Provide a note on 'extern crate' usage in edition 2018 syntax of Rust

src/start/exceptions.md

@@ -62,7 +62,7 @@ times it has been called in the `COUNT` variable and then prints the value of
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use core::fmt::Write;
 
@@ -190,7 +190,7 @@ memory location.
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use core::fmt::Write;
 use core::ptr;

src/start/panicking.md

@@ -51,11 +51,11 @@ profile. For example:
 
 // dev profile: easier to debug panics; can put a breakpoint on `rust_begin_unwind`
 #[cfg(debug_assertions)]
-extern crate panic_halt;
+use panic_halt as _;
 
 // release profile: minimize the binary size of the application
 #[cfg(not(debug_assertions))]
-extern crate panic_abort;
+use panic_abort as _;
 
 // ..
 ```
@@ -64,6 +64,13 @@ In this example the crate links to the `panic-halt` crate when built with the
 dev profile (`cargo build`), but links to the `panic-abort` crate when built
 with the release profile (`cargo build --release`).
 
+> The `use panic_abort as _;` form of the `use` statement is used to ensure the `panic_abort` panic handler is
+> included in our final executable while making it clear to the compiler that we won't explicitly use anything from
+> the crate. Without the `as _` rename, the compiler would warn that we have an unused import.
+> Sometimes you might see `extern crate panic_abort` instead, which is an older style used before the
+> 2018 edition of Rust, and should now only be used for "sysroot" crates (those distributed with Rust itself) such
+> as `proc_macro`, `alloc`, `std`, and `test`.
+
 ## An example
 
 Here's an example that tries to index an array beyond its length. The operation
@@ -73,7 +80,7 @@ results in a panic.
 #![no_main]
 #![no_std]
 
-extern crate panic_semihosting;
+use panic_semihosting as _;
 
 use cortex_m_rt::entry;
 

src/start/qemu.md

@@ -94,7 +94,7 @@ For convenience here are the most important parts of the source code in `src/mai
 #![no_std]
 #![no_main]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use cortex_m_rt::entry;
 
@@ -117,7 +117,7 @@ interface that most Rust programs use. The main (no pun intended) reason to go
 with `no_main` is that using the `main` interface in `no_std` context requires
 nightly.
 
-`extern crate panic_halt;`. This crate provides a `panic_handler` that defines
+`use panic_halt as _;`. This crate provides a `panic_handler` that defines
 the panicking behavior of the program. We will cover this in more detail in the
 [Panicking](panicking.md) chapter of the book.
 
@@ -318,7 +318,7 @@ For convenience here's the source code of `examples/hello.rs`:
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use cortex_m_rt::entry;
 use cortex_m_semihosting::{debug, hprintln};
@@ -509,7 +509,7 @@ list main
 This will show the source code, from the file examples/hello.rs. 
 
 ```text
-6       extern crate panic_halt;
+6       use panic_halt as _;
 7
 8       use cortex_m_rt::entry;
 9       use cortex_m_semihosting::{debug, hprintln};

src/start/registers.md

@@ -53,7 +53,7 @@ We won't get very far with our embedded software development if we restrict ours
 #![no_std]
 #![no_main]
 
-extern crate panic_halt; // panic handler
+use panic_halt as _; // panic handler
 
 use cortex_m_rt::entry;
 use tm4c123x;
@@ -129,7 +129,7 @@ Let's see an example:
 #![no_std]
 #![no_main]
 
-extern crate panic_halt; // panic handler
+use panic_halt as _; // panic handler
 
 use cortex_m_rt::entry;
 use tm4c123x_hal as hal;

src/start/semihosting.md

@@ -16,7 +16,7 @@ world!":
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use cortex_m_rt::entry;
 use cortex_m_semihosting::hprintln;
@@ -67,7 +67,7 @@ until you restart it.
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use cortex_m_rt::entry;
 use cortex_m_semihosting::debug;
@@ -105,7 +105,7 @@ stderr.
 #![no_main]
 #![no_std]
 
-extern crate panic_semihosting; // features = ["exit"]
+use panic_semihosting as _; // features = ["exit"]
 
 use cortex_m_rt::entry;
 use cortex_m_semihosting::debug;