Permissibility heuristics for signatures involving lifetimes · dtolnay/cxx#165

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help wanted

説明

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.

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技術スタック: rust
領域: backenddeveloper experience
Issue 種別: feature
難度: 5
推定時間: over 1 week
活動状況: stale
明確さ: mostly clear
前提条件: RustlifetimesCXX internals
初心者向け度: 10
調査方針: 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.