Move proc_macro_attribute example to an example block

Author: ehuss

src/procedural-macros.md

@@ -228,71 +228,78 @@ r[macro.proc.attribute.intro]
 
 r[macro.proc.attribute.def]
 Attribute macros are defined by a [public] [function] with the `proc_macro_attribute` [attribute] that has a signature of `(TokenStream, TokenStream) -> TokenStream`. The first [`TokenStream`] is the delimited token tree following the attribute's name, not including the outer delimiters. If the attribute is written as a bare attribute name, the attribute [`TokenStream`] is empty. The second [`TokenStream`] is the rest of the [item] including other [attributes] on the [item]. The returned [`TokenStream`] replaces the [item] with an arbitrary number of [items].
+> [!EXAMPLE]
+> The following attribute macro takes the input stream and returns it as is, effectively being the no-op of attributes.
+>
+> <!-- ignore: test doesn't support proc-macro -->
+> ```rust,ignore
+> # #![crate_type = "proc-macro"]
+> # extern crate proc_macro;
+> # use proc_macro::TokenStream;
+>
+> #[proc_macro_attribute]
+> pub fn return_as_is(_attr: TokenStream, item: TokenStream) -> TokenStream {
+>     item
+> }
+> ```
 
 r[macro.proc.attribute.namespace]
 The `proc_macro_attribute` attribute defines the attribute in the [macro namespace] in the root of the crate.
+> [!EXAMPLE]
+> This following example shows the stringified [`TokenStream`s] that the attribute macros see. The output will show in the output of the compiler. The output is shown in the comments after the function prefixed with "out:".
+>
+> <!-- ignore: test doesn't support proc-macro -->
+> ```rust,ignore
+> // my-macro/src/lib.rs
+> # extern crate proc_macro;
+> # use proc_macro::TokenStream;
+>
+> #[proc_macro_attribute]
+> pub fn show_streams(attr: TokenStream, item: TokenStream) -> TokenStream {
+>     println!("attr: \"{attr}\"");
+>     println!("item: \"{item}\"");
+>     item
+> }
+> ```
+>
+> <!-- ignore: requires external crates -->
+> ```rust,ignore
+> // src/lib.rs
+> extern crate my_macro;
+>
+> use my_macro::show_streams;
+>
+> // Example: Basic function
+> #[show_streams]
+> fn invoke1() {}
+> // out: attr: ""
+> // out: item: "fn invoke1() {}"
+>
+> // Example: Attribute with input
+> #[show_streams(bar)]
+> fn invoke2() {}
+> // out: attr: "bar"
+> // out: item: "fn invoke2() {}"
+>
+> // Example: Multiple tokens in the input
+> #[show_streams(multiple => tokens)]
+> fn invoke3() {}
+> // out: attr: "multiple => tokens"
+> // out: item: "fn invoke3() {}"
+>
+> // Example:
+> #[show_streams { delimiters }]
+> fn invoke4() {}
+> // out: attr: "delimiters"
+> // out: item: "fn invoke4() {}"
+> ```
 
-For example, this attribute macro takes the input stream and returns it as is, effectively being the no-op of attributes.
 
-<!-- ignore: test doesn't support proc-macro -->
-```rust,ignore
-# #![crate_type = "proc-macro"]
-# extern crate proc_macro;
-# use proc_macro::TokenStream;
 
-#[proc_macro_attribute]
-pub fn return_as_is(_attr: TokenStream, item: TokenStream) -> TokenStream {
-    item
-}
-```
 
-This following example shows the stringified [`TokenStream`s] that the attribute macros see. The output will show in the output of the compiler. The output is shown in the comments after the function prefixed with "out:".
 
-<!-- ignore: test doesn't support proc-macro -->
-```rust,ignore
-// my-macro/src/lib.rs
-# extern crate proc_macro;
-# use proc_macro::TokenStream;
 
-#[proc_macro_attribute]
-pub fn show_streams(attr: TokenStream, item: TokenStream) -> TokenStream {
-    println!("attr: \"{attr}\"");
-    println!("item: \"{item}\"");
-    item
-}
-```
 
-<!-- ignore: requires external crates -->
-```rust,ignore
-// src/lib.rs
-extern crate my_macro;
-
-use my_macro::show_streams;
-
-// Example: Basic function
-#[show_streams]
-fn invoke1() {}
-// out: attr: ""
-// out: item: "fn invoke1() {}"
-
-// Example: Attribute with input
-#[show_streams(bar)]
-fn invoke2() {}
-// out: attr: "bar"
-// out: item: "fn invoke2() {}"
-
-// Example: Multiple tokens in the input
-#[show_streams(multiple => tokens)]
-fn invoke3() {}
-// out: attr: "multiple => tokens"
-// out: item: "fn invoke3() {}"
-
-// Example:
-#[show_streams { delimiters }]
-fn invoke4() {}
-// out: attr: "delimiters"
-// out: item: "fn invoke4() {}"
-```
 
 r[macro.proc.token]
 ## Declarative macro tokens and procedural macro tokens