DRAFT: coverage of async function bodies should match non-async

The initial commit demonstrates the issue, but the fix is not yet
implemented.

Once corrected...

Fixes: #83985

src/test/run-make-fulldeps/coverage/async2.rs

@@ -0,0 +1,78 @@
+// compile-flags: --edition=2018
+
+use core::{
+    future::Future,
+    marker::Send,
+    pin::Pin,
+};
+
+fn non_async_func() {
+    println!("non_async_func was covered");
+    let b = true;
+    if b {
+        println!("non_async_func println in block");
+    }
+}
+
+// FIXME(#83985): The auto-generated closure in an async function is failing to include
+// the println!() and `let` assignment lines in the coverage code region(s), as it does in the
+// non-async function above, unless the `println!()` is inside a covered block.
+async fn async_func() {
+    println!("async_func was covered");
+    let b = true;
+    if b {
+        println!("async_func println in block");
+    }
+}
+
+// FIXME(#83985): As above, this async function only has the `println!()` macro call, which is not
+// showing coverage, so the entire async closure _appears_ uncovered; but this is not exactly true.
+// It's only certain kinds of lines and/or their context that results in missing coverage.
+async fn async_func_just_println() {
+    println!("async_func_just_println was covered");
+}
+
+fn main() {
+    println!("codecovsample::main");
+
+    non_async_func();
+
+    executor::block_on(async_func());
+    executor::block_on(async_func_just_println());
+
+    // let mut future = Box::pin(async_func());
+    // executor::block_on(future.as_mut());
+
+    // let mut future = Box::pin(async_func());
+    // executor::block_on(future.as_mut());
+
+    // let mut future = Box::pin(async_func_just_println());
+    // executor::block_on(future.as_mut());
+}
+
+mod executor {
+    use core::{
+        future::Future,
+        pin::Pin,
+        task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
+    };
+
+    pub fn block_on<F: Future>(mut future: F) -> F::Output {
+        let mut future = unsafe { Pin::new_unchecked(&mut future) };
+        use std::hint::unreachable_unchecked;
+        static VTABLE: RawWakerVTable = RawWakerVTable::new(
+            |_| unsafe { unreachable_unchecked() }, // clone
+            |_| unsafe { unreachable_unchecked() }, // wake
+            |_| unsafe { unreachable_unchecked() }, // wake_by_ref
+            |_| (),
+        );
+        let waker = unsafe { Waker::from_raw(RawWaker::new(core::ptr::null(), &VTABLE)) };
+        let mut context = Context::from_waker(&waker);
+
+        loop {
+            if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
+                break val;
+            }
+        }
+    }
+}