From e1f073692781c832b4880158515efda8b9e6f48b Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 5 Jun 2022 10:53:35 -0400
Subject: [PATCH] Allow ptr_from_addr_cast to fail

---
 compiler/rustc_const_eval/src/interpret/cast.rs    |  2 +-
 compiler/rustc_const_eval/src/interpret/machine.rs | 11 ++++++-----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index 73cc59ad1e674..fb484fba9fd06 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -221,7 +221,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let addr = addr.to_machine_usize(self)?;
 
         // Then turn address into pointer.
-        let ptr = M::ptr_from_addr_cast(&self, addr);
+        let ptr = M::ptr_from_addr_cast(&self, addr)?;
         Ok(Scalar::from_maybe_pointer(ptr, self).into())
     }
 
diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index 3572a9cc68174..5377535b9fa08 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -294,11 +294,10 @@ pub trait Machine<'mir, 'tcx>: Sized {
     fn ptr_from_addr_cast(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         addr: u64,
-    ) -> Pointer<Option<Self::PointerTag>>;
+    ) -> InterpResult<'tcx, Pointer<Option<Self::PointerTag>>>;
 
-    // FIXME: Transmuting an integer to a pointer should just always return a `None`
-    // provenance, but that causes problems with function pointers in Miri.
     /// Hook for returning a pointer from a transmute-like operation on an addr.
+    /// This is only needed to support Miri's (unsound) "allow-ptr-int-transmute" flag.
     fn ptr_from_addr_transmute(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         addr: u64,
@@ -519,8 +518,10 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     fn ptr_from_addr_cast(
         _ecx: &InterpCx<$mir, $tcx, Self>,
         addr: u64,
-    ) -> Pointer<Option<AllocId>> {
-        Pointer::new(None, Size::from_bytes(addr))
+    ) -> InterpResult<$tcx, Pointer<Option<AllocId>>> {
+        // Allow these casts, but make the pointer not dereferenceable.
+        // (I.e., they behave like transmutation.)
+        Ok(Pointer::new(None, Size::from_bytes(addr)))
     }
 
     #[inline(always)]