feat: initial implementation

it panics way too much and its probably quite flaky, but it is usable if
you take care

src/lib.rs

@@ -0,0 +1,150 @@
+#[proc_macro_attribute]
+pub fn jni(
+	attrs: proc_macro::TokenStream,
+	input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+	generate_jni_wrapper(
+		syn::parse_macro_input!(attrs),
+		syn::parse_macro_input!(input),
+	)
+		.unwrap()
+		.into()
+}
+
+
+
+
+use proc_macro2::{Span, TokenStream, TokenTree};
+use quote::TokenStreamExt;
+use syn::{FnArg, Item, ReturnType, Type};
+
+fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenStream, syn::Error> {
+	let mut out = TokenStream::new();
+
+	let item = syn::parse2(input.clone())?;
+
+	let Item::Fn(fn_item) = item else {
+		panic!("can only be applied to functions"); // TODO throw err instead of panic
+	};
+
+	let mut what_next = WhatNext::Nothing;
+
+	let mut package = None;
+	let mut clazz = None;
+
+	for attr in attrs {
+		match what_next {
+			WhatNext::Nothing => {
+				if let TokenTree::Ident(ref i) = attr {
+					match i.to_string().as_ref() {
+						"package" => what_next = WhatNext::Package,
+						"class" => what_next = WhatNext::Class,
+						_ => panic!("unexpected keyword in attrs: {}", attr),
+					}
+				}
+			},
+			WhatNext::Class => {
+				if let TokenTree::Literal(i) = attr {
+					let raw = i.to_string().replace('"', "");
+					clazz = Some(syn::Ident::new(&raw, Span::call_site()));
+					what_next = WhatNext::Nothing;
+				}
+			},
+			WhatNext::Package => {
+				if let TokenTree::Literal(i) = attr {
+					let raw = i.to_string().replace('"', "").replace(".", "_");
+					package = Some(syn::Ident::new(&raw, Span::call_site()));
+					what_next = WhatNext::Nothing;
+				}
+			},
+		}
+	}
+
+	let package = package.expect("missing attribute 'package'");
+	let clazz = clazz.expect("missing attribute 'class'");
+
+	let (could_error, ret_type) = match fn_item.sig.output {
+		syn::ReturnType::Default => (false, fn_item.sig.output),
+		syn::ReturnType::Type(_tok, ty) => match *ty {
+			syn::Type::Path(ref path) => {
+				let Some(last) = path.path.segments.last() else {
+					panic!("empty type path");
+				};
+
+				// TODO this is terrible, macro returns a function and we call it?? there must be a
+				// better way!!!
+				let mut out = (
+					false,
+					ReturnType::Type(syn::Token![->](Span::call_site()), Box::new(Type::Path(path.clone())))
+				);
+
+				if last.ident == "Result" {
+					match &last.arguments {
+						syn::PathArguments::None => panic!("result without generics is not valid"),
+						syn::PathArguments::Parenthesized(_) => panic!("parenthesized result is not valid"),
+						syn::PathArguments::AngleBracketed(ref generics) => for generic in generics.args.iter() {
+							match generic {
+								syn::GenericArgument::Lifetime(_) => continue,
+								syn::GenericArgument::Type(ty) => {
+									out = (true, ReturnType::Type(syn::Token![->](Span::call_site()), Box::new(ty.clone())));
+									break;
+								},
+								_ => panic!("unexpected type in Result generic"),
+							}
+						}
+					}
+				}
+
+				out
+			},
+			_ => panic!("unsupported return type"),
+		},
+	};
+
+
+	let mut incoming = TokenStream::new();
+	let mut forwarding = TokenStream::new();
+
+	for arg in fn_item.sig.inputs {
+		let FnArg::Typed(ty) = arg else {
+			panic!("jni macro doesn't work on methods");
+		};
+		incoming.append_all(quote::quote!( #ty , ));
+		let pat = ty.pat;
+		forwarding.append_all(quote::quote!( #pat , ));
+	}
+
+	let name = fn_item.sig.ident.to_string();
+	let name_jni = name.replace("_", "_1");
+	let fn_name_inner = syn::Ident::new(&name, Span::call_site());
+	let fn_name = syn::Ident::new(&format!("Java_{package}_{clazz}_{name_jni}"), Span::call_site());
+
+	let wrapped = if could_error {
+		quote::quote! {
+			#[no_mangle]
+			pub extern "system" fn #fn_name<'local>(#incoming) #ret_type {
+				match #fn_name_inner(#forwarding) {
+					Ok(x) => x,
+					Err(e) => panic!("error in JNI!"), // TODO throw java exc
+				}
+			}
+		}
+	} else {
+		quote::quote! {
+			#[no_mangle]
+			pub extern "system" fn #fn_name<'local>(#incoming) #ret_type {
+				#fn_name_inner(#forwarding)
+			}
+		}
+	};
+
+	out.append_all(input);
+	out.append_all(wrapped);
+	Ok(out)
+}
+
+enum WhatNext {
+	Nothing,
+	Package,
+	Class,
+}