Sketch of StrSource trait for access to string data

str_source.rs

use std::borrow::Cow;

pub trait StrSource {
    fn len(&self) -> usize;

    fn str_at(&self, ix: usize) -> &str;

    fn str_before(&self, ix: usize) -> &str;

    fn to_string(&self) -> String {
        let len = self.len();
        let mut s = String::with_capacity(len);
        while s.len() < len {
            s.push_str(self.str_at(s.len()));
        }
        s
    }

    fn len_utf16(&self) -> usize {
        let len = self.len();
        let mut ix = 0;
        let mut len_utf16 = 0;
        while ix < len {
            let s = self.str_at(ix);
            for c in s.chars() {
                len_utf16 += c.len_utf16()
            }
            ix += s.len();
        }
        len_utf16
    }
}

impl<'a> StrSource for &'a str {
    fn len(&self) -> usize {
        str::len(self)
    }

    fn str_at(&self, ix: usize) -> &str {
        &self[ix..]
    }

    fn str_before(&self, ix: usize) -> &str {
        &self[..ix]
    }
}

impl StrSource for String {
    fn len(&self) -> usize {
        str::len(self)
    }

    fn str_at(&self, ix: usize) -> &str {
        &self[ix..]
    }

    fn str_before(&self, ix: usize) -> &str {
        &self[..ix]
    }
}

impl<'a> StrSource for Cow<'a, str> {
    fn len(&self) -> usize {
        str::len(self)
    }

    fn str_at(&self, ix: usize) -> &str {
        &self[ix..]
    }

    fn str_before(&self, ix: usize) -> &str {
        &self[..ix]
    }
}