1 files changed, 31 insertions, 0 deletions
diff --git a/syn-mid/examples/const_fn/lib.rs b/syn-mid/examples/const_fn/lib.rs
new file mode 100644
index 0000000..29255f2
--- /dev/null
+++ b/syn-mid/examples/const_fn/lib.rs
@@ -0,0 +1,31 @@
+#![warn(rust_2018_idioms)]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+use quote::quote;
+use syn_mid::ItemFn;
+
+/// An attribute for easy generation of a const function with conditional compilations.
+#[proc_macro_attribute]
+pub fn const_fn(args: TokenStream, function: TokenStream) -> TokenStream {
+    assert!(!args.is_empty(), "requires an argument");
+
+    let mut function = syn::parse_macro_input!(function as ItemFn);
+    let mut const_function = function.clone();
+
+    if function.constness.is_some() {
+        function.constness = None;
+    } else {
+        const_function.constness = Some(Default::default());
+    }
+
+    let args = TokenStream2::from(args);
+    TokenStream::from(quote! {
+        #[cfg(not(#args))]
+        #function
+        #[cfg(#args)]
+        #const_function
+    })
+}