Rollup merge of #95340 - RalfJung:pnvi, r=oli-obk

interpret: with enforce_number_validity, ensure integers are truly Scalar::Int (i.e., no pointers)

This is required for rust-lang/miri#2040

r? ```@oli-obk```

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -444,6 +444,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match scalar.try_to_int() {
             Ok(int) => int.is_null(),
             Err(_) => {
+                // Can only happen during CTFE.
                 let ptr = self.scalar_to_ptr(scalar);
                 match self.memory.ptr_try_get_alloc(ptr) {
                     Ok((alloc_id, offset, _)) => {
@@ -455,7 +456,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         // Note that one-past-the-end (offset == size) is still inbounds, and never null.
                         offset > size
                     }
-                    Err(offset) => offset == 0,
+                    Err(_offset) => bug!("a non-int scalar is always a pointer"),
                 }
             }
         }

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -21,7 +21,7 @@ use std::hash::Hash;
 
 use super::{
     alloc_range, CheckInAllocMsg, GlobalAlloc, InterpCx, InterpResult, MPlaceTy, Machine,
-    MemPlaceMeta, OpTy, ScalarMaybeUninit, ValueVisitor,
+    MemPlaceMeta, OpTy, Scalar, ScalarMaybeUninit, ValueVisitor,
 };
 
 macro_rules! throw_validation_failure {
@@ -521,8 +521,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // NOTE: Keep this in sync with the array optimization for int/float
                 // types below!
                 if M::enforce_number_validity(self.ecx) {
-                    // Integers/floats in CTFE: Must be scalar bits, pointers are dangerous
-                    let is_bits = value.check_init().map_or(false, |v| v.try_to_int().is_ok());
+                    // Integers/floats with number validity: Must be scalar bits, pointers are dangerous.
+                    // As a special exception we *do* match on a `Scalar` here, since we truly want
+                    // to know its underlying representation (and *not* cast it to an integer).
+                    let is_bits =
+                        value.check_init().map_or(false, |v| matches!(v, Scalar::Int(..)));
                     if !is_bits {
                         throw_validation_failure!(self.path,
                             { "{:x}", value } expected { "initialized plain (non-pointer) bytes" }