fix: incorrect 'unused::constructor'

`ExprTyper` was not incrementing the usage of a constructor
when infering `RecordAccess`.

closes #554

crates/aiken-lang/src/tests/check.rs

@@ -82,6 +82,37 @@ fn validator_illegal_arity() {
     ))
 }
 
+#[test]
+fn mark_constructors_as_used_via_field_access() {
+    let source_code = r#"
+      type Datum {
+        D0(D0Params)
+        D1(D1Params)
+      }
+
+      type D0Params {
+        foo: Int,
+      }
+
+      type D1Params {
+        bar: Int,
+      }
+
+      validator {
+        fn foo(d: Datum, _r, _c) {
+          when d is {
+            D0(params) -> params.foo == 1
+            D1(_params) -> False
+          }
+        }
+      }
+    "#;
+
+    let (warnings, _) = check_validator(parse(source_code)).unwrap();
+
+    assert_eq!(warnings.len(), 1)
+}
+
 #[test]
 fn validator_correct_form() {
     let source_code = r#"

crates/aiken-lang/src/tipo/expr.rs

@@ -782,18 +782,21 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         };
 
         // Check to see if it's a Type that can have accessible fields
-        let accessors = match collapse_links(record.tipo()).as_ref() {
+        let (accessors, name) = match collapse_links(record.tipo()).as_ref() {
             // A type in the current module which may have fields
-            Type::App { module, name, .. } if module == self.environment.current_module => {
-                self.environment.accessors.get(name)
-            }
+            Type::App { module, name, .. } if module == self.environment.current_module => self
+                .environment
+                .accessors
+                .get(name)
+                .map(|t| (t, name.clone())),
 
             // A type in another module which may have fields
             Type::App { module, name, .. } => self
                 .environment
                 .importable_modules
                 .get(module)
-                .and_then(|module| module.accessors.get(name)),
+                .and_then(|module| module.accessors.get(name))
+                .map(|accessors| (accessors, name.clone())),
 
             _something_without_fields => return Err(unknown_field(vec![])),
         }
@@ -826,6 +829,8 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
             false,
         )?;
 
+        self.environment.increment_usage(&name);
+
         Ok(TypedExpr::RecordAccess {
             record,
             label,