dtolnay/cxx

Permissibility heuristics for signatures involving lifetimes

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#165 opened on Apr 28, 2020

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help wanted

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Description

As motivated in https://github.com/dtolnay/cxx/issues/122#issuecomment-615372359, it may be nice to permit a signature like fn(&mut CThing, &str) -> &mut CThingChild even though we don't support explicit lifetime specifications in full generality. In that signature it's obvious the output mutable reference isn't going to be borrowed from inside the contents of the const &str, and it would be a big red flag in the C++ code if it were, so this is fine to allow.

Contributor guide

Research direction
Investigate the current lifetime handling in cxx's signature parsing and code generation. The goal is to add a heuristic that permits a function signature like `fn(&mut CThing, &str) > &mut CThingChild` even without explicit lifetime annotations, by verifying that the output mutable reference does not alias any input const reference. Look at how cxx currently rejects such signatures and implement a relaxation that allows the safe pattern.
Tech stack
rust
Domain
api
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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
Rust
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60