dtolnay/cxx

Permissibility heuristics for signatures involving lifetimes

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#165 opened on Apr 28, 2020

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help wanted

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

Tech stack
rust
Domain
backenddeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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.
mostly clear
Prerequisites
RustlifetimesCXX internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
This issue proposes adding heuristics to permit function signatures with lifetime elision beyond current CXX limitations, as motivated by a comment in issue #122. The goal is to allow patterns like fn(&mut CThing, &str) > &mut CThingChild without explicit lifetime annotations, leveraging the obvious borrowing constraints. To implement, one would study the existing CXX codebase (likely the code generation for signatures and lifetime handling in the `syntax` module) and look at Rust's own lifetime elision rules for references. The key challenge is to safely infer that the output borrows from the relevant input arguments while disallowing ambiguous cases. Discussions in linked comments should be reviewed for design constraints.