rustdocs "The Book" Chapter 3 inaccuracy.

[kkh3049]

I'm not sure if this is the right place or labels for this and didn't find any specific format for errors in "The book", but hope someone can direct me if needed. I'm new to rust completely but excited to learn and contribute where I can.

In "The book" chapter 3 section two, towards the end of the section (https://doc.rust-lang.org/book/ch03-02-data-types.html#invalid-array-element-access), it claims that a certain code snippet will compile fine, but panic at runtime, but the code snippet actually doesn't compile. I imagine it's important to mention that panicking happens at runtime with similar conditions that aren't catchable at compile-time, but it's inaccurate. Here's the error that I get.

$ cargo run
   Compiling book_error v0.1.0 (D:\DDocuments\PersonalProjects\LearningRust\book_error)
error: this operation will panic at runtime
 --> src\main.rs:5:19
  |
5 |     let element = a[index];
  |                   ^^^^^^^^ index out of bounds: the len is 5 but the index is 10
  |
  = note: `#[deny(unconditional_panic)]` on by default

error: aborting due to previous error

error: could not compile `book_error`.

To learn more, run the command again with --verbose.

Here's the error mentioned in the book:

$ cargo run
   Compiling arrays v0.1.0 (file:///projects/arrays)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/arrays`
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:5:19
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[Lonami]

Rust got smarter constant evaluation since that was written it seems. The following doesn't fail to compile but errors at runtime:

fn get_index() -> usize {
    10
}

fn main() {
    let a = [1, 2, 3, 4, 5];
    let index = get_index();

    let element = a[index];

    println!("The value of element is: {}", element);
}

It does make the example a tiny bit worse, but showcases the point.

[workingjubilee]

This should probably be moved to https://github.com/rust-lang/book

[kkh3049]

Turns out there's an open issue about this at rust-lang/book#2417
I'll close this. Thanks!