Improve hir::map::Map::get_parent_node doc #43605
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Improve hir::map::Map::get_parent_node doc
The documentation says
```
/// Similar to get_parent, returns the parent node id or id if there is no
/// parent.
/// This function returns the immediate parent in the AST, whereas get_parent
/// returns the enclosing item.
```
One would think that one can walk up the tree by repeatedly calling `get_parent_node` until it returns the argument, and then work on the `NodeId`s that arise. However, that is not true: `get_parent_node` will return id 0 (the crate itself) for items that sit directly in the crate; calling `get` on that `NodeId` will panic.
So, the fact that `get_parent_node` returns the root when passed the root is actually not really useful, because the root itself is already a somewhat degenerate node. This improves the doc so hopefully people writing code that "walks up the tree" don't run into this issue like I did...
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The documentation says
One would think that one can walk up the tree by repeatedly calling
get_parent_nodeuntil it returns the argument, and then work on theNodeIds that arise. However, that is not true:get_parent_nodewill return id 0 (the crate itself) for items that sit directly in the crate; callinggeton thatNodeIdwill panic.So, the fact that
get_parent_nodereturns the root when passed the root is actually not really useful, because the root itself is already a somewhat degenerate node. This improves the doc so hopefully people writing code that "walks up the tree" don't run into this issue like I did...