Compute the layout of uninhabited structs

src/librustc/mir/interpret/error.rs

@@ -389,10 +389,6 @@ pub enum UnsupportedOpInfo<'tcx> {
     /// Free-form case. Only for errors that are never caught!
     Unsupported(String),
 
-    /// FIXME(#64506) Error used to work around accessing projections of
-    /// uninhabited types.
-    UninhabitedValue,
-
     // -- Everything below is not categorized yet --
     FunctionAbiMismatch(Abi, Abi),
     FunctionArgMismatch(Ty<'tcx>, Ty<'tcx>),
@@ -556,8 +552,6 @@ impl fmt::Debug for UnsupportedOpInfo<'tcx> {
                     not a power of two"),
             Unsupported(ref msg) =>
                 write!(f, "{}", msg),
-            UninhabitedValue =>
-                write!(f, "tried to use an uninhabited value"),
         }
     }
 }

src/librustc/ty/layout.rs

@@ -825,10 +825,14 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     });
                     (present_variants.next(), present_variants.next())
                 };
-                if present_first.is_none() {
+                let present_first = match present_first {
+                    present_first @ Some(_) => present_first,
                     // Uninhabited because it has no variants, or only absent ones.
-                    return tcx.layout_raw(param_env.and(tcx.types.never));
-                }
+                    None if def.is_enum() => return tcx.layout_raw(param_env.and(tcx.types.never)),
+                    // if it's a struct, still compute a layout so that we can still compute the
+                    // field offsets
+                    None => Some(VariantIdx::new(0)),
+                };
 
                 let is_struct = !def.is_enum() ||
                     // Only one variant is present.

src/librustc_mir/interpret/place.rs

@@ -9,7 +9,7 @@ use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{
-    self, Size, Abi, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
+    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
 };
 use rustc::ty::TypeFoldable;
 
@@ -377,20 +377,17 @@ where
             layout::FieldPlacement::Array { stride, .. } => {
                 let len = base.len(self)?;
                 if field >= len {
-                    // This can be violated because this runs during promotion on code where the
-                    // type system has not yet ensured that such things don't happen.
+                    // This can be violated because the index (field) can be a runtime value
+                    // provided by the user.
                     debug!("tried to access element {} of array/slice with length {}", field, len);
                     throw_panic!(BoundsCheck { len, index: field });
                 }
                 stride * field
             }
             layout::FieldPlacement::Union(count) => {
-                // FIXME(#64506) `UninhabitedValue` can be removed when this issue is resolved
-                if base.layout.abi == Abi::Uninhabited {
-                    throw_unsup!(UninhabitedValue);
-                }
                 assert!(field < count as u64,
-                        "Tried to access field {} of union with {} fields", field, count);
+                        "Tried to access field {} of union {:#?} with {} fields",
+                        field, base.layout, count);
                 // Offset is always 0
                 Size::from_bytes(0)
             }

src/librustc_target/abi/mod.rs

@@ -738,7 +738,11 @@ impl FieldPlacement {
 
     pub fn offset(&self, i: usize) -> Size {
         match *self {
-            FieldPlacement::Union(_) => Size::ZERO,
+            FieldPlacement::Union(count) => {
+                assert!(i < count,
+                        "Tried to access field {} of union with {} fields", i, count);
+                Size::ZERO
+            },
             FieldPlacement::Array { stride, count } => {
                 let i = i as u64;
                 assert!(i < count);

src/test/ui/consts/issue-64506.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+#[derive(Copy, Clone)]
+pub struct ChildStdin {
+    inner: AnonPipe,
+}
+
+#[derive(Copy, Clone)]
+enum AnonPipe {}
+
+const FOO: () = {
+    union Foo {
+        a: ChildStdin,
+        b: (),
+    }
+    let x = unsafe { Foo { b: () }.a };
+    let x = &x.inner;
+};
+
+fn main() {}