Allow unused in `macros.rs` docs

The example code in macros.rs is much clearer as it is than changing
it to remove the warnings created by adding the warn attribute to the
`lib.rs` file.

The example code was enclosed within a function block and
`#[allow(unused)]` added.  And a warn attribute added to `lib.rs`.

internals/src/lib.rs

@@ -10,6 +10,7 @@
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 // Coding conventions.
 #![warn(missing_docs)]
+#![doc(test(attr(warn(unused))))]
 // Exclude lints we don't think are valuable.
 #![allow(clippy::needless_question_mark)] // https://github.com/rust-bitcoin/rust-bitcoin/pull/2134
 #![allow(clippy::manual_range_contains)] // More readable than clippy's format.

internals/src/macros.rs

@@ -158,6 +158,8 @@ macro_rules! const_assert {
 /// # Examples
 ///
 /// ```rust
+/// # #[allow(unused)]
+/// # fn main() {
 /// # use core::fmt::{Display, Debug};
 /// use bitcoin_internals::impl_from_infallible;
 ///
@@ -167,7 +169,7 @@ macro_rules! const_assert {
 /// enum BetaEnum<'b> { Item(&'b usize) }
 /// impl_from_infallible!(BetaEnum<'b>);
 ///
-/// enum GammaEnum<T> { Item(T) };
+/// enum GammaEnum<T> { Item(T) }
 /// impl_from_infallible!(GammaEnum<T>);
 ///
 /// enum DeltaEnum<'b, 'a: 'static + 'b, T: 'a, D: Debug + Display + 'a> {
@@ -189,6 +191,7 @@ macro_rules! const_assert {
 ///     what: &'b D,
 /// }
 /// impl_from_infallible!(DeltaStruct<'b, 'a: 'static + 'b, T: 'a, D: Debug + Display + 'a>);
+/// # }
 /// ```
 ///
 /// See <https://stackoverflow.com/a/61189128> for more information about this macro.