allow matching on list arguments

fixes #4806

Author: giacomocavalieri

compiler-core/src/language_server/code_action.rs

@@ -4664,6 +4664,14 @@ where
         match type_ {
             Type::Fn { .. } => None,
             Type::Var { type_ } => self.type_var_to_destructure_patterns(&type_.borrow()),
+
+            // We special case lists, they don't have "regular" constructors
+            // like other types. Instead we always add the two branches covering
+            // the empty and non empty list.
+            Type::Named { .. } if type_.is_list() => {
+                Some(vec1!["[]".into(), "[first, ..rest]".into()])
+            }
+
             Type::Named {
                 module: type_module,
                 name: type_name,

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

@@ -9162,3 +9162,14 @@ fn remove_opaque_from_private_type() {
         find_position_of("Wibble").to_selection()
     );
 }
+
+#[test]
+fn pattern_match_on_list_variable() {
+    assert_code_action!(
+        PATTERN_MATCH_ON_ARGUMENT,
+        "pub fn main(a_list: List(a)) {
+  todo
+}",
+        find_position_of("a_list").to_selection()
+    );
+}

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

@@ -0,0 +1,19 @@
+---
+source: compiler-core/src/language_server/tests/action.rs
+expression: "pub fn main(a_list: List(a)) {\n  todo\n}"
+---
+----- BEFORE ACTION
+pub fn main(a_list: List(a)) {
+            ↑                 
+  todo
+}
+
+
+----- AFTER ACTION
+pub fn main(a_list: List(a)) {
+  case a_list {
+    [] -> todo
+    [first, ..rest] -> todo
+  }
+  todo
+}

compiler-core/src/type_.rs

@@ -320,6 +320,14 @@ impl Type {
         }
     }
 
+    pub fn is_list(&self) -> bool {
+        match self {
+            Self::Named { module, name, .. } if "List" == name && is_prelude_module(module) => true,
+            Self::Var { type_ } => type_.borrow().is_list(),
+            _ => false,
+        }
+    }
+
     pub fn named_type_name(&self) -> Option<(EcoString, EcoString)> {
         match self {
             Self::Named { module, name, .. } => Some((module.clone(), name.clone())),
@@ -1239,6 +1247,13 @@ impl TypeVar {
         }
     }
 
+    pub fn is_list(&self) -> bool {
+        match self {
+            TypeVar::Link { type_ } => type_.is_list(),
+            TypeVar::Unbound { .. } | TypeVar::Generic { .. } => false,
+        }
+    }
+
     pub fn result_ok_type(&self) -> Option<Arc<Type>> {
         match self {
             TypeVar::Link { type_ } => type_.result_ok_type(),