Coroutine variant fields can be uninitialized

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 again requires computing layout of `{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}`, completing the cycle
+   = note: ...which requires computing layout of `core::mem::manually_drop::ManuallyDrop<{async fn body@$DIR/indirect-recursion-issue-112047.rs:33:27: 35:6}>`...
+   = 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