1 files changed, 143 insertions, 0 deletions
diff --git a/syn/examples/lazy-static/lazy-static/src/lib.rs b/syn/examples/lazy-static/lazy-static/src/lib.rs
new file mode 100644
index 0000000..254ca72
--- /dev/null
+++ b/syn/examples/lazy-static/lazy-static/src/lib.rs
@@ -0,0 +1,143 @@
+#![recursion_limit = "128"]
+#![feature(proc_macro_diagnostic)]
+
+extern crate proc_macro;
+use self::proc_macro::TokenStream;
+
+use quote::{quote, quote_spanned};
+use syn::parse::{Parse, ParseStream, Result};
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, Expr, Ident, Token, Type, Visibility};
+
+/// Parses the following syntax, which aligns with the input of the real
+/// `lazy_static` crate.
+///
+///     lazy_static! {
+///         $VISIBILITY static ref $NAME: $TYPE = $EXPR;
+///     }
+///
+/// For example:
+///
+///     lazy_static! {
+///         static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
+///     }
+struct LazyStatic {
+    visibility: Visibility,
+    name: Ident,
+    ty: Type,
+    init: Expr,
+}
+
+impl Parse for LazyStatic {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let visibility: Visibility = input.parse()?;
+        input.parse::<Token![static]>()?;
+        input.parse::<Token![ref]>()?;
+        let name: Ident = input.parse()?;
+        input.parse::<Token![:]>()?;
+        let ty: Type = input.parse()?;
+        input.parse::<Token![=]>()?;
+        let init: Expr = input.parse()?;
+        input.parse::<Token![;]>()?;
+        Ok(LazyStatic {
+            visibility,
+            name,
+            ty,
+            init,
+        })
+    }
+}
+
+#[proc_macro]
+pub fn lazy_static(input: TokenStream) -> TokenStream {
+    let LazyStatic {
+        visibility,
+        name,
+        ty,
+        init,
+    } = parse_macro_input!(input as LazyStatic);
+
+    // The warning looks like this.
+    //
+    //     warning: come on, pick a more creative name
+    //       --> src/main.rs:10:16
+    //        |
+    //     10 |     static ref FOO: String = "lazy_static".to_owned();
+    //        |                ^^^
+    if name == "FOO" {
+        name.span()
+            .unwrap()
+            .warning("come on, pick a more creative name")
+            .emit();
+    }
+
+    // The error looks like this.
+    //
+    //     error: I can't think of a legitimate use for lazily initializing the value `()`
+    //       --> src/main.rs:10:27
+    //        |
+    //     10 |     static ref UNIT: () = ();
+    //        |                           ^^
+    if let Expr::Tuple(ref init) = init {
+        if init.elems.is_empty() {
+            init.span()
+                .unwrap()
+                .error("I can't think of a legitimate use for lazily initializing the value `()`")
+                .emit();
+            return TokenStream::new();
+        }
+    }
+
+    // Assert that the static type implements Sync. If not, user sees an error
+    // message like the following. We span this assertion with the field type's
+    // line/column so that the error message appears in the correct place.
+    //
+    //     error[E0277]: the trait bound `*const (): std::marker::Sync` is not satisfied
+    //       --> src/main.rs:10:21
+    //        |
+    //     10 |     static ref PTR: *const () = &();
+    //        |                     ^^^^^^^^^ `*const ()` cannot be shared between threads safely
+    let assert_sync = quote_spanned! {ty.span()=>
+        struct _AssertSync where #ty: std::marker::Sync;
+    };
+
+    // Check for Sized. Not vital to check here, but the error message is less
+    // confusing this way than if they get a Sized error in one of our
+    // implementation details where it assumes Sized.
+    //
+    //     error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
+    //       --> src/main.rs:10:19
+    //        |
+    //     10 |     static ref A: str = "";
+    //        |                   ^^^ `str` does not have a constant size known at compile-time
+    let assert_sized = quote_spanned! {ty.span()=>
+        struct _AssertSized where #ty: std::marker::Sized;
+    };
+
+    let init_ptr = quote_spanned! {init.span()=>
+        Box::into_raw(Box::new(#init))
+    };
+
+    let expanded = quote! {
+        #visibility struct #name;
+
+        impl std::ops::Deref for #name {
+            type Target = #ty;
+
+            fn deref(&self) -> &#ty {
+                #assert_sync
+                #assert_sized
+
+                static ONCE: std::sync::Once = std::sync::Once::new();
+                static mut VALUE: *mut #ty = 0 as *mut #ty;
+
+                unsafe {
+                    ONCE.call_once(|| VALUE = #init_ptr);
+                    &*VALUE
+                }
+            }
+        }
+    };
+
+    TokenStream::from(expanded)
+}