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Commit b22559f

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Rollup merge of #103346 - HeroicKatora:metadata_of_const_pointer_argument, r=dtolnay
Adjust argument type for mutable with_metadata_of (#75091) The method takes two pointer arguments: one `self` supplying the pointer value, and a second pointer supplying the metadata. The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true. In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. [Example](197g/static-alloc@cad9377). ```rust // Manually taking an unsized object from a `ManuallyDrop` into another allocation. let val: &core::mem::ManuallyDrop<T> = ...; let ptr = val as *const _ as *mut T; let ptr = uninit.as_ptr().with_metadata_of(ptr); ``` This could then instead be simplified to: ```rust // Manually taking an unsized object from a `ManuallyDrop` into another allocation. let val: &core::mem::ManuallyDrop<T> = ...; let ptr = uninit.as_ptr().with_metadata_of(&**val); ``` Tracking issue: #75091 ``@dtolnay`` you're reviewed #95249, would you mind chiming in?
2 parents 3f49f95 + e3606b2 commit b22559f

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3 files changed

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-3
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3 files changed

+7
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‎library/alloc/src/rc.rs‎

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@@ -1386,7 +1386,7 @@ impl<T: ?Sized> Rc<T> {
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Self::allocate_for_layout(
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Layout::for_value(&*ptr),
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|layout| Global.allocate(layout),
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|mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
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|mem| mem.with_metadata_of(ptr as *const RcBox<T>),
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)
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}
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}

‎library/alloc/src/sync.rs‎

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@@ -1204,7 +1204,7 @@ impl<T: ?Sized> Arc<T> {
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Self::allocate_for_layout(
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Layout::for_value(&*ptr),
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|layout| Global.allocate(layout),
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|mem| mem.with_metadata_of(ptr as *mut ArcInner<T>),
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|mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
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)
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}
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}

‎library/core/src/ptr/mut_ptr.rs‎

Lines changed: 5 additions & 1 deletion
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@@ -80,10 +80,14 @@ impl<T: ?Sized> *mut T {
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#[unstable(feature = "set_ptr_value", issue = "75091")]
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#[must_use = "returns a new pointer rather than modifying its argument"]
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#[inline]
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pub fn with_metadata_of<U>(self, mutval: *mut U) -> *mut U
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pub fn with_metadata_of<U>(self, val: *const U) -> *mut U
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where
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U: ?Sized,
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{
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// Prepare in the type system that we will replace the pointer value with a mutable
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// pointer, taking the mutable provenance from the `self` pointer.
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let mut val = val as *mut U;
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// Pointer to the pointer value within the value.
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let target = &mut val as *mut *mut U as *mut *mut u8;
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// SAFETY: In case of a thin pointer, this operations is identical
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// to a simple assignment. In case of a fat pointer, with the current

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