From ae77f4b6dbea62b97cd86273a7c9714b07c02fd5 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Fri, 21 Aug 2026 13:49:32 +0200
Subject: [PATCH] try to reduce impact of scalar size checks

---
 .../rustc_middle/src/mir/interpret/value.rs   | 32 +++++++++++--------
 compiler/rustc_middle/src/ty/consts/int.rs    | 20 ++++++------
 2 files changed, 30 insertions(+), 22 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/interpret/value.rs b/compiler/rustc_middle/src/mir/interpret/value.rs
index 56cded69e469e..e78b000a84c0f 100644
--- a/compiler/rustc_middle/src/mir/interpret/value.rs
+++ b/compiler/rustc_middle/src/mir/interpret/value.rs
@@ -1,4 +1,5 @@
 use std::fmt;
+use std::num::NonZero;
 
 use either::{Either, Left, Right};
 use rustc_abi::{HasDataLayout, Size};
@@ -29,7 +30,7 @@ pub enum Scalar<Prov = CtfeProvenance> {
     /// We also store the size of the pointer, such that a `Scalar` always knows how big it is.
     /// The size is always the pointer size of the current target, but this is not information
     /// that we always have readily available.
-    Ptr(Pointer<Prov>, u8),
+    Ptr(Pointer<Prov>, NonZero<u8>),
 }
 
 #[cfg(target_pointer_width = "64")]
@@ -102,7 +103,8 @@ impl<Prov> From<ScalarInt> for Scalar<Prov> {
 impl<Prov> Scalar<Prov> {
     #[inline(always)]
     pub fn from_pointer(ptr: Pointer<Prov>, cx: &impl HasDataLayout) -> Self {
-        Scalar::Ptr(ptr, u8::try_from(cx.pointer_size().bytes()).unwrap())
+        let ptr_size = u8::try_from(cx.pointer_size().bytes()).ok().and_then(NonZero::new).unwrap();
+        Scalar::Ptr(ptr, ptr_size)
     }
 
     /// Create a Scalar from a pointer with an `Option<_>` provenance (where `None` represents a
@@ -236,17 +238,20 @@ impl<Prov> Scalar<Prov> {
     /// This throws UB (instead of ICEing) on a size mismatch since size mismatches can arise in
     /// Miri when someone declares a function that we shim (such as `malloc`) with a wrong type.
     #[inline]
-    pub fn to_bits_or_ptr_internal(self, target_size: Size) -> Either<u128, Pointer<Prov>> {
-        assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
+    pub fn to_bits_or_ptr_internal(self, expected_size: Size) -> Either<u128, Pointer<Prov>> {
         match self {
-            Scalar::Int(int) => Left(int.to_bits(target_size)),
+            Scalar::Int(int) => Left(int.to_bits(expected_size)),
             Scalar::Ptr(ptr, sz) => {
-                assert_eq!(
-                    target_size.bytes(),
-                    u64::from(sz),
-                    "Scalar is a pointer but expected size {}",
-                    target_size.bytes()
-                );
+                let self_size = u64::from(sz.get());
+                if expected_size.bytes() != self_size {
+                    #[cold]
+                    fn invalid(expected_size: u64, self_size: u64) -> ! {
+                        panic!("Scalar pointer has size {self_size} but expected {expected_size}")
+                    }
+
+                    invalid(expected_size.bytes(), self_size)
+                }
+
                 Right(ptr)
             }
         }
@@ -256,7 +261,7 @@ impl<Prov> Scalar<Prov> {
     pub fn size(self) -> Size {
         match self {
             Scalar::Int(int) => int.size(),
-            Scalar::Ptr(_ptr, sz) => Size::from_bytes(sz),
+            Scalar::Ptr(_ptr, sz) => Size::from_bytes(sz.get()),
         }
     }
 }
@@ -287,7 +292,8 @@ impl<'tcx, Prov: Provenance> Scalar<Prov> {
             Scalar::Int(int) => Ok(int),
             Scalar::Ptr(ptr, sz) => {
                 if Prov::OFFSET_IS_ADDR {
-                    Ok(ScalarInt::try_from_uint(ptr.offset.bytes(), Size::from_bytes(sz)).unwrap())
+                    Ok(ScalarInt::try_from_uint(ptr.offset.bytes(), Size::from_bytes(sz.get()))
+                        .unwrap())
                 } else {
                     // We know `offset` is relative, since `OFFSET_IS_ADDR == false`.
                     let (prov, offset) = ptr.into_raw_parts();
diff --git a/compiler/rustc_middle/src/ty/consts/int.rs b/compiler/rustc_middle/src/ty/consts/int.rs
index ce5ffefe2d4f5..9fa4f157ebffa 100644
--- a/compiler/rustc_middle/src/ty/consts/int.rs
+++ b/compiler/rustc_middle/src/ty/consts/int.rs
@@ -262,15 +262,17 @@ impl ScalarInt {
 
     /// Convert this ScalarInt to the underlying bits.
     #[inline]
-    pub fn to_bits(self, target_size: Size) -> u128 {
-        assert_ne!(target_size.bytes(), 0, "you should never look at the bits of a ZST");
-        assert_eq!(
-            target_size.bytes(),
-            u64::from(self.size.get()),
-            "ScalarInt has size {} but expected {}",
-            self.size,
-            target_size.bytes(),
-        );
+    pub fn to_bits(self, expected_size: Size) -> u128 {
+        let self_size = u64::from(self.size.get());
+        if expected_size.bytes() != self_size {
+            #[cold]
+            fn invalid(expected_size: u64, self_size: u64) -> ! {
+                panic!("ScalarInt has size {self_size} but expected {expected_size}")
+            }
+
+            invalid(expected_size.bytes(), self_size);
+        }
+
         self.check_data();
         self.data
     }