[jeroenhd]

.expect() also takes a message that it prints when the value is empty.

Unlike C++, Rust doesn't support throwing exceptions, so expect() failing would panic. By default, this means dumping a stack trace and terminating the program, and the message provided in "expect" would be printed right before the stack trace.

For example:

    fn main() {
        let x: Option<i32> = None;
        x.expect("Oh no!");
    }

will print:

    thread 'main' panicked at src/main.rs:3:7:
    Oh no!
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

With RUST_BACKTRACE set to "full", it'll print:

    $ RUST_BACKTRACE=full ./target/release/demo
    thread 'main' panicked at src/main.rs:3:7:
    Oh no!
    stack backtrace:
       0:     0x5ff43befa755 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::ha52e99bffe3c0898
       1:     0x5ff43bf1769b - core::fmt::write::h5fdd5156f2480a24
       2:     0x5ff43bef8a5f - std::io::Write::write_fmt::ha2c0b019f448d2c3
       3:     0x5ff43befa52e - std::sys_common::backtrace::print::he84813a4ed1c2825
       4:     0x5ff43befb7e9 - std::panicking::default_hook::{{closure}}::h033521c27c9929b1
       5:     0x5ff43befb52d - std::panicking::default_hook::had42987aad9de78c
       6:     0x5ff43befbc83 - std::panicking::rust_panic_with_hook::h80fc1b429f5a5699
       7:     0x5ff43befbb64 - std::panicking::begin_panic_handler::{{closure}}::h5aa7b89233b1ae33
       8:     0x5ff43befac19 - std::sys_common::backtrace::__rust_end_short_backtrace::h0e4c5e6cee7f8a24
       9:     0x5ff43befb897 - rust_begin_unwind
      10:     0x5ff43bee0b63 - core::panicking::panic_fmt::h3bea7be9b6a41ace
      11:     0x5ff43bf16c6c - core::panicking::panic_display::h20da06138ce63f85
      12:     0x5ff43bee0b2c - core::option::expect_failed::h92448d4f1092eaaa
      13:     0x5ff43bee127a - demo::main::ha244b8f1ce6eaa44
      14:     0x5ff43bee1223 -     std::sys_common::backtrace::__rust_begin_short_backtrace::hfc5c93265480da58
      15:     0x5ff43bee1239 - std::rt::lang_start::{{closure}}::h988fdfb65ef3da3b
      16:     0x5ff43bef6be6 - std::rt::lang_start_internal::h64c4082ce77a6bd6
      17:     0x5ff43bee12a5 - main
      18:     0x741d2a628150 - __libc_start_call_main
                               at ./csu/../sysdeps/nptl/libc_start_call_main.h:    58:16
      19:     0x741d2a628209 - __libc_start_main_impl
                                   at ./csu/../csu/libc-start.c:360:3
      20:     0x5ff43bee1155 - _start
      21:                0x0 - <unknown>

Whether you would want to replicate this behaviour in C++, I don't know; I find panic!() to be quite destructive, and catastrophic when it's used in libraries or frameworks. I think the C++ implementation just throws an exception, but Rust's .expect() does not behave like .value() in C++.