| This is the code to load/save the dependency graph. Loading is assumed |
| to run early in compilation, and saving at the very end. When loading, |
| the basic idea is that we will load up the dependency graph from the |
| previous compilation and compare the hashes of our HIR nodes to the |
| hashes of the HIR nodes that existed at the time. For each node whose |
| hash has changed, or which no longer exists in the new HIR, we can |
| remove that node from the old graph along with any nodes that depend |
| on it. Then we add what's left to the new graph (if any such nodes or |
| edges already exist, then there would be no effect, but since we do |
| this first thing, they do not). |
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