Durability by nikomatsakis · Pull Request #180 · salsa-rs/salsa

We now track the last revision in which constants were modified. When
we see a constant query result, we record the current revision as
well. Then later we can check if the result is "still" constant. This
lets us cut out a lot of intermediate work.

It didn't seem like it was buying us much.

Random bool constants are just so gauche.

This will be important if we want to automatically upgrade the
durability of inputs.

pure refactoring

The old name was really awkward.

Also write some docs explaining its side-effects.