Complexity lint against `some_string.as_bytes().len()`

[LikeLakers2]

What it does

This lint would recommend changing instances of some_string.as_bytes().len() to some_string.len().

Explanation: When getting the length of a string, str::len() already returns the length in bytes. Converting a string to &[u8] using str::as_bytes(), and then getting the length of that, introduces unnecessary complexity where none is needed.

Advantage

Less complex-looking code, while still functioning the same.

Drawbacks

Applying this lint may make it less obvious that the length will be in bytes - because our intuition for getting a string's length is that it will be the number of characters. Yes, the rust documentation addresses this, but it's still quite easy to forget - especially for us english speakers, who probably only expect to deal with ASCII (which is only ever one byte in length).

Example

let string_len_in_bytes = some_string.as_bytes().len();

Could be written as:

let string_len_in_bytes = some_string.len();

[samueltardieu]

@rustbot claim

[cgwalters]

Applying this lint may make it less obvious that the length will be in bytes - because our intuition for getting a string's length is that it will be the number of characters. Yes, the rust documentation addresses this, but it's still quite easy to forget - especially for us english speakers, who probably only expect to deal with ASCII (which is only ever one byte in length).

I got bit recently by forgetting that str::len is bytes, and I went to go check if there was a clippy lint basically warning on str::len and was a bit surprised to find that there's one for the opposite.

I understand where folks are coming from on this, but still...I suspect there's two kinds of uses of str::len - those comparing versus other strings which is fine, and those that are probably buggy and should be counting characters.

Would it be possible to have the opposite lint as an opt-in and have it disable this lint?

[y21]

The category was discussed a fair bit on Zulip and a poll was created to ultimately decide, see the thread: https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/FCP.3A.20needless_as_bytes

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If you want the opposite of this lint, you can disallow the use of str::len in your clippy.toml

disallowed-methods = [
  { path = "str::len", reason = "use <str>.as_bytes().len() instead" }
]

I don't know if it's worth having an opt-in lint for something that disallowed_methods can do by itself (you would need to manually enable it for your project in any case).

[cgwalters]

Thank you! I was not aware of disallowed-methods. I tried it in one of my projects and for what it's worth: every use of str::len was either a bug or a place where we basically wanted to validate more strictly that only ASCII was in use - it's not that we wanted to use .as_bytes().len() either really. I guess this is where https://lib.rs/crates/ascii could be used.