Backport pr 31442 to beta: Split dummy-idx node to fix expand_givens DFS #31768
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…odes. Why do this: The RegionGraph representation previously conflated all of the non-variable regions (i.e. the concrete regions such as lifetime parameters to the current function) into a single dummy node. A single dummy node leads DFS on a graph `'a -> '_#1 -> '_#0 -> 'b` to claim that `'_#1` is reachable from `'_#0` (due to `'a` and `'b` being conflated in the graph representation), which is incorrect (and can lead to soundness bugs later on in compilation, see rust-lang#30438). Splitting the dummy node ensures that DFS will never introduce new ancestor relationships between nodes for variable regions in the graph.
alexcrichton
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Feb 19, 2016
Backport pr 31442 to beta: Split dummy-idx node to fix expand_givens DFS
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Backport pr #31442 to beta