Rework the ByMoveBody shim to actually work correctly

tests/ui/async-await/async-closures/precise-captures.call.run.stdout

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+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured

tests/ui/async-await/async-closures/precise-captures.call_once.run.stdout

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+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+fixed
+after await
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured

tests/ui/async-await/async-closures/precise-captures.force_once.run.stdout

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+after call
+after await
+fixed
+uncaptured
+
+after call
+after await
+fixed
+uncaptured
+
+after call
+fixed
+after await
+uncaptured
+
+after call
+after await
+fixed
+untouched
+
+after call
+drop first
+after await
+uncaptured
+
+after call
+drop first
+after await
+uncaptured

tests/ui/async-await/async-closures/precise-captures.rs

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+//@ aux-build:block-on.rs
+//@ edition:2021
+//@ run-pass
+//@ check-run-results
+//@ revisions: call call_once force_once
+
+// call - Call the closure regularly.
+// call_once - Call the closure w/ `async FnOnce`, so exercising the by_move shim.
+// force_once - Force the closure mode to `FnOnce`, so exercising what was fixed
+//   in <https://github.com/rust-lang/rust/pull/123350>.
+
+#![feature(async_closure)]
+#![allow(unused_mut)]
+
+extern crate block_on;
+
+#[cfg(any(call, force_once))]
+macro_rules! call {
+    ($c:expr) => { ($c)() }
+}
+
+#[cfg(call_once)]
+async fn call_once(f: impl async FnOnce()) {
+    f().await
+}
+
+#[cfg(call_once)]
+macro_rules! call {
+    ($c:expr) => { call_once($c) }
+}
+
+#[cfg(not(force_once))]
+macro_rules! guidance {
+    ($c:expr) => { $c }
+}
+
+#[cfg(force_once)]
+fn infer_fnonce(c: impl async FnOnce()) -> impl async FnOnce() { c }
+
+#[cfg(force_once)]
+macro_rules! guidance {
+    ($c:expr) => { infer_fnonce($c) }
+}
+
+#[derive(Debug)]
+struct Drop(&'static str);
+
+impl std::ops::Drop for Drop {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+
+struct S {
+    a: i32,
+    b: Drop,
+    c: Drop,
+}
+
+async fn async_main() {
+    // Precise capture struct
+    {
+        let mut s = S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture &mut struct
+    {
+        let s = &mut S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct by move
+    {
+        let mut s = S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async move || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture &mut struct by move
+    {
+        let s = &mut S { a: 1, b: Drop("fix me up"), c: Drop("untouched") };
+        let mut c = guidance!(async move || {
+            s.a = 2;
+            let w = &mut s.b;
+            w.0 = "fixed";
+        });
+        // `s` is still captured fully as `&mut S`.
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct, consume field
+    {
+        let mut s = S { a: 1, b: Drop("drop first"), c: Drop("untouched") };
+        let c = guidance!(async move || {
+            // s.a = 2; // FIXME(async_closures): Figure out why this fails
+            drop(s.b);
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+    println!();
+
+    // Precise capture struct by move, consume field
+    {
+        let mut s = S { a: 1, b: Drop("drop first"), c: Drop("untouched") };
+        let c = guidance!(async move || {
+            // s.a = 2; // FIXME(async_closures): Figure out why this fails
+            drop(s.b);
+        });
+        s.c.0 = "uncaptured";
+        let fut = call!(c);
+        println!("after call");
+        fut.await;
+        println!("after await");
+    }
+}
+
+fn main() {
+    block_on::block_on(async_main());
+}