Document the lazy-static example
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+An example of parsing a custom syntax within a `functionlike!(...)` procedural
+macro. Demonstrates how to trigger custom warnings and error messages on
+individual tokens of the input.
+
+- [`lazy-static/src/lib.rs`](lazy-static/src/lib.rs)
+- [`example/src/main.rs`](example/src/main.rs)
+
+The library implements a `lazy_static!` macro similar to the one from the real
+[`lazy_static`](https://docs.rs/lazy_static/1.0.0/lazy_static/) crate on
+crates.io.
+
+```rust
+lazy_static {
+ static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
+}
+```
+
+Compile and run the example by doing `cargo run` in the directory of the
+`example` crate.
+
+The implementation shows how to trigger custom warnings and error messages on
+the macro input. For example if you try adding an uncreatively named `FOO` lazy
+static, the macro will scold you with the following warning.
+
+```
+warning: come on, pick a more creative name
+ --> src/main.rs:10:16
+ |
+10 | static ref FOO: String = "lazy_static".to_owned();
+ | ^^^
+```
+
+And if you try to lazily initialize `() = ()`, the macro will outright refuse
+the compile it for you.
+
+```
+error: I can't think of a legitimate use for lazily initializing the value `()`
+ --> src/main.rs:10:27
+ |
+10 | static ref UNIT: () = ();
+ | ^^
+```