Formally reasoning about Rust programs requires a rigorous formal semantics, especially in the context of deductive verification and concurrent programming. We present a modular, flexible semantics for a significant subset of (close to) source code level Rust, based on the recent locally abstract, globally concrete semantics framework, separating local evaluation of expressions from their composition into concrete traces. The semantics is extended to model Rust's asynchronous programming features and Rust's most popular async runtime, Tokio. Based on our more abstract formalization, we establish the fairness of Tokio's scheduler. Further, we show the applicability of our semantics to deductive verification of Rust by providing soundness proofs for a Rust program logic.

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