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Add more explanation for why the assumes are there
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src/liballoc/rc.rs

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Original file line numberDiff line numberDiff line change
@@ -909,6 +909,8 @@ trait RcBoxPtr<T> {
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fn inc_strong(&self) {
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let strong = self.strong();
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// The reference count is always at least one unless we're about to drop the type
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// This allows the bulk of the destructor to be omitted in cases where we know that
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// the reference count must be > 0.
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unsafe { assume(strong > 0); }
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self.inner().strong.set(strong + 1);
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}
@@ -917,6 +919,8 @@ trait RcBoxPtr<T> {
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fn dec_strong(&self) {
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let strong = self.strong();
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// The reference count is always at least one unless we're about to drop the type
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// This allows the bulk of the destructor to be omitted in cases where we know that
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// the reference count must be > 0
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unsafe { assume(strong > 0); }
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self.inner().strong.set(strong - 1);
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}
@@ -936,7 +940,9 @@ impl<T> RcBoxPtr<T> for Rc<T> {
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fn inner(&self) -> &RcBox<T> {
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unsafe {
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// Safe to assume this here, as if it weren't true, we'd be breaking
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// the contract anyway
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// the contract anyway.
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// This allows the null check to be elided in the destructor if we
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// manipulated the reference count in the same function.
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assume(!self._ptr.is_null());
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&(**self._ptr)
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}
@@ -949,6 +955,8 @@ impl<T> RcBoxPtr<T> for Weak<T> {
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unsafe {
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// Safe to assume this here, as if it weren't true, we'd be breaking
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// the contract anyway
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// This allows the null check to be elided in the destructor if we
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// manipulated the reference count in the same function.
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assume(!self._ptr.is_null());
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&(**self._ptr)
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}

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