Update unit testing post for new panic handler

blog/content/second-edition/posts/04-unit-testing/index.md

@@ -24,23 +24,20 @@ Unfortunately it's a bit more complicated for `no_std` applications such as our
 
 ```
 > cargo test
-   Compiling blog_os v0.2.0 (file:///home/philipp/Documents/blog_os)
-error[E0152]: duplicate lang item found: `panic_fmt`.
-  --> src/main.rs:26:1
+   Compiling blog_os v0.2.0 (file:///…/blog_os)
+error[E0152]: duplicate lang item found: `panic_impl`.
+  --> src/main.rs:35:1
    |
-26 | / pub extern "C" fn rust_begin_panic(
-27 | |     _msg: core::fmt::Arguments,
-28 | |     _file: &'static str,
-29 | |     _line: u32,
-...  |
-32 | |     loop {}
-33 | | }
+35 | / pub fn panic(info: &PanicInfo) -> ! {
+36 | |     println!("{}", info);
+37 | |     loop {}
+38 | | }
    | |_^
    |
    = note: first defined in crate `std`.
 ```
 
-The problem is that unit tests are built for the host machine, with the `std` library included. This makes sense because they should be able to run as a normal application on the host operating system. Since the standard library has it's own implementation of the `panic_fmt` language item, we get the above error. To fix it, we use [conditional compilation] to include our implementation of the language item only in non-test environments:
+The problem is that unit tests are built for the host machine, with the `std` library included. This makes sense because they should be able to run as a normal application on the host operating system. Since the standard library has it's own `panic_implementation` function, we get the above error. To fix it, we use [conditional compilation] to include our implementation of the panic handler only in non-test environments:
 
 [conditional compilation]: https://doc.rust-lang.org/reference/attributes.html#conditional-compilation
 
@@ -49,14 +46,10 @@ The problem is that unit tests are built for the host machine, with the `std` li
 // in src/main.rs
 
 #[cfg(not(test))] // only compile when the test flag is not set
-#[lang = "panic_fmt"]
+#[panic_implementation]
 #[no_mangle]
-pub extern "C" fn rust_begin_panic(
-    _msg: core::fmt::Arguments,
-    _file: &'static str,
-    _line: u32,
-    _column: u32,
-) -> ! {
+pub fn panic(info: &PanicInfo) -> ! {
+    println!("{}", info);
     loop {}
 }
 ```

src/main.rs

@@ -29,6 +29,7 @@ pub extern "C" fn _start() -> ! {
 }
 
 /// This function is called on panic.
+#[cfg(not(test))]
 #[panic_implementation]
 #[no_mangle]
 pub fn panic(info: &PanicInfo) -> ! {