In the talk "Proving Macro Expansion with Expandable," Sasha Pourcelot discusses the intricacies of Rust's declarative macros and introduces a tool called "expandable" designed to validate macro expansions. The presentation covers the differences between declarative and procedural macros, the concept of purity in macros, and the challenges of ensuring that macro expansions produce syntactically valid code. Pourcelot explains the underlying algorithms and state machine concepts used in the expandable tool, which can be integrated into Rust projects to enhance macro reliability, while also acknowledging its current limitations in handling complex macro scenarios.