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Auto merge of #37192 - cristicbz:rust-rc-into-raw, r=brson
Add `{into,from}_raw` to Rc and Arc These methods convert to and from a `*const T` for `Rc` and `Arc` similar to the way they work on `Box`. The only slight complication is that `from_raw` needs to offset the pointer back to find the beginning of the `RcBox`/`ArcInner`. I felt this is a fairly small addition, filling in a gap (when compared to `Box`) so it wouldn't need an RFC. The motivation is primarily for FFI. (I'll create an issue and update a PR with the issue number if reviewers agree with the change in principle **Edit: done #37197**) ~~Edit: This was initially `{into,from}_raw` but concerns were raised about the possible footgun if mixed with the methods of the same name of `Box`.~~ Edit: This was went from `{into,from}_raw` to `{into,from}_inner_raw` then back to `{into,from}_raw` during review.
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// Copyright 2013-2016 The Rust Project Developers. See the COPYRIGHT | ||
// file at the top-level directory of this distribution and at | ||
// http://rust-lang.org/COPYRIGHT. | ||
// | ||
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
// option. This file may not be copied, modified, or distributed | ||
// except according to those terms. | ||
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// Private macro to get the offset of a struct field in bytes from the address of the struct. | ||
macro_rules! offset_of { | ||
($container:path, $field:ident) => {{ | ||
// Make sure the field actually exists. This line ensures that a compile-time error is | ||
// generated if $field is accessed through a Deref impl. | ||
let $container { $field : _, .. }; | ||
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// Create an (invalid) instance of the container and calculate the offset to its | ||
// field. Using a null pointer might be UB if `&(*(0 as *const T)).field` is interpreted to | ||
// be nullptr deref. | ||
let invalid: $container = ::core::mem::uninitialized(); | ||
let offset = &invalid.$field as *const _ as usize - &invalid as *const _ as usize; | ||
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// Do not run destructors on the made up invalid instance. | ||
::core::mem::forget(invalid); | ||
offset as isize | ||
}}; | ||
} |
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