Need some help to understand the behavior of Index trait on StringRecord

[meretciel]

rust version: rustc 1.71.0 (8ede3aae2 2023-07-12)
rust-csv version: 1.2.2

I don't know if I read the error message correctly, it seems that the index operator of StringBuilder class returns str instead of &str. Moreover, the implementation of the index function just calls the get method and the get returns &str. (see code here).

Sample Code:

    let mut reader = ReaderBuilder::new().from_path("your-csv-file.csv").unwrap();
    let header = reader.headers().unwrap();
    let t = header[0];  // This does not build, see attached error message below. If this line is 
                                  // commented out, then the program can build.
    let t2 = header.get(0);

It give the following error:

164 |     let t = header[0];
    |         ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature
help: consider borrowing here
    |
164 |     let t = &header[0];
    |    

I'm wondering if there is any explanation of this behavior (It's surprising that we can even return a str in a function). Appreciate any helps in advance. Thanks!

[BurntSushi]

The error is telling you what you should do: use &header[0] instead of header[0]. The key here is that header[0] is actually syntactic sugar for *header.index(0). Since the Index impl for StringRecord returns a &str, it follows that the type of *header.index(0) is str. Since str is a dynamically sized type, you get a compiler error here because the compiler doesn't know how much stack space to allocate for t.

This quirk tends to only apply to Index impls that return fat pointers, such as &str. In cases where the Index impl returns a regular pointer, say, &u8, then slice[0] would just return a u8 which is typically what you want.

The docs for std::ops::Index are a little sparse, but they might help a little.

[meretciel]

Thank you so much for the explanation! I didn't realize container[index] is actually syntactic sugar for *container.index(index).