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Improve raw Box conversions #44877

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Merged
merged 7 commits into from
Oct 10, 2017
Merged

Improve raw Box conversions #44877

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edb2c2d
Remove uses of mem::transmute in Box methods
nvzqz Sep 27, 2017
8f167b3
Merge remote-tracking branch 'upstream/master' into box-conversions
nvzqz Sep 27, 2017
904133e
Revert to using mem::transmute in Box::into_unique
nvzqz Oct 6, 2017
452b71a
Revert to using mem::transmute in Box::from_raw
nvzqz Oct 6, 2017
5ce5b2f
Create Box::from_unique function
nvzqz Oct 6, 2017
5af88ee
Add missing word in Box::from_unique docs
nvzqz Oct 6, 2017
22298b8
Add unique feature in Box::from_unique docs
nvzqz Oct 6, 2017
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8 changes: 5 additions & 3 deletions src/liballoc/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -269,7 +269,7 @@ impl<T: ?Sized> Box<T> {
#[stable(feature = "box_raw", since = "1.4.0")]
#[inline]
pub unsafe fn from_raw(raw: *mut T) -> Self {
mem::transmute(raw)
Box(Unique::new_unchecked(raw))
}

/// Consumes the `Box`, returning the wrapped raw pointer.
Expand All @@ -295,7 +295,7 @@ impl<T: ?Sized> Box<T> {
#[stable(feature = "box_raw", since = "1.4.0")]
#[inline]
pub fn into_raw(b: Box<T>) -> *mut T {
unsafe { mem::transmute(b) }
Box::into_unique(b).as_ptr()
}

/// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
Expand Down Expand Up @@ -326,7 +326,9 @@ impl<T: ?Sized> Box<T> {
issue = "27730")]
#[inline]
pub fn into_unique(b: Box<T>) -> Unique<T> {
unsafe { mem::transmute(b) }
let u = b.0;
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@nagisa nagisa Sep 28, 2017 •
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The error is most likely arising due to this line in particular.

If it is indeed the culprit, it is pretty easy to see why is this a case -- namely within the compiler it is considered that Box is pretty much equivalent to a non-aliasable *mut T, and you cannot just take fields of *mut Ts without deferencing first -- hence the ICE you see.

If you want, you could try fixing it in the compiler, but I’m fairly sure it would be very messy to do so properly.

Disclaimer: This is a guess.

mem::forget(b);
u
}
}

Expand Down