PCC: switch "max" facts to "range" facts with two-sided ranges. #7263
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Oct 17, 2023
This is needed for soundness when verifying accesses to memtype fields: it's not enough to know that we're accessing an offset in `0` up to `field_offset` inclusive, we need to know the access is actually to `field_offset`. The simplest change that validates this turned out to be the most general one: making ranges two-sided rather than one-sided. The transform is *mostly* mechanical, but a few new tests verify that ranges are updated on both sides, and some fail-tests verify that "fuzzily imprecise" pointers to struct fields fail to validate.
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This is needed for soundness when verifying accesses to memtype fields: it's not enough to know that we're accessing an offset in
0up tofield_offsetinclusive, we need to know the access is actually tofield_offset.The simplest change that validates this turned out to be the most general one: making ranges two-sided rather than one-sided. The transform is mostly mechanical, but a few new tests verify that ranges are updated on both sides, and some fail-tests verify that "fuzzily imprecise" pointers to struct fields fail to validate.