Use char as &str in HashMap - string

Question

I want to create a HashMap which maps words — a Vec of &str — and letters of those words each to another. For example, vec!["ab", "b", "abc"] will be converted to the following HashMap
{
// Letters are keys, words which contain the keys are values
"a" => ["ab", "abc"],
"b" => ["ab", "bc", "abc"],
"c" => ["bc", "abc"],
// Words are keys, letters which are in the words are values
"ab" => ["a", "b"],
"abc" => ["a", "b", "c"],
}
I tried this code [playground]:
let words = vec!["ab", "bc", "abc"];
let mut map: HashMap<&str, Vec<&str>> = HashMap::new();
for word in words.iter() {
for letter in word.chars() {
map.entry(letter).or_default().push(word);
map.entry(word).or_default().push(letter);
}
}
but there is a problem: letter is of type char but I need a &str because map accepts only &strs as keys. I also tried to convert letter to a &str:
for word in words.iter() {
for letter in word.chars() {
let letter = letter.to_string()
// no changes
but this code creates a new String which has a smaller lifetime than map's one. In other words, letter is dropped after the nested for loop but and I get compiler error.
How can I use a char in HashMap which accepts only &strs as keys?

Answer 1

I would separate the map into two:
One is char -> Vec<&str>. It maps letters to a list of words.
The second one would be &str -> Vec<char>, but I do not know if you really need it: Why not just iterate over the chars of a &str directly?
Storing a Vec<char> essentially just doubles the amount of memory you use, unless the Vec<char> is e.g. sorted in a particular order (which may or may not be necessary).
If you really want to keep them in one map, I think it is easier to have a HashMap<String, Vec<String>>.
The problem seems to be that chars gives you one char after another, but what you actually want is a &str after another, where each &str actually encompasses a single character. I did not find anything like that in the docs for &str, but maybe there is something somewhere that iterates like this.
You could work-around it using matches:
let words = vec!["ab", "bc", "abc"];
let mut map: HashMap<&str, Vec<&str>> = HashMap::new();
for word in words.iter() {
for letter in word.matches(|_| true) { // iterates over &str's that encompass one single character
map.entry(letter).or_default().push(word);
map.entry(word).or_default().push(letter);
}
}