str::array_split for splitting a string into a constant number of substrings

[pitaj]

Related to:

• str::split_once
• str::splitn
• slice::as_chunks

The idea is to provide an API similar to split_once, but allowing the programmer to specify any number of substrings at compile time, instead of being limited to two (one split) or relying on splitn at runtime.

This is helpful for many cases where a programmer wants exactly N substrings, no more and no less, and knows N at compile time. splitn can work for this use case, but since you get an iterator out, there's no concise way to get the substrings into an array. You could also have less than N substrings.

API

pub fn array_split<const N: usize, 'a, P: Pattern<'a>>(&'a self, delimiter: P) -> Option<[&'a str; N]>

Usage example

let static_format = "abc 123 xyz The rest of the string";
let parts: [&str; 4] = static_format.array_split(char::is_whitespace).unwrap();
// or using turbofish to specify # of substrings
let parts = static_format.array_split::<4>(char::is_whitespace).unwrap();

assert_eq!(parts, ["abc", "123", "xyz", "The rest of the string"]);

Returning an array enables nice features like infallible destructuring:

let line = "John Doe,Engineer,Kansas,555-555-5555,This is a comment about John, who is a hard worker, and loves to fish.";
let [name, occupation, state, phone_num, comment] = line.array_split().unwrap();

assert_eq!(name, "John Doe");
assert_eq!(phone_num, "555-555-5555");
assert_eq!(comment, "This is a comment about John, who is a hard worker, and loves to fish.");

@rustbot label +T-libs-api +A-str +A-array +A-const-generics

[camsteffen]

I think it would be better to have a good solution for #81615.

[pitaj]

That would still require some new associated string function to return a const generic iterator, right?

[camsteffen]

I think str::split would work since it's lazy.

[pitaj]

But str::split would not give you the rest of the string as the last element.

[camsteffen]

How about Split::as_str?

[pitaj]

Sure, but that requires either special handling in the FromIterator (or whatever) adapter, or manual handling by the user. The first is unlikely to be included in std, and the second is not ergonomic.