rename y to _y to get the correct compile error

[aviad-dev]

Without this change, the error is:

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `y`
  --> src/main.rs:13:9
   |
13 |     let y: &'a i32 = &_x;
   |         ^ help: if this is intentional, prefix it with an underscore: `_y`
   |
   = note: `#[warn(unused_variables)]` on by default

error[E0597]: `_x` does not live long enough
  --> src/main.rs:13:22
   |
9  | fn failed_borrow<'a>() {
   |                  -- lifetime `'a` defined here
10 |     let _x = 12;
   |         -- binding `_x` declared here
...
13 |     let y: &'a i32 = &_x;
   |            -------   ^^^ borrowed value does not live long enough
   |            |
   |            type annotation requires that `_x` is borrowed for `'a`
...
17 | }
   | - `_x` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.
warning: `playground` (bin "playground") generated 1 warning
error: could not compile `playground` (bin "playground") due to previous error; 1 warning emitted

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) soon.

[kberov]

True. Although the message for the y variable is only a warning, the learner may get confused.

[marioidival]

Can you change that reference?

[aviad-dev]

see update in 3dffa1e

If that is what you meant, let me know and I'll squash the commits to keep git history tidy.