baseline version with comments

wordcount.rs

use std::io::{self, Read, Write, BufWriter}; // Bring some IO stuff into scope
use std::collections::HashMap; // Bring Hashmap into scope

fn main() { // special entry function
    let mut buffer = String::new(); // Initialize a string
    io::stdin().read_to_string(&mut buffer).ok(); // fill string with input from stdin

    let mut words: HashMap<&str, usize> = HashMap::new(); // initialize HashMap
    for word in buffer.split(is_whitespace).filter(is_not_empty) {
        if let Some(count) = words.get_mut(word) {
            *count += 1;
            continue;
        }
        words.insert(word, 1);
    }

    // we gather all entries of the dictionary in a Vector of Tuples
    // the first field  is the word, the second the number of occurrences
    let mut sortable: Vec<(&str, usize)> = words
      .iter()
      .map(|(word, count)| (*word, *count))
      .collect();

    sortable.sort_by_key(|a| a.0); //sort words
    sortable.sort_by(|a, b| b.1.cmp(&a.1)); // sort occurences

    let mut output = BufWriter::new(io::stdout());
    for word in sortable {
        writeln!(output, "{}\t{}", word.0, word.1).ok(); // output to stdout
    }
}

#[inline] // this instructs the compiler to inline this function
fn is_not_empty(s: &&str) -> bool {
    !s.is_empty()
}

#[inline]
fn is_whitespace(c: char) -> bool {
    c == ' ' || c == '\t' || c == '\n'
}