libexpr: Remember intermediate values on exception

[roberth]

Motivation

Preserve sharing in case of an exception. When an exception occurs, the original expression, or a similar expression with shared values may be retried. Intermediate values should be retained as much as possible.

TODO:

• Thoroughly check that this code is correct and that the tests cover all the changes. I originally wrote this as an experiment and I don't think it is ready for production.

Context

• Does not solve throwing/asserting in tryEval has exponential complexity after evaluating certain values #8530 (yet?)

We want the rules about memoization of let bindings to hold even when an exception occurs.

This is slightly counterintuitive, because we might think that we'd have better luck next time if we didn't memoize the result of the let binding.
However this contradicts the design goal of having a declarative language and therefore idempotent evaluation.

So instead of maintaining doubt about this situation, we choose to lean into the language semantics and exploit the idempotency. This improves the performance asymptotically and significantly in scenarios where we may evaluate more than once. These scenarios include:

• tryEval has been used, such as in these test cases, or
• traversal of multiple attributes, such as in nix-build with recurseForDerivations,
• applications that handle missing information by throwing an exception to be handled by a convergent main loop,
• re-parsing a file that used to have a syntax error in nix repl, without using :r.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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