docs: Add template deep dive for src/utils.rs

Issue rustwasm#346

Author: danwilhelm

docs/src/tutorial/template-deep-dive/src-utils-rs.md

@@ -1,3 +1,66 @@
 # src/utils.rs
 
-🚧 COMING SOON 🚧
+The purpose of `utils.rs` is to define the `utils` module, which contains a single function `set_panic_hook`. This function becomes part of the `utils` module in `lib.rs`, as described in the preceding section.
+
+If the `console_error_panic_hook` feature is not enabled, then `set_panic_hook` is defined to be an inlined empty function. So, there is no run-time performance or code-size penalty incurred by its use.
+
+We will discuss:
+1. [Defining `set_panic_hook`](#a1-defining-set_panic_hook)
+2. [What is `console_error_panic_hook`?](#a2-what-is-console_error_panic_hook)
+
+
+---
+
+## 1. Defining `set_panic_hook`
+
+```
+use cfg_if::cfg_if;
+```
+
+This allows us to write `cfg_if!` instead of `cfg_if::cfg_if!`, identically to the line in `src/lib.rs`.
+
+```
+cfg_if! {
+    if #[cfg(feature = "console_error_panic_hook")] {
+        extern crate console_error_panic_hook;
+        pub use self::console_error_panic_hook::set_once as set_panic_hook;
+    } else {
+        #[inline]
+        pub fn set_panic_hook() {}
+    }
+}
+```
+
+As described in the preceding section, the macro `cfg_if!` evaluates the `if` statement during compile time. This is possible because it is essentially testing whether `"console_error_panic_hook"` is defined in the `[features]` section of `Cargo.toml`, which is available during compile time.
+
+The entire macro block will either be replaced with the statements in the `if` block or with those in the `else` block. These two cases are now described in turn:
+
+```
+extern crate console_error_panic_hook;
+pub use self::console_error_panic_hook::set_once as set_panic_hook;
+```
+
+Due to the `use` statement, the function `self::console_error_panic_hook::set_once` can now be accessed more conveniently as `set_panic_hook`. Due to `pub`, this function will be publicly accessible outside of the `utils` module as `utils::set_panic_hook`.
+
+```
+#[inline]
+pub fn set_panic_hook() {}
+```
+
+An inline function replaces the function call with the contents of the function during compile time. Here, `set_panic_hook` is defined to be an empty inline function. This allows the use of `set_panic_hook` without any run-time or code-size performance penalty if the feature is not enabled.
+
+## 2. What is `console_error_panic_hook`?
+
+The crate `console_error_panic_hook` enhances error messages in the web browser. This allows you to easily debug WebAssembly code.
+
+Let's compare error messages before and after enabling the feature:
+
+**Before:** `"RuntimeError: Unreachable executed"`
+
+**After:** `"panicked at 'index out of bounds: the len is 3 but the index is 4', libcore/slice/mod.rs:2046:10"`
+
+To do this, a panic hook for WebAssembly is provided that logs panics to the developer console via the JavaScript `console.error` function. 
+
+Note that although the template sets up the function, your error messages will not automatically be enhanced. To enable the enhanced errors, call the function `utils::set_panic_hook()` in `lib.rs` when your code first runs. The function may be called multiple times if needed.
+
+For more details, see the [`console_error_panic_hook` repository](https://github.com/rustwasm/console_error_panic_hook).