feat: Refactor hint system

Hints are now accessible using the CLI subcommand `rustlings hint
<exercise name`.

BREAKING CHANGE: This fundamentally changes the way people interact with exercises.

exercises/enums/enums1.rs

@@ -1,5 +1,5 @@
 // enums1.rs
-// Make me compile! Scroll down for hints!
+// Make me compile! Execute `rustlings hint enums1` for hints!
 
 #[derive(Debug)]
 enum Message {
@@ -12,31 +12,3 @@ fn main() {
     println!("{:?}", Message::Move);
     println!("{:?}", Message::ChangeColor);
 }
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-// Hint: The declaration of the enumeration type has not been defined yet.

exercises/enums/enums2.rs

@@ -1,5 +1,5 @@
 // enums2.rs
-// Make me compile! Scroll down for hints
+// Make me compile! Execute `rustlings hint enums2` for hints!
 
 #[derive(Debug)]
 enum Message {
@@ -24,38 +24,3 @@ fn main() {
         message.call();
     }
 }
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-// Hint: you can create enumerations that have different variants with different types
-// such as no data, anonymous structs, a single string, tuples, ...etc

exercises/error_handling/errors1.rs

@@ -4,7 +4,7 @@
 // was, instead of just sometimes returning `None`. The 2nd test currently
 // does not compile or pass, but it illustrates the behavior we would like
 // this function to have.
-// Scroll down for hints!!!
+// Execute `rustlings hint errors1` for hints!
 
 pub fn generate_nametag_text(name: String) -> Option<String> {
     if name.len() > 0 {
@@ -38,36 +38,3 @@ mod tests {
         );
     }
 }
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-// `Err` is one of the variants of `Result`, so what the 2nd test is saying
-// is that `generate_nametag_text` should return a `Result` instead of an
-// `Option`.
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-// To make this change, you'll need to:
-// - update the return type in the function signature to be a Result<String, String> that
-//   could be the variants `Ok(String)` and `Err(String)`
-// - change the body of the function to return `Ok(stuff)` where it currently
-//   returns `Some(stuff)`
-// - change the body of the function to return `Err(error message)` where it
-//   currently returns `None`
-// - change the first test to expect `Ok(stuff)` where it currently expects
-//   `Some(stuff)`.

exercises/error_handling/errors2.rs

@@ -14,7 +14,7 @@
 // and add.
 
 // There are at least two ways to implement this that are both correct-- but
-// one is a lot shorter! Scroll down for hints to both ways.
+// one is a lot shorter! Execute `rustlings hint errors2` for hints to both ways.
 
 use std::num::ParseIntError;
 
@@ -43,27 +43,3 @@ mod tests {
         );
     }
 }
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-// One way to handle this is using a `match` statement on
-// `item_quantity.parse::<i32>()` where the cases are `Ok(something)` and
-// `Err(something)`. This pattern is very common in Rust, though, so there's
-// a `?` operator that does pretty much what you would make that match statement
-// do for you! Take a look at this section of the Error Handling chapter:
-// https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
-// and give it a try!

exercises/error_handling/errors3.rs

@@ -1,7 +1,8 @@
 // errors3.rs
 // This is a program that is trying to use a completed version of the
 // `total_cost` function from the previous exercise. It's not working though!
-// Why not? What should we do to fix it? Scroll for hints!
+// Why not? What should we do to fix it?
+// Execute `rustlings hint errors3` for hints!
 
 use std::num::ParseIntError;
 
@@ -26,22 +27,3 @@ pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
 
     Ok(qty * cost_per_item + processing_fee)
 }
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-// If other functions can return a `Result`, why shouldn't `main`?

exercises/error_handling/errorsn.rs

@@ -13,7 +13,7 @@
 // type goes where the question marks are, and how do we return
 // that type from the body of read_and_validate?
 //
-// Scroll down for hints :)
+// Execute `rustlings hint errors4` for hints :)
 
 use std::error;
 use std::fmt;
@@ -110,138 +110,3 @@ impl error::Error for CreationError {
         }
     }
 }
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-// First hint: To figure out what type should go where the ??? is, take a look
-// at the test helper function `test_with_str`, since it returns whatever
-// `read_and_validate` returns and`test_with_str` has its signature fully
-// specified.
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-// Next hint: There are three places in `read_and_validate` that we call a
-// function that returns a `Result` (that is, the functions might fail).
-// Apply the `?` operator on those calls so that we return immediately from
-// `read_and_validate` if those function calls fail.
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-// Another hint: under the hood, the `?` operator calls `From::from`
-// on the error value to convert it to a boxed trait object, a Box<dyn error::Error>,
-// which is polymorphic-- that means that lots of different kinds of errors
-// can be returned from the same function because all errors act the same
-// since they all implement the `error::Error` trait.
-// Check out this section of the book:
-// https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
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-// Another another hint: Note that because the `?` operator returns
-// the *unwrapped* value in the `Ok` case, if we want to return a `Result` from
-// `read_and_validate` for *its* success case, we'll have to rewrap a value
-// that we got from the return value of a `?`ed call in an `Ok`-- this will
-// look like `Ok(something)`.
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-// Another another another hint: `Result`s must be "used", that is, you'll
-// get a warning if you don't handle a `Result` that you get in your
-// function. Read more about that in the `std::result` module docs:
-// https://doc.rust-lang.org/std/result/#results-must-be-used

exercises/error_handling/option1.rs

@@ -2,7 +2,7 @@
 // This example panics because the second time it calls `pop`, the `vec`
 // is empty, so `pop` returns `None`, and `unwrap` panics if it's called
 // on `None`. Handle this in a more graceful way than calling `unwrap`!
-// Scroll down for hints :)
+// Execute `rustlings hint option1` for hints :)
 
 pub fn pop_too_much() -> bool {
     let mut list = vec![3];
@@ -27,31 +27,3 @@ mod tests {
         assert!(pop_too_much());
     }
 }
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-// Try using a `match` statement where the arms are `Some(thing)` and `None`.
-// Or set a default value to print out if you get `None` by using the
-// function `unwrap_or`.
-// Or use an `if let` statement on the result of `pop()` to both destructure
-// a `Some` value and only print out something if we have a value!