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-rw-r--r--syn-mid/examples/const_fn/Cargo.toml16
-rw-r--r--syn-mid/examples/const_fn/lib.rs31
2 files changed, 47 insertions, 0 deletions
diff --git a/syn-mid/examples/const_fn/Cargo.toml b/syn-mid/examples/const_fn/Cargo.toml
new file mode 100644
index 0000000..d823e76
--- /dev/null
+++ b/syn-mid/examples/const_fn/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "const_fn"
+version = "0.0.0"
+authors = ["Taiki Endo <te316e89@gmail.com>"]
+edition = "2018"
+publish = false
+
+[lib]
+proc-macro = true
+path = "lib.rs"
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = "1.0"
+syn-mid = { version = "0.4", path = "../..", features = ["clone-impls"] }
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
+ })
+}