A way to display *why* a predicate failed

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Output of case_tree example:

var is 5
((! var != 5) && var >= 5)
├── (! var != 5)
|   └── var != 5
└── var >= 5

An extension trait is added to make a Displayable struct that will print out the above tree.

• This is built off of find_case which will find the Case that prove whether the Predicate succeeds or fails.
• To reduce overhead, Case only owns by-Products of the Predicate evaluation, parameters are made available as references on the PredicateReflection trait
  • This had the benefit of simplifying the Case API because Parameter and Product have different ownership requirements.
• PredicateReflection is also use to homogenize the predicate tree
  • Because of predicate adapters, you could have a tree that includes Predicate<[u8]>, Predicate<str>, etc.

Once this is in, to reduce API breaks from impacting the assert-ecosystem, #30 will be implemented and this repo will be turned into a workspace for

• predicate-core: core.rs, reflection.rs
• predicate-tree: tree.rs
• predicates
  • All existing predicates
  • Re-export predicate-core

aka the only change to most end-users will be predicate-tree being removed from the API while the assert-ecosystem will use predicate_core::Predicate in its public API.

This supersedes #39 due to having an API that we are more comfortable moving to 1.0

• Unlike WIP: Pretty error printer for predicates. #39, we do not include item in the Display, allowing us to avoid requiring Item: Debug. Instead we just rely on every Predicate implementing Display, usually doing so by having an Item: Debug requirement but clients are free to bypass that by creating their own Predicate.

BREAKING CHANGES

• Predicates must also implement PredicateReflection. Provided methods make this a impl PredicateReflection for X {}

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@nastevens my main concern with having this reviewed is regarding vision

• Does this approach to An eval that tells me why the predicate failed #7 raise concerns with where this crate is going?
  • like the additional requirements to implement a Predicate
  • like Predicate::find_case
  • like the additional predicate policy in reflection.rs to describe predicate reflection and the results of Predicate::find_case.
• Are there concerns with the plan for Decouple Predicate from implementations #30?

Luckily, I was able to avoid trait Predicate where Item: Debug but I do have Predicate : PredicateReflection : Display. I tried to keep the requirements for implementing a Predicate to a minimum, allowing people to write more code to opt-in to richer reporting features.

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@nastevens btw for time table, the goal is to have this PR in, #30 in, and assert_fs / assert_cmd switched over to these changes by the end of the month in preparation for the next 2018 Edition preview.