Add hints to the variable bindings exercises

variables/variables1.rs

@@ -1,6 +1,41 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     x = 5;
     println!("x has the value {}", x);
 }
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+// Hint: The declaration on line 4 is missing a keyword that is needed in Rust
+// to create a new variable binding.

variables/variables2.rs

@@ -1,4 +1,4 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x;
@@ -8,3 +8,39 @@ fn main() {
         println!("Not ten!");
     }
 }
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+// The compiler message is saying that Rust cannot infer the type that the
+// variable binding `x` has with what is given here.
+// What happens if you annotate line 4 with a type annotation?
+// What if you give x a value?
+// What if you do both?
+// What type should x be, anyway?
+// What if x is the same type as 10? What if it's a different type?

variables/variables3.rs

@@ -1,4 +1,4 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x = 3;
@@ -6,3 +6,37 @@ fn main() {
     x = 5;
     println!("Number {}", x);
 }
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+// In Rust, variable bindings are immutable by default. But here we're trying
+// to reassign a different value to x! There's a keyword we can use to make
+// a variable binding mutable instead.

variables/variables4.rs

@@ -1,6 +1,44 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x: i32;
     println!("Number {}", x);
 }
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+// Oops! In this exercise, we have a variable binding that we've created on
+// line 4, and we're trying to use it on line 5, but we haven't given it a
+// value. We can't print out something that isn't there; try giving x a value!
+// This is an error that can cause bugs that's very easy to make in any
+// programming language -- thankfully the Rust compiler has caught this for us!