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diff --git a/syn/examples/lazy-static/README.md b/syn/examples/lazy-static/README.md new file mode 100644 index 0000000..bc64585 --- /dev/null +++ b/syn/examples/lazy-static/README.md @@ -0,0 +1,42 @@ +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 to +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: () = (); + | ^^ +``` |