std::from_chars will still accept "123abc". You have to manually check if all parts of the string have been consumed. On the other hand, " 123" is not accepted, because it starts with an invalid character, so the behaviour isn't "take the first acceptable number and parse that" either.

To get the equivalent of Rust's

    if let Ok(x) = input.parse::<i32>() {
         println!("You entered {x}");
    } else {
        eprintln!("You did not enter a number");
    }

you need something like:

     int x{};
     auto [ptr, ec] = std::from_chars(input.data(), input.data() + input.size(), x);
     if (ec == std::errc() && ptr == input.data() + input.size()) { 
         std::cout << "You entered " << x << std::endl;
     } else {
         std::cerr << "You did not enter a valid number" << std::endl;
     }

I find the choice to always require a start and and end position, and not to provide a method that simply passes or fails, to be quite baffling. In C++26, they also added an automatic boolean conversion for from_chars' return type to indicate success, which considers "only consumed half the input from the start" to be a success.

Maybe I'm weird for mostly writing code that does straightforward input-to-number conversions and not partial string parsers, but I have yet to see a good alternative for Rust's parse().