Return-position noalias · Issue #385 · rust-lang/unsafe-code-guidelines (original) (raw)

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Description

LLVM supports the noalias attribute in return position, and Rust uses that for functions that return Box. However the semantics of that are mostly unclear I think -- this has very little to do with argument-position noalias.

I think in Stacked Borrows terms it corresponds to something like: give the return value a fresh tag, and remove all other tags from the stack.

Questions:

• Is that the right semantics?
• Should there be some way to get return-position noalias without using Box? @gnzlbg recently mentioned a usecase for that.