Unfortunate comparison between const and #define in Rust Book 5.27

[matthieu-m]

At the end of Rust Book chapter 5.27 const and static is the following extract:

A const can be thought of as a #define in C: it has metadata overhead but it has no runtime overhead. “Should I use a #define or a static in C,” is largely the same question as whether you should use a const or a static in Rust.

Unfortunately, this is rather misleading. An experienced C programmer will try and avoid #define as much as possible (they do have their use) because:

• a #define is unscoped, and the C preprocessor is just a text preprocessor
• a #define is untyped

You can check constants - "static const" vs "define" in C for more context and relevant advices; specifically Jonathan Leffer's answer (yep, just skip mine). Note: in C++, the answer is static const because C++ lifts a lot of restrictions for where one can use static const variables.

Since in Rust const is scoped and typed, it is really no equivalent to #define. It is, however, much closer to enum.

Maybe simply comparing to enum instead of #define would avoid confusing the reader?

[steveklabnik]

We had a chat about this on Reddit, I wanted an issue here for more visibility. Thoughts, anyone else?

[alexcrichton]

I think it would be fine to add some clarification here, but the emphasis is that the definition is inlined everywhere (like #define), not so much the odd semantics of #define