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Commit60c2d80

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Rollup merge of#125739 - RalfJung:drop-in-place-docs, r=workingjubilee
drop_in_place: weaken the claim of equivalence with drop(ptr.read())The two are *not* semantically equivalent in all cases, so let's not be so definite about this.Fixes#112015
2 parents7ed5d46 +5c68a15 commit60c2d80

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‎library/core/src/ptr/mod.rs‎

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@@ -450,8 +450,13 @@ mod mut_ptr;
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/// Executes the destructor (if any) of the pointed-to value.
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///
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/// This issemantically equivalent to calling [`ptr::read`] and discarding
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/// This isalmost the same as calling [`ptr::read`] and discarding
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/// the result, but has the following advantages:
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// FIXME: say something more useful than "almost the same"?
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// There are open questions here: `read` requires the value to be fully valid, e.g. if `T` is a
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// `bool` it must be 0 or 1, if it is a reference then it must be dereferenceable. `drop_in_place`
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// only requires that `*to_drop` be "valid for dropping" and we have not defined what that means. In
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// Miri it currently (May 2024) requires nothing at all for types without drop glue.
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///
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/// * It is *required* to use `drop_in_place` to drop unsized types like
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/// trait objects, because they can't be read out onto the stack and
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// Miri currently doesn't require types without drop glue to be
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// valid when dropped. This test confirms that behavior.
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// This is not a stable guarantee!
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use std::ptr;
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fnmain(){
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letmut not_a_bool =13u8;
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unsafe{
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ptr::drop_in_place(&mut not_a_boolas*mutu8as*mutbool)
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};
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}

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