From 2e0ca9472ba3ec1532bb752f7ea1f477f8c34c90 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sat, 9 Jul 2022 10:48:43 -0400 Subject: [PATCH] add a concrete example --- library/core/src/intrinsics.rs | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs index 4c8619f3135..d7ed82e71b6 100644 --- a/library/core/src/intrinsics.rs +++ b/library/core/src/intrinsics.rs @@ -2550,14 +2550,23 @@ pub const unsafe fn copy(src: *const T, dst: *mut T, count: usize) { /// /// * `dst` must be properly aligned. /// -/// Additionally, note that changing `*dst` in this way can lead to undefined behavior later if the -/// written bytes are not a valid representation of some `T`. For instance, if `dst: *mut bool`, a -/// `dst.write_bytes(0xFFu8, 1)` followed by `dst.read()` is undefined behavior since the `read` -/// tries to construct a `bool` value from `0xFF` which does not represent any `bool`. -/// /// Note that even if the effectively copied size (`count * size_of::()`) is /// `0`, the pointer must be non-null and properly aligned. /// +/// Additionally, note that changing `*dst` in this way can easily lead to undefined behavior (UB) +/// later if the written bytes are not a valid representation of some `T`. For instance, the +/// follwing is an **incorrect** use of this function: +/// +/// ```rust,no_run +/// unsafe { +/// let mut value: u8 = 0; +/// let ptr: *mut bool = &mut value as *mut u8 as *mut bool; +/// let _bool = ptr.read(); // This is fine, `ptr` points to a valid `bool`. +/// ptr.write_bytes(42u8, 1); // This function itself does not cause UB... +/// let _bool = ptr.read(); // ...but it makes this operation UB! ⚠️ +/// } +/// ``` +/// /// [valid]: crate::ptr#safety /// /// # Examples