Forms a slice from a pointer and a length. This function is safe, but actually using the return value is unsafe. Function std::slice::from_raw_parts_mut pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T]Notable traits for &[u8]impl Read for &[u8]impl Write for &mut [u8] Performs the same functionality as from_raw_parts, except that a mutable slice is returned. Function std::slice::from_raw_parts pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T]Notable traits for &'_ [u8]impl<'_> Read for &'_ [u8]impl<'_> Write for &'_ mut [u8] Forms a slice from a pointer and a length. This function is safe but the returned pointer is not necessarily safe to dereference. from_raw_parts, except that a mutable slice is returned. Specifically, T must have the same size as alignment as the original T. u32 is four times as big as u8, so this again breaks this requirement. Rust emphasizes performance, type safety, and concurrency. Forms a slice from a pointer range. When you write std::slice::from_raw_parts_mut (data.as_ptr (), len) you are building a value of type &mut [i32]. Converts a reference to T into a slice of length 1 (without copying). Use from_raw_parts to convert the pointer and length into a slice. For slices, see the documentation of slice::from_raw_parts for safety requirements. To simultaneously enforce memory safety and prevent concurrent data races, Rust. C Even if capacity*size remains the same, size isn't, and capacity isn't. This function will never be sound as implemented. from a Vec), it is the base parameter to that method. Rust could gain some memory utilization there, removing clown shoes, without breaking any current Vec API. This is an unsafe operation because we may be dereferencing invalid memory. You could also do this using Vec::from_raw_parts (ptr, len, len).into_boxed_slice () Guaranteed to work and not touch the . let slice = slice::from_raw_parts_mut(ptr, capacity); let boxed_slice: Box<[T]> = Box::from_raw(slice); . 1 Answer. Performs the same functionality as from_raw_parts, except that a mutable slice is returned.. Safety. The len argument is the number of elements, not the number of bytes.. Safety. This would require a change in the ptr module as well, as slice module just forwards to it.. slice::from_raw_parts[mut] is used in alot of places (e.g slice::from_ref[mut], which would get one step closer into constification if slice::from_raw_parts[mut] is a const fn. range Experimental Performs bounds-checking of a range. 14 comments . Behavior is undefined if any of the following conditions are violated: data must be valid for both reads and writes for len * mem::size_of::<T>() many bytes, and it must be properly aligned. The theory is that all capacities in the same bin . For trait objects, the metadata must come from a pointer to the same underlying erased type. from_mut Converts a reference to T into a slice of length 1 (without copying). Rust enforces memory safetythat is, that all references point to valid memorywithout requiring the use of a garbage collector or reference counting present in other memory-safe languages. I was quite sure that this is sufficiently documented. The len argument is the number of elements, not the number of bytes. First, the function as written is disregarding another requirement of from_raw_parts. Edit: Never mind, the solution is simply to do Box::from_raw (slice::from_raw_parts_mut (ptr, len))) (ref -> ptr coercion means this works). It requires looking at jemalloc's size bins and working out the relevant details. Safety Behavior is undefined if any of the following conditions are violated: data must be valid for both reads . If you want to obtain a slice from a raw pointer, use std::slice::from_raw_parts (): let slice = unsafe { std::slice::from_raw_parts (some_pointer, count_of_items) }; If you want to obtain a mutable slice from a raw pointer, use std::slice::from_raw_parts_mut (): let slice = unsafe { std::slice::from_raw_parts_mut (some_pointer, count . This function is unsafe as there is no guarantee that the given pointer is valid for len elements, nor whether the lifetime inferred is a suitable lifetime for the returned slice.. p must be non-null, even for zero-length slices, because non-zero bits are required to . While I could do this in my code, there's no guarantee of layout stability. Behavior is undefined if any of the following conditions are violated: data must be valid for reads for len * mem::size_of::<T>() many bytes, and it must be properly aligned. Then you are passing it to drop_in_place () that is defined more or less as: So you are coercing a &mut [i32] into a *mut T, that is solved in two steps: there is an automatic coercion from reference to pointer, and then T is . Performs the same functionality as slice_from_raw_parts, except that a raw mutable slice is returned, as opposed to a raw immutable slice. The len argument is the number of elements, not the number of bytes. Converting an array is a two-step process: Assert that the C pointer is not NULL as Rust references may never be NULL. This means in particular: The entire memory range of this slice must be contained within a single . See the documentation of slice . See the documentation of slice_from_raw_parts for more details. The len argument is the number of elements, not the number of bytes.. Safety. Forms a slice from a pointer and a length. Function std :: ptr :: slice_from_raw_parts. This means in particular: This function is safe, but actually using the return value is unsafe. pub const fn slice_from_raw_parts<T> (data: *const T, len: usize) -> *const [T] Forms a raw slice from a pointer and a length. Make slice::from_raw_parts and slice::from_raw_parts_mut a const fn available under a feature flag. Safety If the pointer was created by from_raw_parts (e.g. Forms a (possibly-wide) raw pointer from a data address and metadata. 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