How can I read one character from stdin without having to hit enter?

Use one of the 'ncurses' libraries now available, for instance this one.

Add the dependency in Cargo

[dependencies]
ncurses = "5.86.0"

and include in main.rs:

extern crate ncurses;
use ncurses::*; // watch for globs

Follow the examples in the library to initialize ncurses and wait for single character input like this:

initscr();
/* Print to the back buffer. */
printw("Hello, world!");

/* Update the screen. */
refresh();

/* Wait for a key press. */
getch();

/* Terminate ncurses. */
endwin();

While @Jon's solution using ncurses works, ncurses clears the screen by design. I came up with this solution that uses the termios crate for my little project to learn Rust. The idea is to modify ECHO and ICANON flags by accessing tcsetattr through termios bindings.

extern crate termios;
use std::io;
use std::io::Read;
use std::io::Write;
use termios::{Termios, TCSANOW, ECHO, ICANON, tcsetattr};

fn main() {
    let stdin = 0; // couldn't get std::os::unix::io::FromRawFd to work 
                   // on /dev/stdin or /dev/tty
    let termios = Termios::from_fd(stdin).unwrap();
    let mut new_termios = termios.clone();  // make a mutable copy of termios 
                                            // that we will modify
    new_termios.c_lflag &= !(ICANON | ECHO); // no echo and canonical mode
    tcsetattr(stdin, TCSANOW, &mut new_termios).unwrap();
    let stdout = io::stdout();
    let mut reader = io::stdin();
    let mut buffer = [0;1];  // read exactly one byte
    print!("Hit a key! ");
    stdout.lock().flush().unwrap();
    reader.read_exact(&mut buffer).unwrap();
    println!("You have hit: {:?}", buffer);
    tcsetattr(stdin, TCSANOW, & termios).unwrap();  // reset the stdin to 
                                                    // original termios data
}

One advantage of reading a single byte is capturing arrow keys, ctrl etc. Extended F-keys are not captured (although ncurses can capture these).

This solution is intended for UNIX-like platforms. I have no experience with Windows, but according to this forum perhaps something similar can be achieved using SetConsoleMode in Windows.