coverage: Add a test for async blocks

We have coverage tests that use async functions, but none that use async
blocks.

tests/coverage/async_block.cov-map

@@ -0,0 +1,32 @@
+Function name: async_block::main
+Raw bytes (38): 0x[01, 01, 02, 01, 05, 03, 05, 06, 01, 05, 01, 00, 0b, 05, 01, 09, 00, 0a, 03, 00, 0e, 00, 13, 05, 00, 14, 01, 16, 05, 07, 0a, 02, 06, 06, 03, 01, 00, 02]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Expression(0, Add), rhs = Counter(1)
+Number of file 0 mappings: 6
+- Code(Counter(0)) at (prev + 5, 1) to (start + 0, 11)
+- Code(Counter(1)) at (prev + 1, 9) to (start + 0, 10)
+- Code(Expression(0, Add)) at (prev + 0, 14) to (start + 0, 19)
+    = (c0 + c1)
+- Code(Counter(1)) at (prev + 0, 20) to (start + 1, 22)
+- Code(Counter(1)) at (prev + 7, 10) to (start + 2, 6)
+- Code(Expression(1, Sub)) at (prev + 3, 1) to (start + 0, 2)
+    = ((c0 + c1) - c1)
+
+Function name: async_block::main::{closure#0}
+Raw bytes (28): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, 07, 1c, 01, 17, 05, 01, 18, 02, 0e, 02, 02, 14, 02, 0e, 07, 03, 09, 00, 0a]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 7, 28) to (start + 1, 23)
+- Code(Counter(1)) at (prev + 1, 24) to (start + 2, 14)
+- Code(Expression(0, Sub)) at (prev + 2, 20) to (start + 2, 14)
+    = (c0 - c1)
+- Code(Expression(1, Add)) at (prev + 3, 9) to (start + 0, 10)
+    = (c1 + (c0 - c1))
+

tests/coverage/async_block.coverage

@@ -0,0 +1,37 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |#![feature(noop_waker)]
+   LL|       |// edition: 2021
+   LL|       |
+   LL|      1|fn main() {
+   LL|     17|    for i in 0..16 {
+                      ^16
+   LL|     16|        let future = async {
+   LL|     16|            if i >= 12 {
+   LL|      4|                println!("big");
+   LL|     12|            } else {
+   LL|     12|                println!("small");
+   LL|     12|            }
+   LL|     16|        };
+   LL|     16|        executor::block_on(future);
+   LL|     16|    }
+   LL|      1|}
+   LL|       |
+   LL|       |mod executor {
+   LL|       |    use core::future::Future;
+   LL|       |    use core::pin::pin;
+   LL|       |    use core::task::{Context, Poll, Waker};
+   LL|       |
+   LL|       |    #[coverage(off)]
+   LL|       |    pub fn block_on<F: Future>(mut future: F) -> F::Output {
+   LL|       |        let mut future = pin!(future);
+   LL|       |        let waker = Waker::noop();
+   LL|       |        let mut context = Context::from_waker(&waker);
+   LL|       |
+   LL|       |        loop {
+   LL|       |            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
+   LL|       |                break val;
+   LL|       |            }
+   LL|       |        }
+   LL|       |    }
+   LL|       |}
+

tests/coverage/async_block.rs

@@ -0,0 +1,35 @@
+#![feature(coverage_attribute)]
+#![feature(noop_waker)]
+// edition: 2021
+
+fn main() {
+    for i in 0..16 {
+        let future = async {
+            if i >= 12 {
+                println!("big");
+            } else {
+                println!("small");
+            }
+        };
+        executor::block_on(future);
+    }
+}
+
+mod executor {
+    use core::future::Future;
+    use core::pin::pin;
+    use core::task::{Context, Poll, Waker};
+
+    #[coverage(off)]
+    pub fn block_on<F: Future>(mut future: F) -> F::Output {
+        let mut future = pin!(future);
+        let waker = Waker::noop();
+        let mut context = Context::from_waker(&waker);
+
+        loop {
+            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
+                break val;
+            }
+        }
+    }
+}