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Update ptr docs with regards to ptr::addr_of!
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This updates the documentation since `ptr::addr_of!` and
`ptr::addr_of_mut!` are now stable. One might remove the distinction
between the sections `# On packed structs` and `# Examples`, as the old
section on packed structs was primarily to prevent users of doing unde-
fined behavior, which is not necessary anymore.
There is also a new section in "how to obtain a pointer", which referen-
ces the `ptr::addr_of!` macros.

This commit contains squashed commits from code review.

Co-authored-by: Joshua Nelson <[email protected]>
Co-authored-by: Mara Bos <[email protected]>
Co-authored-by: Soveu <[email protected]>
Co-authored-by: Ralf Jung <[email protected]>
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5 people committed May 3, 2021
1 parent 5208f63 commit 389333a
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61 changes: 23 additions & 38 deletions library/core/src/ptr/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -742,9 +742,6 @@ pub const unsafe fn read<T>(src: *const T) -> T {
///
/// ## On `packed` structs
///
/// It is currently impossible to create raw pointers to unaligned fields
/// of a packed struct.
///
/// Attempting to create a raw pointer to an `unaligned` struct field with
/// an expression such as `&packed.unaligned as *const FieldType` creates an
/// intermediate unaligned reference before converting that to a raw pointer.
Expand All @@ -753,9 +750,13 @@ pub const unsafe fn read<T>(src: *const T) -> T {
/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
/// *undefined behavior* in your program.
///
/// Instead you must use the [`ptr::addr_of!`](addr_of) macro to
/// create the pointer. You may use that returned pointer together with this
/// function.
///
/// An example of what not to do and how this relates to `read_unaligned` is:
///
/// ```no_run
/// ```
/// #[repr(packed, C)]
/// struct Packed {
/// _padding: u8,
Expand All @@ -767,24 +768,15 @@ pub const unsafe fn read<T>(src: *const T) -> T {
/// unaligned: 0x01020304,
/// };
///
/// #[allow(unaligned_references)]
/// let v = unsafe {
/// // Here we attempt to take the address of a 32-bit integer which is not aligned.
/// let unaligned =
/// // A temporary unaligned reference is created here which results in
/// // undefined behavior regardless of whether the reference is used or not.
/// &packed.unaligned
/// // Casting to a raw pointer doesn't help; the mistake already happened.
/// as *const u32;
/// // Take the address of a 32-bit integer which is not aligned.
/// // In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
/// let unaligned = std::ptr::addr_of!(packed.unaligned);
///
/// let v = std::ptr::read_unaligned(unaligned);
///
/// v
/// };
/// let v = unsafe { std::ptr::read_unaligned(unaligned) };
/// assert_eq!(v, 0x01020304);
/// ```
///
/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
// FIXME: Update docs based on outcome of RFC #2582 and friends.
///
/// # Examples
///
Expand Down Expand Up @@ -938,9 +930,6 @@ pub const unsafe fn write<T>(dst: *mut T, src: T) {
///
/// ## On `packed` structs
///
/// It is currently impossible to create raw pointers to unaligned fields
/// of a packed struct.
///
/// Attempting to create a raw pointer to an `unaligned` struct field with
/// an expression such as `&packed.unaligned as *const FieldType` creates an
/// intermediate unaligned reference before converting that to a raw pointer.
Expand All @@ -949,36 +938,32 @@ pub const unsafe fn write<T>(dst: *mut T, src: T) {
/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
/// *undefined behavior* in your program.
///
/// An example of what not to do and how this relates to `write_unaligned` is:
/// Instead you must use the [`ptr::addr_of_mut!`](addr_of_mut)
/// macro to create the pointer. You may use that returned pointer together with
/// this function.
///
/// An example of how to do it and how this relates to `write_unaligned` is:
///
/// ```no_run
/// ```
/// #[repr(packed, C)]
/// struct Packed {
/// _padding: u8,
/// unaligned: u32,
/// }
///
/// let v = 0x01020304;
/// let mut packed: Packed = unsafe { std::mem::zeroed() };
///
/// #[allow(unaligned_references)]
/// let v = unsafe {
/// // Here we attempt to take the address of a 32-bit integer which is not aligned.
/// let unaligned =
/// // A temporary unaligned reference is created here which results in
/// // undefined behavior regardless of whether the reference is used or not.
/// &mut packed.unaligned
/// // Casting to a raw pointer doesn't help; the mistake already happened.
/// as *mut u32;
/// // Take the address of a 32-bit integer which is not aligned.
/// // In contrast to `&packed.unaligned as *mut _`, this has no undefined behavior.
/// let unaligned = std::ptr::addr_of_mut!(packed.unaligned);
///
/// std::ptr::write_unaligned(unaligned, v);
/// unsafe { std::ptr::write_unaligned(unaligned, 42) };
///
/// v
/// };
/// assert_eq!({packed.unaligned}, 42); // `{...}` forces copying the field instead of creating a reference.
/// ```
///
/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
// FIXME: Update docs based on outcome of RFC #2582 and friends.
/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however
/// (as can be seen in the `assert_eq!` above).
///
/// # Examples
///
Expand Down
22 changes: 21 additions & 1 deletion library/std/src/primitive_docs.rs
Original file line number Diff line number Diff line change
Expand Up @@ -445,7 +445,27 @@ mod prim_unit {}
/// Note that here the call to [`drop`] is for clarity - it indicates
/// that we are done with the given value and it should be destroyed.
///
/// ## 3. Get it from C.
/// ## 3. Create it using `ptr::addr_of!`
///
/// Instead of coercing a reference to a raw pointer, you can use the macros
/// [`ptr::addr_of!`] (for `*const T`) and [`ptr::addr_of_mut!`] (for `*mut T`).
/// These macros allow you to create raw pointers to fields to which you cannot
/// create a reference (without causing undefined behaviour), such as an
/// unaligned field. This might be necessary if packed structs or uninitialized
/// memory is involved.
///
/// ```
/// #[derive(Debug, Default, Copy, Clone)]
/// #[repr(C, packed)]
/// struct S {
/// aligned: u8,
/// unaligned: u32,
/// }
/// let s = S::default();
/// let p = std::ptr::addr_of!(s.unaligned); // not allowed with coercion
/// ```
///
/// ## 4. Get it from C.
///
/// ```
/// # #![feature(rustc_private)]
Expand Down

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