Adjust `transmute{,_copy}` to be clearer about which of `T` and `U` is input vs output

library/core/src/intrinsics.rs

@@ -994,14 +994,14 @@ extern "rust-intrinsic" {
     /// `transmute` is semantically equivalent to a bitwise move of one type
     /// into another. It copies the bits from the source value into the
     /// destination value, then forgets the original. Note that source and destination
-    /// are passed by-value, which means if `T` or `U` contain padding, that padding
+    /// are passed by-value, which means if `Src` or `Dst` contain padding, that padding
     /// is *not* guaranteed to be preserved by `transmute`.
     ///
     /// Both the argument and the result must be [valid](../../nomicon/what-unsafe-does.html) at
     /// their given type. Violating this condition leads to [undefined behavior][ub]. The compiler
     /// will generate code *assuming that you, the programmer, ensure that there will never be
     /// undefined behavior*. It is therefore your responsibility to guarantee that every value
-    /// passed to `transmute` is valid at both types `T` and `U`. Failing to uphold this condition
+    /// passed to `transmute` is valid at both types `Src` and `Dst`. Failing to uphold this condition
     /// may lead to unexpected and unstable compilation results. This makes `transmute` **incredibly
     /// unsafe**. `transmute` should be the absolute last resort.
     ///
@@ -1012,7 +1012,7 @@ extern "rust-intrinsic" {
     ///
     /// Because `transmute` is a by-value operation, alignment of the *transmuted values
     /// themselves* is not a concern. As with any other function, the compiler already ensures
-    /// both `T` and `U` are properly aligned. However, when transmuting values that *point
+    /// both `Src` and `Dst` are properly aligned. However, when transmuting values that *point
     /// elsewhere* (such as pointers, references, boxes…), the caller has to ensure proper
     /// alignment of the pointed-to values.
     ///
@@ -1248,7 +1248,7 @@ extern "rust-intrinsic" {
     #[rustc_allowed_through_unstable_modules]
     #[rustc_const_stable(feature = "const_transmute", since = "1.56.0")]
     #[rustc_diagnostic_item = "transmute"]
-    pub fn transmute<T, U>(e: T) -> U;
+    pub fn transmute<Src, Dst>(src: Src) -> Dst;
 
     /// Returns `true` if the actual type given as `T` requires drop
     /// glue; returns `false` if the actual type provided for `T`

library/core/src/mem/mod.rs

@@ -1008,18 +1008,18 @@ pub fn copy<T: Copy>(x: &T) -> T {
     *x
 }
 
-/// Interprets `src` as having type `&U`, and then reads `src` without moving
+/// Interprets `src` as having type `&Dst`, and then reads `src` without moving
 /// the contained value.
 ///
-/// This function will unsafely assume the pointer `src` is valid for [`size_of::<U>`][size_of]
-/// bytes by transmuting `&T` to `&U` and then reading the `&U` (except that this is done in a way
-/// that is correct even when `&U` has stricter alignment requirements than `&T`). It will also
-/// unsafely create a copy of the contained value instead of moving out of `src`.
+/// This function will unsafely assume the pointer `src` is valid for [`size_of::<Dst>`][size_of]
+/// bytes by transmuting `&Src` to `&Dst` and then reading the `&Dst` (except that this is done
+/// in a way that is correct even when `&Dst` has stricter alignment requirements than `&Src`).
+/// It will also unsafely create a copy of the contained value instead of moving out of `src`.
 ///
-/// It is not a compile-time error if `T` and `U` have different sizes, but it
-/// is highly encouraged to only invoke this function where `T` and `U` have the
-/// same size. This function triggers [undefined behavior][ub] if `U` is larger than
-/// `T`.
+/// It is not a compile-time error if `Src` and `Dst` have different sizes, but it
+/// is highly encouraged to only invoke this function where `Src` and `Dst` have the
+/// same size. This function triggers [undefined behavior][ub] if `Dst` is larger than
+/// `Src`.
 ///
 /// [ub]: ../../reference/behavior-considered-undefined.html
 ///
@@ -1052,19 +1052,22 @@ pub fn copy<T: Copy>(x: &T) -> T {
 #[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_transmute_copy", issue = "83165")]
-pub const unsafe fn transmute_copy<T, U>(src: &T) -> U {
-    assert!(size_of::<T>() >= size_of::<U>(), "cannot transmute_copy if U is larger than T");
+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"
+    );
 
-    // If U has a higher alignment requirement, src might not be suitably aligned.
-    if align_of::<U>() > align_of::<T>() {
+    // If Dst has a higher alignment requirement, src might not be suitably aligned.
+    if align_of::<Dst>() > align_of::<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 T as *const U) }
+        unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }
     } else {
         // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
-        // We just checked that `src as *const U` was properly aligned.
+        // We just checked that `src as *const Dst` was properly aligned.
         // The caller must guarantee that the actual transmutation is safe.
-        unsafe { ptr::read(src as *const T as *const U) }
+        unsafe { ptr::read(src as *const Src as *const Dst) }
     }
 }
 

library/core/tests/mem.rs

@@ -130,7 +130,11 @@ fn test_transmute_copy_grow_panics() {
             payload
                 .downcast::<&'static str>()
                 .and_then(|s| {
-                    if *s == "cannot transmute_copy if U is larger than T" { Ok(s) } else { Err(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));
         }

src/test/ui/issues/issue-6458-3.stderr

@@ -2,12 +2,12 @@ error[E0282]: type annotations needed
   --> $DIR/issue-6458-3.rs:4:5
    |
 LL |     mem::transmute(0);
-   |     ^^^^^^^^^^^^^^ cannot infer type of the type parameter `U` declared on the function `transmute`
+   |     ^^^^^^^^^^^^^^ cannot infer type of the type parameter `Dst` declared on the function `transmute`
    |
 help: consider specifying the generic arguments
    |
-LL |     mem::transmute::<i32, U>(0);
-   |                   ++++++++++
+LL |     mem::transmute::<i32, Dst>(0);
+   |                   ++++++++++++
 
 error: aborting due to previous error