Update transmute_copy to ub_checks and ?Sized

library/core/src/mem/mod.rs

@@ -35,6 +35,7 @@ use crate::clone::TrivialClone;
 use crate::cmp::Ordering;
 use crate::marker::{Destruct, DiscriminantKind};
 use crate::panic::const_assert;
+use crate::ub_checks::assert_unsafe_precondition;
 use crate::{clone, cmp, fmt, hash, intrinsics, ptr};
 
 mod alignment;
@@ -1040,6 +1041,18 @@ pub const fn copy<T: Copy>(x: &T) -> T {
 ///
 /// [ub]: ../../reference/behavior-considered-undefined.html
 ///
+/// If you have a raw pointer instead of a reference, you might be looking for
+/// [`ptr::read_unaligned`]`::<Dst>(ptr.cast::<Dst>())` instead.
+///
+/// # Safety
+///
+/// - Requires `size_of_val::<Src>(src) >= size_of::<Dst>()`
+/// - The first `size_of::<Dst>()` bytes behind `src` must be *readable*
+/// - The first `size_of::<Dst>()` bytes behind `src` must be *valid*
+///   when interpreted as a `Dst`.
+/// - All safety invariants of the `Dst` type must be upheld.  (For example,
+///   `{ transmute_copy::<String, String>(&x); x; }` is UB for the double-drop.)
+///
 /// # Examples
 ///
 /// ```
@@ -1064,20 +1077,32 @@ pub const fn copy<T: Copy>(x: &T) -> T {
 ///
 /// // The contents of 'foo_array' should not have changed
 /// assert_eq!(foo_array, [10]);
+///
+/// let bytes: &[u8] = &[1, 2, 3, 4, 5, 6, 7];
+/// assert_eq!(
+///     unsafe { mem::transmute_copy::<[u8], u32>(bytes) },
+///     u32::from_ne_bytes(*bytes.first_chunk().unwrap()),
+/// );
 /// ```
 #[inline]
 #[must_use]
 #[track_caller]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "const_transmute_copy", since = "1.74.0")]
-pub const unsafe fn transmute_copy<Src, Dst>(src: &Src) -> Dst {
-    assert!(
-        size_of::<Src>() >= size_of::<Dst>(),
-        "cannot transmute_copy if Dst is larger than Src"
+pub const unsafe fn transmute_copy<Src: ?Sized, Dst>(src: &Src) -> Dst {
+    // library UB because it's possible for the `Src` to be only a subset of the allocation
+    // and thus for a failure to not be immediate language UB
+    assert_unsafe_precondition!(
+        check_library_ub,
+        "cannot transmute_copy if Dst is larger than Src",
+        (
+            src_size: usize = size_of_val::<Src>(src),
+            dst_size: usize = Dst::SIZE,
+        ) => src_size >= dst_size
     );
 
     // If Dst has a higher alignment requirement, src might not be suitably aligned.
-    if align_of::<Dst>() > align_of::<Src>() {
+    if align_of::<Dst>() > align_of_val::<Src>(src) {
         // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
         // The caller must guarantee that the actual transmutation is safe.
         unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }

library/coretests/tests/mem.rs

@@ -138,32 +138,6 @@ fn test_transmute_copy_unaligned() {
     assert_eq!(0_u64, unsafe { transmute_copy(&u.b) });
 }
 
-#[test]
-#[cfg(panic = "unwind")]
-fn test_transmute_copy_grow_panics() {
-    use std::panic;
-
-    let err = panic::catch_unwind(panic::AssertUnwindSafe(|| unsafe {
-        let _unused: u64 = transmute_copy(&1_u8);
-    }));
-
-    match err {
-        Ok(_) => unreachable!(),
-        Err(payload) => {
-            payload
-                .downcast::<&'static str>()
-                .and_then(|s| {
-                    if *s == "cannot transmute_copy if Dst is larger than Src" {
-                        Ok(s)
-                    } else {
-                        Err(s)
-                    }
-                })
-                .unwrap_or_else(|p| panic::resume_unwind(p));
-        }
-    }
-}
-
 #[test]
 #[allow(dead_code)]
 fn test_discriminant_send_sync() {

tests/ui/precondition-checks/transmute_copy.rs

@@ -0,0 +1,9 @@
+//@ run-crash
+//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
+//@ error-pattern: unsafe precondition(s) violated: cannot transmute_copy if Dst is larger than Src
+
+fn main() {
+    unsafe {
+        let _unused: u64 = std::mem::transmute_copy(&1_u8);
+    }
+}