fix(intro2): Changed intro2 to be a name error, not a format string error

[0atman]

I think intro2 is too difficult to be the very first real exercise, and we're scaring away some students unnecessarily.

TL;DR: I've made a tweak to intro2 to cause a much friendlier compiler error.

My Case Against The Existing Exercise

The existing error for intro2 is more difficult to understand compared with subsequent exercises. That's not right, the Rustlings exercises are otherwise in ascending order of difficulty. The first one should be the easiest!

The existing intro2 error is:

error: 1 positional argument in format string, but no arguments were given
  --> exercises/intro/intro2.rs:11:21
     println!("Hello {}!");
                     ^^
error: aborting due to previous error

This is terrifying as an error! There's not even a hint or compiler help to aid us! Even existing programmers coming from other languages might be stumped:

• How do they know what a format string is?
• How are they supposed to know that {} is called a 'positional argument'?
• There IS an argument there, it's the string argument to println! (they might reasonably think).

I don't think there's a need to teach the student about format strings right here, we're about to use them multiple times in a much friendlier way in the very next exercise: variables.

If this exercise were named formatstring1 I would understand, but it's not, it's named intro2:
This is a poor intro to Rustlings, but one we can fix easily.

My Case For The Proposed Change

I propose the first exercise should both showcase Rust's fantastic errors AND show that the compiler can often tell you how to solve your mistakes with help text.

I propose changing intro2 to the following, but I would agree with any proposal that has a clear error with correct compiler help text.

printline!("Hello there!");

The compiler error for this is much simpler, AND comes with a correct help suggestion. The error is:

error: cannot find macro `printline` in this scope
   --> exercises/00_intro/intro2.rs:11:5
    |
11  |     printline!("Hello there!")
    |     ^^^^^^^^^ help: a macro with a similar name exists: `println`
    |
   ::: /home/oatman/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/macros.rs:138:1
    |
138 | macro_rules! println {
    | -------------------- similarly named macro `println` defined here

error: aborting due to previous error

The student only needs to read the help line to know how to fix the error. 🎉

[0atman]

As a background to this:

I tutor students 1:1 in Rust, and my primary resource for new students is Rustlings.

Though they mostly go through the exercises in their own time, I often help them install rustup and Rustlings on their computer. To confirm everything works and that they've understood the system, I ask them to complete intro1 and intro2.

I have seen MANY students struggle with intro2, and I think we can simply fix that.

[0atman]

Though I have read CONTRIBUTION.MD, this is my first PR here, and so I may need help to do things by the books. I have updated the exercise and the hint text, and would welcome any suggestions that adheres to the the goal of making this first exercise as simple and friendly as possible.

Thanks very much to everyone in the Rustlings project, it's the best code kata system I've ever used 😃

[chrischtel]

Hello everyone,
I think this pull request is actually a very good Idea. I personally had a problem fixing the second Intro when I started out learning rust and it scared me a little bit. The updated version by @0atman seems to be a lot easier for new rustaceans.

[shadows-withal]

Agreed, sorry for the delay!

[shadows-withal]

@all-contributors please add @0atman for content

[allcontributors]

@shadows-withal

I've put up a pull request to add @0atman! 🎉