Some local unit tests to track progress and capture interesting cases as I identify them.

issue-62958-c.rs was reduced from the tracing-attributes proc-macro crate.

issue-62958-d.rs was reduced from the doc test attached to `AtomicPtr::from_mut_slice`.

issue-62958-e.rs covers some important operational characteristics.

Author: pnkfelix

tests/ui/mir/issue-62958-a.rs

@@ -0,0 +1,28 @@
+// revisions: both_off just_prop both_on
+// ignore-tidy-linelength
+// run-pass
+// [both_off]  compile-flags: -Z mir-enable-passes=-UpvarToLocalProp,-InlineFutureIntoFuture
+// [just_prop] compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,-InlineFutureIntoFuture
+// [both_on]   compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,+InlineFutureIntoFuture
+// edition:2018
+
+async fn wait() {}
+#[allow(dropping_copy_types)]
+async fn test(arg: [u8; 8192]) {
+    wait().await;
+    drop(arg);
+}
+
+#[cfg(both_off)]
+fn main() {
+    let expected = 16000..=17000;
+    let actual = std::mem::size_of_val(&test([0; 8192]));
+    assert!(expected.contains(&actual));
+}
+
+#[cfg(any(just_prop, both_on))]
+fn main() {
+    let expected = 8192..=9999;
+    let actual = std::mem::size_of_val(&test([0; 8192]));
+    assert!(expected.contains(&actual));
+}

tests/ui/mir/issue-62958-b.rs

@@ -0,0 +1,27 @@
+// revisions: both_off just_prop both_on
+// ignore-tidy-linelength
+// run-pass
+// [both_off]  compile-flags: -Z mir-enable-passes=-UpvarToLocalProp,-InlineFutureIntoFuture
+// [just_prop] compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,-InlineFutureIntoFuture
+// [both_on]   compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,+InlineFutureIntoFuture
+// edition:2018
+
+async fn test(_arg: [u8; 8192]) {}
+
+async fn use_future(fut: impl std::future::Future<Output = ()>) {
+    fut.await
+}
+
+#[cfg(both_on)]
+fn main() {
+    let expected = 8192..=9999;
+    let actual = std::mem::size_of_val(&use_future(use_future(use_future(test([0; 8192])))));
+    assert!(expected.contains(&actual), "expected: {:?} actual: {}", expected, actual);
+}
+
+#[cfg(any(both_off, just_prop))]
+fn main() {
+    let expected = 16000..=8192*(2*2*2*2); // O(2^n) LOL
+    let actual = std::mem::size_of_val(&use_future(use_future(use_future(test([0; 8192])))));
+    assert!(expected.contains(&actual), "expected: {:?} actual: {}", expected, actual);
+}

tests/ui/mir/issue-62958-c.rs

@@ -0,0 +1,23 @@
+// revisions: both_off just_prop both_on
+// ignore-tidy-linelength
+// build-pass
+// [both_off]  compile-flags: -Z mir-enable-passes=-UpvarToLocalProp,-InlineFutureIntoFuture
+// [just_prop] compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,-InlineFutureIntoFuture
+// [both_on]   compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,+InlineFutureIntoFuture
+
+#![crate_type="rlib"]
+
+// FIXME: I should be able to expand the below into something that actually does
+// some interesting work. The crucial thing is to enforce a rule that we never
+// replace `_3` with `_1.0` on a place that holds a Deref.
+
+pub struct G { p: () }
+pub struct S { g: G }
+pub struct R<'a> { s: &'a S, b: () }
+
+pub fn gen_function(input: R<'_>) {
+    let R { s, b: _b } = input;
+    let S { g, .. } = s;
+    let G { p: _p, .. } = g;
+    loop { }
+}

tests/ui/mir/issue-62958-d.rs

@@ -0,0 +1,34 @@
+// revisions: both_off just_prop both_on
+// ignore-tidy-linelength
+// run-pass
+// [both_off]  compile-flags: -Z mir-enable-passes=-UpvarToLocalProp,-InlineFutureIntoFuture
+// [just_prop] compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,-InlineFutureIntoFuture
+// [both_on]   compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,+InlineFutureIntoFuture
+// edition:2018
+
+#![feature(atomic_from_mut)]
+
+// FIXME: I should be able to reduce the below further now that I understand the
+// nature of the problem. (Namely, the fact that the old upvar_to_local_prop
+// code was ignoring locals in projections.)
+
+use std::ptr::null_mut;
+use std::sync::atomic::{AtomicPtr, Ordering};
+
+fn main() {
+    let mut some_ptrs = [null_mut::<String>(); 10];
+    let a = &*AtomicPtr::from_mut_slice(&mut some_ptrs);
+    std::thread::scope(|s| {
+        for i in 0..a.len() {
+            s.spawn(move || {
+                let name = Box::new(format!("thread{i}"));
+                a[i].store(Box::into_raw(name), Ordering::Relaxed);
+            });
+        }
+    });
+    for p in some_ptrs {
+        assert!(!p.is_null());
+        let name = unsafe { Box::from_raw(p) };
+        println!("Hello, {name}!");
+    }
+}

tests/ui/mir/issue-62958-e.rs

@@ -0,0 +1,108 @@
+// revisions: both_off both_on
+// ignore-tidy-linelength
+// run-pass
+// [both_off]  compile-flags: -Z mir-enable-passes=-UpvarToLocalProp,-InlineFutureIntoFuture
+// [both_on]   compile-flags: -Z mir-enable-passes=+UpvarToLocalProp,+InlineFutureIntoFuture
+// edition:2018
+
+use std::future::Future;
+
+async fn wait() {}
+
+fn test_shrinks_1(arg: [u8; 1000]) -> impl std::future::Future<Output=()> {
+    async move {
+        let mut local = arg;
+        local[2] = 3;
+        wait().await;
+        assert_eq!(local[2], 3);
+    }
+}
+
+fn test_noshrinks_1(mut arg: [u8; 1000]) -> impl std::future::Future<Output=()> {
+    async move {
+        let mut local = arg;
+        local[2] = 3;
+        let l2 = &mut arg;
+        l2[2] = 4;
+        wait().await;
+        assert_eq!(local[2], 3);
+        assert_eq!(l2[2], 4);
+    }
+}
+
+fn test_noshrinks_2(arg: [u8; 1000]) -> impl std::future::Future<Output=()> {
+    async move {
+        let mut local = arg;
+        local[2] = 1;
+        let l2 = arg;
+        wait().await;
+        assert_eq!(local[2], 1);
+        assert_eq!(l2[2], 0);
+    }
+}
+
+fn test_noshrinks_3(arg: [u8; 1000]) -> impl std::future::Future<Output=()> {
+    async move {
+        let bor = &arg[2];
+        let mut local = arg;
+        local[2] = 1;
+        wait().await;
+        assert_eq!(local[2], 1);
+        assert_eq!(*bor, 0);
+    }
+}
+
+#[cfg(both_on)]
+fn check_shrinks(which: &str, fut: impl std::future::Future<Output=()>) {
+    let sz = std::mem::size_of_val(&fut);
+    println!("{which}: {sz}");
+    assert!((1000..=1500).contains(&sz));
+    run_fut(fut)
+}
+
+fn check_no_shrinks(which: &str, fut: impl std::future::Future<Output=()>) {
+    let sz = std::mem::size_of_val(&fut);
+    println!("{which}: {sz}");
+    assert!((2000..).contains(&sz));
+    run_fut(fut);
+}
+
+#[cfg(both_on)]
+fn main() {
+    check_shrinks("s1", test_shrinks_1([0; 1000]));
+
+    check_no_shrinks("n1", test_noshrinks_1([0; 1000]));
+    check_no_shrinks("n2", test_noshrinks_2([0; 1000]));
+    check_no_shrinks("n3", test_noshrinks_3([0; 1000]));
+}
+
+#[cfg(both_off)]
+fn main() {
+    check_no_shrinks("s1", test_shrinks_1([0; 1000]));
+    check_no_shrinks("n1", test_noshrinks_1([0; 1000]));
+    check_no_shrinks("n2", test_noshrinks_2([0; 1000]));
+    check_no_shrinks("n3", test_noshrinks_3([0; 1000]));
+}
+
+fn run_fut<T>(fut: impl Future<Output = T>) -> T {
+    use std::sync::Arc;
+    use std::task::{Context, Poll, Wake, Waker};
+
+    struct MyWaker;
+    impl Wake for MyWaker {
+        fn wake(self: Arc<Self>) {
+            unimplemented!()
+        }
+    }
+
+    let waker = Waker::from(Arc::new(MyWaker));
+    let mut context = Context::from_waker(&waker);
+
+    let mut pinned = Box::pin(fut);
+    loop {
+        match pinned.as_mut().poll(&mut context) {
+            Poll::Pending => continue,
+            Poll::Ready(v) => return v,
+        }
+    }
+}