fix crash in "pattern match on variable code action"

Author: giacomocavalieri

CHANGELOG.md

@@ -25,6 +25,10 @@
   the final step of a pipeline.
   ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
 
+- Fixed a bug where the "pattern match on variable" code action would crash when
+  used on a variable followed by a case expression.
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
 ## v1.13.0-rc1 - 2025-09-29
 
 ### Compiler

compiler-core/src/language_server/code_action.rs

@@ -4923,6 +4923,13 @@ impl<'ast, IO> ast::visit::Visit<'ast> for PatternMatchOnValue<'ast, IO> {
     }
 
     fn visit_typed_assignment(&mut self, assignment: &'ast TypedAssignment) {
+        // If we're not inside the assignment there's no point in exploring its
+        // ast further.
+        let assignment_range = self.edits.src_span_to_lsp_range(assignment.location);
+        if !within(self.params.range, assignment_range) {
+            return;
+        }
+
         ast::visit::visit_typed_assignment(self, assignment);
         if let Some((name, _, ref type_)) = self.pattern_variable_under_cursor {
             self.selected_value = Some(PatternMatchedValue::LetVariable {
@@ -4934,6 +4941,13 @@ impl<'ast, IO> ast::visit::Visit<'ast> for PatternMatchOnValue<'ast, IO> {
     }
 
     fn visit_typed_clause(&mut self, clause: &'ast ast::TypedClause) {
+        // If we're not inside the clause there's no point in exploring its
+        // ast further.
+        let clause_range = self.edits.src_span_to_lsp_range(clause.location);
+        if !within(self.params.range, clause_range) {
+            return;
+        }
+
         for pattern in clause.pattern.iter() {
             self.visit_typed_pattern(pattern);
         }

compiler-core/src/language_server/tests/action.rs

@@ -7538,6 +7538,33 @@ fn map(list: List(a), fun: fn(a) -> b) { todo }
     );
 }
 
+#[test]
+// https://github.com/gleam-lang/gleam/issues/5042
+fn pattern_match_on_variable_crashes() {
+    assert_code_action!(
+        PATTERN_MATCH_ON_VARIABLE,
+        r#"
+pub type Wibble {
+    Wibble(Wobble)
+}
+
+pub type Wobble {
+    Wobble
+    Wubble
+}
+
+pub fn main() {
+    let Wibble(wobble) = todo
+
+    case todo {
+      _ -> todo
+    }
+}
+"#,
+        find_position_of("wobble").to_selection()
+    );
+}
+
 #[test]
 fn generate_function_works_with_invalid_call() {
     assert_code_action!(

compiler-core/src/language_server/tests/snapshots/gleam_core__language_server__tests__action__pattern_match_on_variable_crashes.snap

@@ -0,0 +1,47 @@
+---
+source: compiler-core/src/language_server/tests/action.rs
+expression: "\npub type Wibble {\n    Wibble(Wobble)\n}\n\npub type Wobble {\n    Wobble\n    Wubble\n}\n\npub fn main() {\n    let Wibble(wobble) = todo\n\n    case todo {\n      _ -> todo\n    }\n}\n"
+---
+----- BEFORE ACTION
+
+pub type Wibble {
+    Wibble(Wobble)
+}
+
+pub type Wobble {
+    Wobble
+    Wubble
+}
+
+pub fn main() {
+    let Wibble(wobble) = todo
+               ↑             
+
+    case todo {
+      _ -> todo
+    }
+}
+
+
+----- AFTER ACTION
+
+pub type Wibble {
+    Wibble(Wobble)
+}
+
+pub type Wobble {
+    Wobble
+    Wubble
+}
+
+pub fn main() {
+    let Wibble(wobble) = todo
+    case wobble {
+      Wobble -> todo
+      Wubble -> todo
+    }
+
+    case todo {
+      _ -> todo
+    }
+}