Coroutine variant fields can be uninitialized

Wrap coroutine variant fields in MaybeUninit to indicate that they
might be uninitialized. Otherwise an uninhabited field will make
the entire variant uninhabited and introduce undefined behaviour.

The analogous issue in the prefix of coroutine layout was addressed by
6fae7f8071.

compiler/rustc_ty_utils/src/layout.rs

@@ -831,7 +831,10 @@ fn coroutine_layout<'tcx>(
                     Assigned(_) => bug!("assignment does not match variant"),
                     Ineligible(_) => false,
                 })
-                .map(|local| subst_field(info.field_tys[*local].ty));
+                .map(|local| {
+                    let field_ty = subst_field(info.field_tys[*local].ty);
+                    Ty::new_maybe_uninit(tcx, field_ty)
+                });
 
             let mut variant = univariant_uninterned(
                 cx,

tests/ui/async-await/future-sizes/async-awaiting-fut.stdout

@@ -52,10 +52,16 @@ print-type-size     variant `Panicked`: 1024 bytes
 print-type-size         upvar `.arg`: 1024 bytes
 print-type-size type: `std::mem::ManuallyDrop<bool>`: 1 bytes, alignment: 1 bytes
 print-type-size     field `.value`: 1 bytes
+print-type-size type: `std::mem::ManuallyDrop<{async fn body@$DIR/async-awaiting-fut.rs:6:17: 6:19}>`: 1 bytes, alignment: 1 bytes
+print-type-size     field `.value`: 1 bytes
 print-type-size type: `std::mem::MaybeUninit<bool>`: 1 bytes, alignment: 1 bytes
 print-type-size     variant `MaybeUninit`: 1 bytes
 print-type-size         field `.uninit`: 0 bytes
 print-type-size         field `.value`: 1 bytes
+print-type-size type: `std::mem::MaybeUninit<{async fn body@$DIR/async-awaiting-fut.rs:6:17: 6:19}>`: 1 bytes, alignment: 1 bytes
+print-type-size     variant `MaybeUninit`: 1 bytes
+print-type-size         field `.uninit`: 0 bytes
+print-type-size         field `.value`: 1 bytes
 print-type-size type: `std::task::Poll<()>`: 1 bytes, alignment: 1 bytes
 print-type-size     discriminant: 1 bytes
 print-type-size     variant `Ready`: 0 bytes

tests/ui/async-await/in-trait/indirect-recursion-issue-112047.stderr

@@ -1,7 +1,9 @@
-error[E0391]: cycle detected when computing layout of `{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}`
+error[E0391]: cycle detected when computing layout of `core::mem::maybe_uninit::MaybeUninit<{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}>`
    |
-   = note: ...which requires computing layout of `<<A as First>::Second as Second>::{opaque#0}`...
+   = note: ...which requires computing layout of `core::mem::manually_drop::ManuallyDrop<{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}>`...
-   = note: ...which again requires computing layout of `{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}`, completing the cycle
+   = note: ...which requires computing layout of `{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}`...
+   = note: ...which requires computing layout of `core::mem::maybe_uninit::MaybeUninit<<<A as First>::Second as Second>::{opaque#0}>`...
+   = note: ...which again requires computing layout of `core::mem::maybe_uninit::MaybeUninit<{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}>`, completing the cycle
    = note: cycle used when computing layout of `{async block@$DIR/indirect-recursion-issue-112047.rs:6:13: 8:6}`
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 

tests/ui/coroutine/uninhabited-field.rs

@@ -0,0 +1,37 @@
+// Test that uninhabited saved local doesn't make the entire variant uninhabited.
+// run-pass
+#![allow(unused)]
+#![feature(assert_matches)]
+#![feature(coroutine_trait)]
+#![feature(coroutines)]
+#![feature(never_type)]
+use std::assert_matches::assert_matches;
+use std::ops::Coroutine;
+use std::ops::CoroutineState;
+use std::pin::Pin;
+
+fn conjure<T>() -> T { loop {} }
+
+fn run<T>(x: bool, y: bool) {
+    let mut c = || {
+        if x {
+            let a : T;
+            if y {
+                a = conjure::<T>();
+            }
+            yield ();
+        } else {
+            let a : T;
+            if y {
+                a = conjure::<T>();
+            }
+            yield ();
+        }
+    };
+    assert_matches!(Pin::new(&mut c).resume(()), CoroutineState::Yielded(()));
+    assert_matches!(Pin::new(&mut c).resume(()), CoroutineState::Complete(()));
+}
+
+fn main() {
+    run::<!>(false, false);
+}

tests/ui/print_type_sizes/coroutine_discr_placement.stdout

@@ -9,3 +9,9 @@ print-type-size         padding: 3 bytes
 print-type-size         local `.z`: 4 bytes, alignment: 4 bytes
 print-type-size     variant `Returned`: 0 bytes
 print-type-size     variant `Panicked`: 0 bytes
+print-type-size type: `std::mem::ManuallyDrop<i32>`: 4 bytes, alignment: 4 bytes
+print-type-size     field `.value`: 4 bytes
+print-type-size type: `std::mem::MaybeUninit<i32>`: 4 bytes, alignment: 4 bytes
+print-type-size     variant `MaybeUninit`: 4 bytes
+print-type-size         field `.uninit`: 0 bytes
+print-type-size         field `.value`: 4 bytes