Correctly destructure patterns that are assigned to a thunk'd value

Closes #4309

crates/compiler/mono/src/ir.rs

@@ -2604,9 +2604,11 @@ fn from_can_let<'a>(
         );
     }
 
-    if let roc_can::expr::Expr::Var(outer_symbol) = def.loc_expr.value {
+    match def.loc_expr.value {
+        roc_can::expr::Expr::Var(outer_symbol) if !procs.is_module_thunk(outer_symbol) => {
             store_pattern(env, procs, layout_cache, &mono_pattern, outer_symbol, stmt)
-    } else {
+        }
+        _ => {
             let outer_symbol = env.unique_symbol();
             stmt = store_pattern(env, procs, layout_cache, &mono_pattern, outer_symbol, stmt);
 
@@ -2621,6 +2623,7 @@ fn from_can_let<'a>(
                 env.arena.alloc(stmt),
             )
         }
+    }
 }
 
 /// turn record/tag patterns into a when expression, e.g.

crates/compiler/test_gen/src/gen_str.rs

@@ -1982,3 +1982,46 @@ fn str_with_prefix() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn destructure_pattern_assigned_from_thunk_opaque() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            MyCustomType := Str
+            myMsg = @MyCustomType "Hello"
+
+            main =
+                @MyCustomType msg = myMsg
+
+                msg
+            "#
+        ),
+        RocStr::from("Hello"),
+        RocStr
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn destructure_pattern_assigned_from_thunk_tag() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            myMsg = A "hello " "world"
+
+            main =
+                A m1 m2 = myMsg
+
+                Str.concat m1 m2
+            "#
+        ),
+        RocStr::from("hello world"),
+        RocStr
+    );
+}