Clarify preconditions of raw size/align methods

library/core/src/alloc/layout.rs

@@ -226,7 +226,9 @@ impl Layout {
     ///
     /// # Safety
     ///
-    /// This function is only safe to call if the following conditions hold:
+    /// This function is safe to call if the pointer is safe to reborrow as `&T`
+    /// (in which case you could also call [`for_value`][Self::for_value]).
+    /// Otherwise, the following conditions must hold:
     ///
     /// - If `T` is `Sized`, this function is always safe to call.
     /// - If the unsized tail of `T` is:

library/core/src/mem/alignment.rs

@@ -114,7 +114,9 @@ impl Alignment {
     ///
     /// # Safety
     ///
-    /// This function is only safe to call if the following conditions hold:
+    /// This function is safe to call if the pointer is safe to reborrow as `&T`
+    /// (in which case you could also call [`of_val`][Self::of_val]).
+    /// Otherwise, the following conditions must hold:
     ///
     /// - If `T` is `Sized`, this function is always safe to call.
     /// - If the unsized tail of `T` is:

library/core/src/mem/mod.rs

@@ -419,7 +419,9 @@ pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
 ///
 /// # Safety
 ///
-/// This function is only safe to call if the following conditions hold:
+/// This function is safe to call if the pointer is safe to reborrow as `&T`
+/// (in which case you could also call [`size_of_val`]).
+/// Otherwise, the following conditions must hold:
 ///
 /// - If `T` is `Sized`, this function is always safe to call.
 /// - If the unsized tail of `T` is:
@@ -592,7 +594,9 @@ pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
 ///
 /// # Safety
 ///
-/// This function is only safe to call if the following conditions hold:
+/// This function is safe to call if the pointer is safe to reborrow as `&T`
+/// (in which case you could also call [`align_of_val`]).
+/// Otherwise, the following conditions must hold:
 ///
 /// - If `T` is `Sized`, this function is always safe to call.
 /// - If the unsized tail of `T` is: