crates/engine/src/parser/oracle_effect/lower.rs	Where-X CDA quantity parsing now uses a ScopedPlayer context. Tests verify that “they control” resolves to ControllerRef::ScopedPlayer, while “you control” remains ControllerRef::You.
Citadel of Pain integration validation crates/engine/tests/integration/issue_6508_citadel_of_pain_each_players_end_step.rs, crates/engine/tests/integration/main.rs	Integration tests verify end-step damage for each player, inspect the parsed damage controller, and register the new test module.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to 6c6cf

The token-creation variant is covered only at the parser-helper level, so a regression in actual gameplay could pass the current tests. Add runtime integration coverage before merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main parser fix: binding a where-X count's "they" to the scoped player for Citadel of Pain.
Linked Issues check	✅ Passed	Issue #6508 requires Citadel of Pain to deal damage to the player whose end step triggered the ability, using that player's untapped-land count. The parser now supplies a shared context with `relative…
Out of Scope Changes check	✅ Passed	The changes are limited to the where-X parser binding and tests for issue #6508. The create-of-those-tokens parser test covers the shared parsing path. No unrelated runtime behavior, data structure, o…
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 2 files. (1 skipped: 1 t…

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Actionable comments posted: 1

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Inline comments:
In `@crates/engine/src/parser/oracle_effect/lower.rs`:
- Line 12884: Add an integration test under crates/engine/tests/integration/ for
the create x of those tokens behavior, register it in main.rs, and exercise the
real Oracle parsing, token-effect rewriting, and runtime quantity-resolution
pipeline using ControllerRef::ScopedPlayer. Keep the focused unit test
match_create_of_those_tokens_where_x_binds_they_to_scoped_player unchanged.

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📒 Files selected for processing (3)

• crates/engine/src/parser/oracle_effect/lower.rs
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[matthewevans]

Generated for head b63e6ca128a33ebd07b736b6c6328bcc9c8371b1.

Parse changes introduced by this PR · 4 card(s), 5 signature(s) (baseline: main 9d919c6f692e)

🟡 Modified fields (5 signatures)

• 1 card · 🔄 ability/DealDamage · changed field amount: # of untapped you control land → # of untapped scoped player controls land
  • Affected (first 3): Citadel of Pain
• 1 card · 🔄 ability/DealDamage · changed field amount: 2*# of white you control creature → 2*# of white scoped player controls creature
  • Affected (first 3): Jovial Evil
• 1 card · 🔄 ability/Draw · changed field count: # of chosen creature type you control creature → # of chosen creature type scoped player controls creature
  • Affected (first 3): Pact of the Serpent
• 1 card · 🔄 ability/LoseLife · changed field amount: # of chosen creature type you control creature → # of chosen creature type scoped player controls creature
  • Affected (first 3): Pact of the Serpent
• 1 card · 🔄 ability/Token · changed field token: # of you control artifact× +3/+3 Green Beast (Creature Beast) → # of scoped player controls artifact× +3/+3 Green Beast (Creature Beast)
  • Affected (first 3): Curious Herd

[matthewevans]

Changes requested for current head 6c6cf4c42f75683fb2e31daf171e65788611f55d.

1. HIGH — the token-anaphor path has no real-pipeline regression coverage. lower.rs:12884-12909 only calls match_create_of_those_tokens on a hand-built Effect::unimplemented; production reaches rewrite_those_tokens_from_antecedent at lower.rs:3026-3043. Add a registered integration test that parses and resolves the real create X of those tokens anaphor pipeline, with distinct player counts, so it fails if the scoped binding/rewrite is removed.

2. HIGH — universal ScopedPlayer loses target-player provenance. where_x_count_context hard-codes ScopedPlayer at lower.rs:10181-10196 and passes it to the CDA parser at 10350-10353. That is not correct for target-player wording such as Curious Herd's “that player controls”: ScopedPlayer falls back to the source controller (filter.rs:1383-1397), while TargetPlayer resolves the declared target (filter.rs:1537-1555). Carry the originating player scope/target referent into the where-X parser and add a P0/P1 target-player regression proving the count uses the declared target rather than the source controller.

[matthewevans]

Changes requested for current head 6c6cf4c42f75683fb2e31daf171e65788611f55d.

1. HIGH — the token-anaphor path has no real-pipeline regression coverage. lower.rs:12884-12909 only calls match_create_of_those_tokens on a hand-built Effect::unimplemented; production reaches rewrite_those_tokens_from_antecedent at lower.rs:3026-3043. Add a registered integration test that parses and resolves the real create X of those tokens anaphor pipeline, with distinct player counts, so it fails if the scoped binding/rewrite is removed.

2. HIGH — universal ScopedPlayer loses target-player provenance. where_x_count_context hard-codes ScopedPlayer at lower.rs:10181-10196 and passes it to the CDA parser at 10350-10353. That is not correct for target-player wording such as Curious Herd's “that player controls”: ScopedPlayer falls back to the source controller (filter.rs:1383-1397), while TargetPlayer resolves the declared target (filter.rs:1537-1555). Carry the originating player scope/target referent into the where-X parser and add a P0/P1 target-player regression proving the count uses the declared target rather than the source controller.

Duplicate submission caused by a local temporary-directory failure; the immediately preceding current-head changes request is the canonical review.

[matthewevans]

Changes requested for current head 6c6cf4c42f75683fb2e31daf171e65788611f55d.

1. HIGH — the token-anaphor path has no real-pipeline regression coverage. lower.rs:12884-12909 only calls match_create_of_those_tokens on a hand-built Effect::unimplemented; production reaches rewrite_those_tokens_from_antecedent at lower.rs:3026-3043. Add a registered integration test that parses and resolves the real create X of those tokens anaphor pipeline, with distinct player counts, so it fails if the scoped binding/rewrite is removed.

2. HIGH — universal ScopedPlayer loses target-player provenance. where_x_count_context hard-codes ScopedPlayer at lower.rs:10181-10196 and passes it to the CDA parser at 10350-10353. That is not correct for target-player wording such as Curious Herd's “that player controls”: ScopedPlayer falls back to the source controller (filter.rs:1383-1397), while TargetPlayer resolves the declared target (filter.rs:1537-1555). Carry the originating player scope/target referent into the where-X parser and add a P0/P1 target-player regression proving the count uses the declared target rather than the source controller.

Superseded by the canonical current-head changes request after a local temporary-directory retry; the later review carries the same findings.

[matthewevans]

Changes requested for current head b63e6ca128a33ebd07b736b6c6328bcc9c8371b1.

HIGH — where_x_count_context discards target-player provenance. crates/engine/src/parser/oracle_effect/lower.rs:10181-10196 always constructs a new ParseContext with ControllerRef::ScopedPlayer, and both where-X paths pass it to parse_cda_quantity_with_context at lower.rs:3237-3242 and 10350-10353. That is correct for the per-player/phase iteration case covered by Citadel of Pain, but not for a preceding target-player referent.

Curious Herd says, “Choose target opponent. You create X 3/3 green Beast creature tokens, where X is the number of artifacts that player controls.” The current parse-diff receipt reports its token count as scoped player controls artifact, rather than target-player scoped. At resolution ScopedPlayer searches a scoped/trigger binding and then falls back to the source controller (crates/engine/src/game/filter.rs:1383-1397,1547-1555), so this spell can count the caster's artifacts instead of the chosen opponent's.

Please preserve and thread the antecedent provenance into the where-X quantity parser: use TargetPlayer for target-opponent/player paths and ScopedPlayer only for genuine per-player/phase iteration. Add a Curious Herd P0/P1 regression with unequal artifact counts that verifies the chosen opponent's count. The new token-anaphor integration coverage addresses the separate prior test-coverage request; this review does not repeat it.