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Improve the example for ptr::copy #77385

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Oct 2, 2020
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Improve the example for ptr::copy #77385

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Improve the example for ptr::copy
scottmcm Oct 1, 2020
e58f3d3
Things are only moved if non-copy
scottmcm Oct 1, 2020
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12 changes: 11 additions & 1 deletion library/core/src/intrinsics.rs
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Expand Up @@ -1901,11 +1901,21 @@ pub unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
/// ```
/// use std::ptr;
///
/// /// # Safety:
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@pickfire pickfire Oct 1, 2020 •
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/// /// # Safety:
/// /// # Safety
/// ///

We usually have a space after Safety for heading and don't have : at the end.

/// /// * `ptr` must be correctly aligned for its type and non-zero.
/// /// * `ptr` must be valid for reads of `elts` contiguous objects of type `T`.
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@pickfire pickfire Oct 1, 2020 •
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I never heard of "contiguous objects"? Is this perhaps a new term? Also, s/of/for.

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/// /// * `ptr` must be valid for reads of `elts` contiguous objects of type `T`.
/// /// * `ptr` must be valid for reads of `elts` contiguous objects for type `T`.

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@jyn514 jyn514 Oct 1, 2020 •
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Try ctrl+f 'contiguous' on https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#guarantees

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Yes, contiguous is there but I never heard of contiguous objects. Try rg 'contiguous objects' in rust or https://github.com/rust-lang/rust/search?q=%22contiguous+objects%22

/// /// * Those elements must not be used after calling this function unless `T: Copy`.
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@pickfire pickfire Oct 1, 2020 •
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Is this required? Is it only limited for Copy?

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Yes, if these are not Copy you'll have a double-free. ptr::read does not drop or move the source buffer.

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Okay, I guess that's true but should this be part of safety? I think it could be but I wonder if this should be here.

/// # #[allow(dead_code)]
/// unsafe fn from_buf_raw<T>(ptr: *const T, elts: usize) -> Vec<T> {
/// let mut dst = Vec::with_capacity(elts);
/// dst.set_len(elts);
///
/// // SAFETY: Our precondition ensures the source is aligned and valid,
/// // and `Vec::with_capacity` ensures that we have usable space to write them.
/// ptr::copy(ptr, dst.as_mut_ptr(), elts);
///
/// // SAFETY: We created it with this much capacity earlier,
/// // and the previous `copy` has initialized these elements.
/// dst.set_len(elts);
/// dst
/// }
/// ```
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