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Clarify Box<T> representation and its use in FFI #62514

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merged 8 commits into from
Dec 12, 2019
Merged

Clarify Box<T> representation and its use in FFI #62514

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318c5d6
Clarify `Box<T>` representation and its use in FFI
stephaneyfx Jul 9, 2019
aea9423
Update Box representation comment based on reviews
stephaneyfx Aug 26, 2019
812ec6a
Update FFI example
stephaneyfx Oct 30, 2019
ead1159
Use Niko's wording
stephaneyfx Dec 10, 2019
fe6ddd5
Specify behavior when passed a null pointer
stephaneyfx Dec 10, 2019
1a26df7
Remove trailing whitespace
stephaneyfx Dec 10, 2019
cb1cc11
Fix description based on review
stephaneyfx Dec 11, 2019
fafa489
clarify that `Box<T>` should only be used when defined *in Rust*
Dec 11, 2019
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22 changes: 22 additions & 0 deletions src/liballoc/boxed.rs
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Expand Up @@ -63,6 +63,28 @@
//! T` obtained from `Box::<T>::into_raw` may be deallocated using the
//! [`Global`] allocator with `Layout::for_value(&*value)`.
//!
//! `Box<T>` has the same representation as `*mut T`. In particular, when
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//! `T: Sized`, this means that `Box<T>` has the same representation as
//! a C pointer, making the following code valid in FFI:
//!
//! ```c
//! /* C header */
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//! struct Foo* foo(); /* Returns ownership */
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"Returns" is ambiguous - can be construed as returning ownership to the function. I would say "Returns ownership to the caller." or "Gives ownership to the caller.".

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Proper C syntax is struct Foo* foo(void);.

//! void bar(struct Foo*); /* `bar` takes ownership */
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This comment has the format "bar takes.." but the comment above is not "foo returns ...". So to be consistent, would change the comment to "Takes ownership from the caller.".

//! ```
//!
//! ```
//! #[repr(C)]
//! pub struct Foo;
//!
//! #[no_mangle]
//! pub extern "C" fn foo() -> Box<Foo> {
//! Box::new(Foo)
//! }
//!
//! #[no_mangle]
//! pub extern "C" fn bar(_: Option<Box<Foo>>) {}
//! ```
//!
//! [dereferencing]: ../../std/ops/trait.Deref.html
//! [`Box`]: struct.Box.html
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