Add lightning-macros crate

Previously, we used the `bdk_macros` dependency for some simple proc
macros in `lightning-transaction-sync`. However, post-1.0 BDK doesn't
further maintain this crate and will at some point probably yank it
together with the old `bdk` crate that was split up.

Here, we create a new crate for utility proc macros and ~~steal~~ add
what we currently use (slightly modified for the latest `syn` version's
API though). In the future we may want to expand this crate, e.g., for
some `maybe_async` macros in the context of an `async KVStore`
implementation.

Cargo.toml

@@ -14,6 +14,7 @@ members = [
     "lightning-rapid-gossip-sync",
     "lightning-custom-message",
     "lightning-transaction-sync",
+    "lightning-macros",
     "possiblyrandom",
 ]
 

ci/ci-tests.sh

@@ -46,6 +46,7 @@ WORKSPACE_MEMBERS=(
 	lightning-rapid-gossip-sync
 	lightning-custom-message
 	lightning-transaction-sync
+	lightning-macros
 	possiblyrandom
 )
 

lightning-macros/Cargo.toml

@@ -0,0 +1,26 @@
+[package]
+name = "lightning-macros"
+version = "0.1.0"
+authors = ["Elias Rohrer"]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/lightningdevkit/rust-lightning/"
+description = """
+Proc macros used by LDK
+"""
+edition = "2021"
+
+[package.metadata.docs.rs]
+rustdoc-args = ["--cfg", "docsrs"]
+
+[lib]
+proc-macro = true
+
+[features]
+
+[dependencies]
+syn = { version = "2.0.77", default-features = false, features = ["parsing", "printing", "proc-macro", "full"] }
+proc-macro2 = { version = "1.0.86", default-features = false, features = ["proc-macro"] }
+quote = { version = "1.0", default-features = false, features = ["proc-macro"] }
+
+[lints]
+workspace = true

lightning-macros/src/lib.rs

@@ -0,0 +1,129 @@
+// This file is Copyright its original authors, visible in version control
+// history.
+//
+// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
+// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// You may not use this file except in accordance with one or both of these
+// licenses.
+
+#![crate_name = "lightning_macros"]
+
+//! Proc macros used by LDK
+
+#![cfg_attr(not(test), no_std)]
+#![deny(missing_docs)]
+#![forbid(unsafe_code)]
+#![deny(rustdoc::broken_intra_doc_links)]
+#![deny(rustdoc::private_intra_doc_links)]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::spanned::Spanned;
+use syn::{parse, ImplItemFn, ItemImpl, ItemTrait, Token};
+
+fn add_async_trait(mut parsed: ItemTrait) -> TokenStream {
+	let output = quote! {
+		#[cfg(not(feature = "async-interface"))]
+		#parsed
+	};
+
+	for mut item in &mut parsed.items {
+		if let syn::TraitItem::Fn(f) = &mut item {
+			f.sig.asyncness = Some(Token![async](f.span()));
+		}
+	}
+
+	let output = quote! {
+		#output
+
+		#[cfg(feature = "async-interface")]
+		#[async_trait(?Send)]
+		#parsed
+	};
+
+	output.into()
+}
+
+fn add_async_method(mut parsed: ImplItemFn) -> TokenStream {
+	let output = quote! {
+		#[cfg(not(feature = "async-interface"))]
+		#parsed
+	};
+
+	parsed.sig.asyncness = Some(Token![async](parsed.span()));
+
+	let output = quote! {
+		#output
+
+		#[cfg(feature = "async-interface")]
+		#parsed
+	};
+
+	output.into()
+}
+
+fn add_async_impl_trait(mut parsed: ItemImpl) -> TokenStream {
+	let output = quote! {
+		#[cfg(not(feature = "async-interface"))]
+		#parsed
+	};
+
+	for mut item in &mut parsed.items {
+		if let syn::ImplItem::Fn(f) = &mut item {
+			f.sig.asyncness = Some(Token![async](f.span()));
+		}
+	}
+
+	let output = quote! {
+		#output
+
+		#[cfg(feature = "async-interface")]
+		#[async_trait(?Send)]
+		#parsed
+	};
+
+	output.into()
+}
+
+/// Makes a method or every method of a trait `async`, if the `async-interface` feature is enabled.
+///
+/// Requires the `async-trait` crate as a dependency whenever this attribute is used on a trait
+/// definition or trait implementation.
+#[proc_macro_attribute]
+pub fn maybe_async(_attr: TokenStream, item: TokenStream) -> TokenStream {
+	if let Ok(parsed) = parse(item.clone()) {
+		add_async_trait(parsed)
+	} else if let Ok(parsed) = parse(item.clone()) {
+		add_async_method(parsed)
+	} else if let Ok(parsed) = parse(item) {
+		add_async_impl_trait(parsed)
+	} else {
+		(quote! {
+			compile_error!("#[maybe_async] can only be used on methods, trait or trait impl blocks")
+		})
+		.into()
+	}
+}
+
+/// Awaits, if the `async-interface` feature is enabled.
+#[proc_macro]
+pub fn maybe_await(expr: TokenStream) -> TokenStream {
+	let expr: proc_macro2::TokenStream = expr.into();
+	let quoted = quote! {
+		{
+			#[cfg(not(feature = "async-interface"))]
+			{
+				#expr
+			}
+
+			#[cfg(feature = "async-interface")]
+			{
+				#expr.await
+			}
+		}
+	};
+
+	quoted.into()
+}