Compute localized outlives constraints lazily #150551
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now that the analysis is only using the regular liveness shape, we don't need to store it transposed, and thus don't need the container where it was stored: the liveness context would be alone in the polonius context. we thus remove the latter, and rename the former.
we may need to traverse the lazy graph multiple times: - to record loan liveness - to dump the localized outlives constraint in the polonius MIR dump to do that we extract the previous loan liveness code into an abstract traversal + visitor handling the liveness-specific parts, while the MIR dump will be able to record constraints in its own visitor.
now that we need to hold the graph for MIR dumping, and the associated data to traverse it, there is no difference between the main context and diagnostics context, so we merge them.
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This PR rewrites the loan liveness algorithm to compute localized constraints lazily during traversal, to avoid the sometimes costly conversion and indexing happening in the current eager algorithm (or for now as well, the need to reshape the liveness data to better list regions live at a given point).
It thus greatly reduces the current alpha overhead, although it wasn't entirely removed of course (we're obviously doing more work to improve precision): the worst offending benchmark has a +5-6% wall-time regression — but icounts are worse looking (+13%) rn.
Best reviewed per-commit, as steps are in sequence to simplify the process or unify some things.
r? @jackh726