Convert a char to upper case

Question

I have a variable which contains a single char. I want to convert this char to upper case. However, the to_uppercase function returns a rustc_unicode::char::ToUppercase struct instead of a char.

Answer 1

Explanation

ToUppercase is an Iterator, that may yield more than one char. This is necessary, because some Unicode characters consist of multiple "Unicode Scalar Values" (which a Rust char represents).

A nice example are the so called ligatures. Try this for example (on playground):

let fi_upper: Vec<_> = 'fi'.to_uppercase().collect();
println!("{:?}", fi_upper);   // prints: ['F', 'I']

The 'fi' ligature is a single character whose uppercase version consists of two letters/characters.

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Solution

There are multiple possibilities how to deal with that:

1. Work on &str: if your data is actually in string form, use str::to_uppercase which returns a String which is easier to work with.
2. Use ASCII methods: if you are sure that your data is ASCII only and/or you don't care about unicode symbols you can use std::ascii::AsciiExt::to_ascii_uppercase which returns just a char. But it only changes the letters 'a' to 'z' and ignores all other characters!
3. Deal with it manually: Collect into a String or Vec like in the example above.