Fix Debug for Value during GC

Summary:
Context:

```
value = pointer ->
  union {
    [ vtable ptr                     | value data    ] # normal value
    [ target ptr with lowest bit set | old self size ] # "forward" value during GC
  }
```

During GC, `Value` after value is moved, data is replaced with forward value.

All normal `Value` operations crash on forward value. But debug should work.

Reviewed By: rajneesh

Differential Revision: D62771488

fbshipit-source-id: aba664c8ea56957a37db32b74e571a0e7ea3e4c1

starlark/src/values/layout/heap/repr.rs

@@ -102,6 +102,7 @@ impl ForwardPtr {
 
 /// This is object written over [`AValueRepr`] during GC.
 #[repr(C)]
+#[derive(Debug)]
 pub(crate) struct AValueForward {
     /// Moved object pointer with lowest bit set.
     forward_ptr: usize,

starlark/src/values/layout/value.rs

@@ -78,6 +78,7 @@ use crate::values::iter::StarlarkIterator;
 use crate::values::layout::avalue::AValue;
 use crate::values::layout::avalue::AValueImpl;
 use crate::values::layout::heap::repr::AValueHeader;
+use crate::values::layout::heap::repr::AValueOrForwardUnpack;
 use crate::values::layout::heap::repr::AValueRepr;
 use crate::values::layout::pointer::FrozenPointer;
 use crate::values::layout::pointer::Pointer;
@@ -176,6 +177,14 @@ impl Display for FrozenValue {
 }
 
 fn debug_value(typ: &str, v: Value, f: &mut fmt::Formatter) -> fmt::Result {
+    // When value is being moved during GC or freeze,
+    // `Value` pointee is not a proper value, but a GC-related information.
+    // Regular operations like `.to_repr()` crash, but `Debug` should work.
+    if let Some(x) = v.0.unpack_ptr() {
+        if let AValueOrForwardUnpack::Forward(fwd) = x.unpack() {
+            return f.debug_tuple(typ).field(&fwd).finish();
+        }
+    }
     f.debug_tuple(typ).field(v.get_ref().as_debug()).finish()
 }