mozilla/mentat

Panic when leaving unused `:in` variable unbound

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#654 opened on Apr 23, 2018

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Description

When leaving an :in variable unbound, like in:

[:find
 ?e
 :in
 ?f ?unused
 :where
 [(= e f)]
]

After binding only ?f, we panic at https://github.com/mozilla/mentat/blob/0c31fc7875294fdf80f4caf253590999a9214e0d/query-algebrizer/src/clauses/mod.rs#L147

That code is trying to be careful, but the issue is that the subtraction underflows in the above circumstance, since we're subtracting usize instances.

Contributor guide

Research direction
Inspect the panic location at query algebrizer/src/clauses/mod.rs:147 and ensure the subtraction of usize does not underflow when an `:in` variable is left unbound.
Tech stack
rust
Domain
databasebackend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
RustUnderstanding of usize arithmetic
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70