Add test of thread_local containing multiline const block

Before making thread_local accept statements inside the const block,
this test would fail to compile as follows:

    error: no rules expected the token `let`
       --> library/std/tests/thread.rs:26:13
        |
    26  |             let value = 1;
        |             ^^^ no rules expected this token in macro call
        |
    note: while trying to match meta-variable `$init:expr`
       --> library/std/src/thread/local.rs:189:69
        |
    189 |     ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const { $init:expr }; $($rest:tt)*) => (
        |                                                                     ^^^^^^^^^^

library/std/tests/thread.rs

@@ -1,3 +1,4 @@
+use std::cell::{Cell, RefCell};
 use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
@@ -14,3 +15,24 @@ fn sleep() {
     thread::sleep(Duration::from_millis(100));
     assert_eq!(*finished.lock().unwrap(), false);
 }
+
+#[test]
+fn thread_local_containing_const_statements() {
+    // This exercises the `const $init:block` cases of the thread_local macro.
+    // Despite overlapping with expression syntax, the `const { ... }` is not
+    // parsed as `$init:expr`.
+    thread_local! {
+        static CELL: Cell<u32> = const {
+            let value = 1;
+            Cell::new(value)
+        };
+
+        static REFCELL: RefCell<u32> = const {
+            let value = 1;
+            RefCell::new(value)
+        };
+    }
+
+    assert_eq!(CELL.get(), 1);
+    assert_eq!(REFCELL.take(), 1);
+}