use a better name for patterns with a single unlabelled field

Instead of being called `value_0` it just uses `value` which looks way
better when there's just a single unlabelled field:

  let Wibble(value) = arg
  // instead of
  let Wibble(value_0) = arg

compiler-core/src/language_server/code_action.rs

@@ -3398,12 +3398,21 @@ where
         }
 
         pattern.push('(');
-        let args = (0..*constructor_arity as u32)
-            .map(|i| match index_to_label.get(&i) {
-                Some(label) => format!("{label}:"),
-                None => format!("value_{i}"),
-            })
-            .join(", ");
+        let args = if *constructor_arity <= 1 && index_to_label.get(&0).is_none() {
+            // If there's a single argument and its not labelled we don't add a
+            // number suffix to it and just call it "value".
+            String::from("value")
+        } else {
+            // Otherwise all unlabelled arguments will be called "value_<n>".
+            // Labelled arguments, on the other hand, will always use the
+            // shorthand syntax.
+            (0..*constructor_arity as u32)
+                .map(|i| match index_to_label.get(&i) {
+                    Some(label) => format!("{label}:"),
+                    None => format!("value_{i}"),
+                })
+                .join(", ")
+        };
         pattern.push_str(&args);
         pattern.push(')');
         Some(pattern)

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

@@ -4606,3 +4606,18 @@ fn pattern_match_on_argument_nicely_formats_code_when_used_on_function_with_empt
         find_position_of("arg").to_selection()
     );
 }
+
+#[test]
+fn pattern_match_on_argument_single_unlabelled_field_is_not_numbered() {
+    assert_code_action!(
+        PATTERN_MATCH_ON_ARGUMENT,
+        "
+pub type Wibble {
+  Wibble(Int)
+}
+
+pub fn main(arg: Wibble) {}
+",
+        find_position_of(":").to_selection()
+    );
+}

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

@@ -0,0 +1,23 @@
+---
+source: compiler-core/src/language_server/tests/action.rs
+expression: "\npub type Wibble {\n  Wibble(Int)\n}\n\npub fn main(arg: Wibble) {}\n"
+---
+----- BEFORE ACTION
+
+pub type Wibble {
+  Wibble(Int)
+}
+
+pub fn main(arg: Wibble) {}
+               ↑           
+
+
+----- AFTER ACTION
+
+pub type Wibble {
+  Wibble(Int)
+}
+
+pub fn main(arg: Wibble) {
+  let Wibble(value) = arg
+}