The Reference is not a formal spec, but is more detailed and comprehensive than the book.

This book is the primary reference for the Rust programming language. It provides three kinds of material:

• Chapters that informally describe each language construct and their use.
• Chapters that informally describe the memory model, concurrency model, runtime services, linkage model, and debugging facilities.
• Appendix chapters providing rationale and references to languages that influenced the design.

Warning:

• This book is incomplete. Documenting everything takes a while.
• This document is not normative. It may include details that are specific to rustc itself, and should not be taken as a specification for the Rust language. We intend to produce such a document someday, but this is what we have for now.