Rollup merge of #64726 - andrewbanchich:unimplemented, r=rkruppe

rewrite documentation for unimplemented! to clarify use

The current docs for `unimplemented!` seem to miss the point of this macro.

> This can be useful if you are prototyping and are just looking to have your code type-check, or if you're implementing a trait that requires multiple methods, and you're only planning on using one of them.

You could also return a `()` if you just want your code to type-check.

I think `unimplemented!` is useful for when you want your program to exit when it reaches an unimplemented area.

I rewrote the explanation and gave examples of both forms of this macro that I think clarify its use a little better.

src/libcore/macros.rs

@@ -520,18 +520,20 @@ macro_rules! unreachable {
     });
 }
 
-/// Indicates unfinished code.
+/// Indicates unfinished code by panicking with a message of "not yet implemented".
 ///
-/// This can be useful if you are prototyping and are just looking to have your
-/// code type-check, or if you're implementing a trait that requires multiple
-/// methods, and you're only planning on using one of them.
+/// This allows the your code to type-check, which is useful if you are prototyping or
+/// implementing a trait that requires multiple methods which you don't plan of using all of.
 ///
 /// There is no difference between `unimplemented!` and `todo!` apart from the
 /// name.
 ///
 /// # Panics
 ///
-/// This will always [panic!](macro.panic.html)
+/// This will always [panic!](macro.panic.html) because `unimplemented!` is just a
+/// shorthand for `panic!` with a fixed, specific message.
+///
+/// Like `panic!`, this macro has a second form for displaying custom values.
 ///
 /// # Examples
 ///
@@ -539,38 +541,53 @@ macro_rules! unreachable {
 ///
 /// ```
 /// trait Foo {
-///     fn bar(&self);
+///     fn bar(&self) -> u8;
 ///     fn baz(&self);
+///     fn qux(&self) -> Result<u64, ()>;
 /// }
 /// ```
 ///
-/// We want to implement `Foo` on one of our types, but we also want to work on
-/// just `bar()` first. In order for our code to compile, we need to implement
-/// `baz()`, so we can use `unimplemented!`:
+/// We want to implement `Foo` for 'MyStruct', but so far we only know how to
+/// implement the `bar()` function. `baz()` and `qux()` will still need to be defined
+/// in our implementation of `Foo`, but we can use `unimplemented!` in their definitions
+/// to allow our code to compile.
+///
+/// In the meantime, we want to have our program stop running once these
+/// unimplemented functions are reached.
 ///
 /// ```
 /// # trait Foo {
-/// #     fn bar(&self);
+/// #     fn bar(&self) -> u8;
 /// #     fn baz(&self);
+/// #     fn qux(&self) -> Result<u64, ()>;
 /// # }
 /// struct MyStruct;
 ///
 /// impl Foo for MyStruct {
-///     fn bar(&self) {
-///         // implementation goes here
+///     fn bar(&self) -> u8 {
+///         1 + 1
 ///     }
 ///
 ///     fn baz(&self) {
-///         // let's not worry about implementing baz() for now
+///         // We aren't sure how to even start writing baz yet,
+///         // so we have no logic here at all.
+///         // This will display "thread 'main' panicked at 'not yet implemented'".
 ///         unimplemented!();
 ///     }
+///
+///     fn qux(&self) -> Result<u64, ()> {
+///         let n = self.bar();
+///         // We have some logic here,
+///         // so we can use unimplemented! to display what we have so far.
+///         // This will display:
+///         // "thread 'main' panicked at 'not yet implemented: we need to divide by 2'".
+///         unimplemented!("we need to divide by {}", n);
+///     }
 /// }
 ///
 /// fn main() {
 ///     let s = MyStruct;
 ///     s.bar();
-///
-///     // we aren't even using baz() yet, so this is fine.
 /// }
 /// ```
 #[macro_export]