Remove str::is_whitespace and str::is_alphanumeric

[dtolnay]

These methods were added as stable in #49381 (currently in beta for 1.27.0) but we would prefer not to add them as discussed in #49381 (comment).

[withoutboats]

Strongly prefer to have this method (unsurprisngly). I assumed it was an oversight that it was missing; I've probably tried s.is_whitespace() a half dozen times and then been surprised it was nightly only.

[oli-obk]

We could add a system to be able to add suggestions to missing method errors. Thus the use of is_whitespace would suggest .chars().all(char::is_whitespace)

Deleting these methods should of course not be blocked on such a system being in place

[LukasKalbertodt]

@oli-obk I like this idea a lot! (independently from this discussion)

---

About is_alphanumeric: I think I prefer to remove str::is_alphanumeric as it might lead to subtle Unicode bugs. Example:

println!("{}", "böb".chars().all(char::is_alphanumeric));    // true
println!("{}", "böb".chars().all(char::is_alphanumeric));    // false

The second string uses a Combining Diaeresis with an o instead of an ö. Sure, the explicit variant chars().all(...) also has this problem, but being explicit makes the bug a bit less subtle. Having is_alphanumeric on a string might suggest that the method "does the right thing" (whatever that is) instead of looking at characters individually.

---

I think str::is_whitespace doesn't have a problem with subtle Unicode bugs, but I don't quite like the name: is_whitespace sounds singular to me and thus doesn't translate perfectly to a string.

If you are interested, I did a bit of research how .chars().all(char::is_whitespace) is called in other languages (sometimes with libraries):

• Ruby on Rails: blank?
• Java (Apache Commons Lang): StringUtils.isBlank
• Python: isspace
• C# with .NET: IsNullOrWhiteSpace
• I didn't find such a method for C++, JavaScript, Haskell, Swift and Go

Maybe str::is_blank is a better name for Rust?