mirror of https://github.com/rust-lang/rust.git
BTree: evaluate static type-related check at compile time
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@ -315,7 +315,7 @@ impl<BorrowType: marker::BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type>
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pub fn ascend(
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self,
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) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
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assert!(BorrowType::PERMITS_TRAVERSAL);
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let _ = BorrowType::TRAVERSAL_PERMIT;
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// We need to use raw pointers to nodes because, if BorrowType is marker::ValMut,
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// there might be outstanding mutable references to values that we must not invalidate.
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let leaf_ptr: *const _ = Self::as_leaf_ptr(&self);
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@ -986,7 +986,7 @@ impl<BorrowType: marker::BorrowType, K, V>
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/// `edge.descend().ascend().unwrap()` and `node.ascend().unwrap().descend()` should
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/// both, upon success, do nothing.
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pub fn descend(self) -> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
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assert!(BorrowType::PERMITS_TRAVERSAL);
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let _ = BorrowType::TRAVERSAL_PERMIT;
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// We need to use raw pointers to nodes because, if BorrowType is
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// marker::ValMut, there might be outstanding mutable references to
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// values that we must not invalidate. There's no worry accessing the
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@ -1637,15 +1637,17 @@ pub mod marker {
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pub struct ValMut<'a>(PhantomData<&'a mut ()>);
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pub trait BorrowType {
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// Whether node references of this borrow type allow traversing
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// to other nodes in the tree.
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const PERMITS_TRAVERSAL: bool = true;
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// If node references of this borrow type allow traversing to other
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// nodes in the tree, this constant can be evaluated. Thus reading it
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// serves as a compile-time assertion.
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const TRAVERSAL_PERMIT: () = ();
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}
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impl BorrowType for Owned {
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// Traversal isn't needed, it happens using the result of `borrow_mut`.
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// Reject evaluation, because traversal isn't needed. Instead traversal
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// happens using the result of `borrow_mut`.
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// By disabling traversal, and only creating new references to roots,
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// we know that every reference of the `Owned` type is to a root node.
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const PERMITS_TRAVERSAL: bool = false;
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const TRAVERSAL_PERMIT: () = panic!();
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}
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impl BorrowType for Dying {}
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impl<'a> BorrowType for Immut<'a> {}
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