add cargo manifest reflection

Signed-off-by: Jason Volk <jason@zemos.net>

src/macros/cargo.rs

@@ -0,0 +1,51 @@
+use std::{fs::read_to_string, path::PathBuf};
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, AttributeArgs, ItemConst, Lit, NestedMeta};
+
+pub(super) fn manifest(args: TokenStream, item: TokenStream) -> TokenStream {
+	let item = parse_macro_input!(item as ItemConst);
+	let args = parse_macro_input!(args as AttributeArgs);
+	let member = args.into_iter().find_map(|arg| {
+		let NestedMeta::Lit(arg) = arg else {
+			return None;
+		};
+		let Lit::Str(arg) = arg else {
+			return None;
+		};
+		Some(arg.value())
+	});
+
+	let path = manifest_path(member.as_deref());
+	let manifest = read_to_string(&path).unwrap_or_default();
+
+	let name = item.ident;
+	let val = manifest.as_str();
+	let ret = quote! {
+		const #name: &'static str = #val;
+	};
+
+	ret.into()
+}
+
+#[allow(clippy::option_env_unwrap)]
+fn manifest_path(member: Option<&str>) -> PathBuf {
+	let mut path: PathBuf = option_env!("CARGO_MANIFEST_DIR")
+		.expect("missing CARGO_MANIFEST_DIR in environment")
+		.into();
+
+	// conduwuit/src/macros/ -> conduwuit/src/
+	path.pop();
+
+	if let Some(member) = member {
+		// conduwuit/$member/Cargo.toml
+		path.push(member);
+	} else {
+		// conduwuit/src/ -> conduwuit/
+		path.pop();
+	}
+
+	path.push("Cargo.toml");
+	path
+}