diff options
| -rw-r--r-- | library/core/src/intrinsics.rs | 6 |
1 files changed, 4 insertions, 2 deletions
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs index f4234105ac6..37940c79213 100644 --- a/library/core/src/intrinsics.rs +++ b/library/core/src/intrinsics.rs @@ -1212,8 +1212,8 @@ 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` contains padding, that padding - /// might *not* be preserved by `transmute`. + /// are passed by-value, which means if `T` or `U` 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 @@ -1225,6 +1225,8 @@ extern "rust-intrinsic" { /// /// Transmuting pointers to integers in a `const` context is [undefined behavior][ub]. /// Any attempt to use the resulting value for integer operations will abort const-evaluation. + /// (And even outside `const`, such transmutation is touching on many unspecified aspects of the + /// Rust memory model and should be avoided. See below for alternatives.) /// /// 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 |