Is there a difference between string.as_str() and &string[..]

[Igor4Er]

Are these expressions the same? Maybe one of them is better?

[H2CO3]

There's no difference in result. It's purely a stylistic preference, basically.

I like the explicitness of as_str, but some programmers prefer the brevity of indexing.

[jbe]

They may differ if string is not a String (though if string is a &str, you should use none of them):

fn main() {
    let s = "Hello";
    let t = &s[..];
    let s = "Hello";
    let u = s.as_str();
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0599]: no method named `as_str` found for reference `&str` in the current scope
 --> src/main.rs:5:15
  |
5 |     let u = s.as_str();
  |              -^^^^^^-- help: remove this method call

For more information about this error, try `rustc --explain E0599`.
error: could not compile `playground` (bin "playground") due to previous error

[Igor4Er]

I also tried to ask ChatGPT about this, and here's what it said:

In Rust, both &text[..] and text.as_str() can be used to obtain an immutable reference to a string slice (&str) from a String or a string literal. However, there are some differences between the two.

Cloning behavior: &text[..] creates a new string slice without cloning the underlying data, while text.as_str() does not create a new object but returns a reference to the existing string data. This means that &text[..] may be slightly more efficient if you don't need to share the reference across different parts of your code.

Required type: &text[..] works with any type that can be sliced (Deref trait implementation), including String, &str, and arrays. On the other hand, text.as_str() is specific to String and requires an owned String object.

Readability and expressiveness: &text[..] might be considered more concise and idiomatic in some cases, especially when dealing with string slices extracted from arrays or when you prefer the slicing syntax.

Ultimately, choosing between &text[..] and text.as_str() depends on the context and your specific requirements. Both methods are commonly used, and it's a matter of personal preference and readability in most cases.

Is it true or is it just making up facts out of thin air as usual

[H2CO3]

I guess that's not what OP intended to ask, though. It is true of every method that there's a difference between it existing on some types and not existing on others.

[steffahn]

Assuming the variable string is a String, in many contexts, instead of &string[…] or string.as_str(), just writing &string will work, too, namely: Whenever the fact that a &str value is required is already apparent from context. Then type inference will detect the different type (&String vs &str) and the compiler tries to insert an implicit coercion. A so-called “deref coercion” is available in this case, powered by the Deref implementation of String with Target = str.

Whenever this implicit coercion works in your use case, then it’s arguably the most idiomatic approach to create a &str from a String (or &String, or other similar types).

Fun fact: This implementation does by the way also give rise to yet another equivalent expression, you can write &*string for string: String, or &**string for a string: &String variable.

[H2CO3]

ChatGPT's answers are weird and/or wrong.

The "difference in cloning behavior" it cites basically means "no difference". The first half of its answer taken in itself is wrong, and the second half (which is consistent with reality) is inconsistent with the first half.

Wrt. the second bulletpoint: it makes it look like "slicing" comes from a Deref impl, but that's just false. Indexing is implemented by the Index trait, not Deref.

Wrt. the third paragraph, I don't think it's more idiomatic to slice than to write as_str. Both are fine.

So ChatGPT is spitting out plausibly-sounding nonsense in the majority of its answer.

[steffahn]

@Igor4Er

Is it true or is it just making up facts out of thin air as usual

Well let’s see…

@Igor4Er

Cloning behavior: &text[..] creates a new string slice without cloning the underlying data, while text.as_str() does not create a new object but returns a reference to the existing string data. This means that &text[..] may be slightly more efficient if you don't need to share the reference across different parts of your code.

This paragraph seems a bit contradictory / nonsensical, and can be safely ignored. There are no performance differences.

@Igor4Er

Required type: &text[..] works with any type that can be sliced (Deref trait implementation), including String, &str, and arrays. On the other hand, text.as_str() is specific to String and requires an owned String object.

It is true that slicing notation is more general, so for example to turn a Vec<T> into &[T], let's say from a variable v, the slicing notation will also look like &v[..], but using a method would use a method called v.as_slice(), not v.as_str(). This information is largely irrelevant for your question though, any maybe being more specific can be seen as a benefit for the reader.

@Igor4Er

Ultimately, choosing between &text[..] and text.as_str() depends on the context and your specific requirements.

I’d disagree, there’s barely any “requirements” that influence the decision, it’s largely a question of personal taste (or perhaps taste of your collaborators, if you aim for some consistent “style” in a code-base.

---

Edit: I didn’t notice the reference to Deref in the answer that @H2CO3 correctly pointed out, too; yeah that’s arguably an actual plain error. In any case I agree with the general sentiment that the ChatGPT answer is almost entirely useless in this case. Whilst containing not much actually 100% factually wrong information, it doesn’t give a cohesive interpretation or presentation of the situation and deduces the wrong suggestions / conclusions.

[simonbuchan]

GPT seems to really like putting "ultimately the answer depends on your situation" type paragraphs at the end of every "which is better question", no matter how nonsense it is:

Ultimately, both football and the Beatles have their own merits and can bring joy and inspiration to different individuals in their respective ways. It's a matter of personal preference and interests when deciding which is better for you.

Determining which is "better" between the Beatles and a hammer is subjective and depends on individual preferences and needs. If you appreciate music and cultural impact, you might consider the Beatles to be better. However, if you require a tool for practical tasks, a hammer would be more suitable.

In summary, it is not appropriate to compare ice cream and skin cancer in terms of being "better" or "worse." They are completely different entities with different implications. Enjoying ice cream in moderation can be a pleasurable experience, while preventing skin cancer is essential for maintaining good health.

[H2CO3]

I mean… that sort of evasive answer is probably naïvely classified as "not wrong", so I can imagine it's being accidentally reinforced by simple loss functions unless explicitly penalized. (Disclaimer: I have no idea if this actually is how ChatGPT was trained.)

[simonbuchan]

I liked the accidental pun of the Beatles cultural impact, at least.