Correctly perform record updates that come from thunks

Closes #3908

crates/compiler/mono/src/ir.rs

@@ -4874,7 +4874,10 @@ pub fn with_hole<'a>(
                             stmt =
                                 Stmt::Let(*symbol, access_expr, *field_layout, arena.alloc(stmt));
 
-                            if record_needs_specialization {
+                            // If the records needs specialization or it's a thunk, we need to
+                            // create the specialized definition or force the thunk, respectively.
+                            // Both cases are handled below.
+                            if record_needs_specialization || procs.is_module_thunk(structure) {
                                 stmt = specialize_symbol(
                                     env,
                                     procs,

crates/compiler/test_gen/src/gen_records.rs

@@ -1044,3 +1044,43 @@ fn generalized_accessor() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn update_record_that_is_a_thunk() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            main = Num.toStr fromOriginal.birds
+            
+            original = { birds: 5, iguanas: 7, zebras: 2, goats: 1 }
+            
+            fromOriginal = { original & birds: 4, iguanas: 3 }
+            "#
+        ),
+        RocStr::from("4"),
+        RocStr
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn update_record_that_is_a_thunk_single_field() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            main = Num.toStr fromOriginal.birds
+            
+            original = { birds: 5 }
+            
+            fromOriginal = { original & birds: 4 }
+            "#
+        ),
+        RocStr::from("4"),
+        RocStr
+    );
+}