feat: return compiler errors, throw java err

no more random panicing

README.md

@@ -15,10 +15,12 @@ fn your_function_name(arg: i32) -> Result<(), String> {
 
 by specifying package and class, this crate will write an appropriate wrapper with the right function name. If inner function returns a `Result<>`, wrapper will also handle it. (currently just panics, soon will throw exceptions!)
 
+note that input/output arguments must be natively FFI safe: there will be no hidden translations! you will have to un-marshal strings yourself
+
 ## examples
 the following function:
 ```rust
-#[java_easy_jni::jni(package = "mp.code", class = "Client")]
+#[jni_macro::jni(package = "mp.code", class = "Client")]
 fn connect(env: JNIEnv, cacca: JString) -> Result<(), ConnectionError> {
   let config = codemp::api::Config::new("asd".into(), "dsa".into());
   tokio().block_on(codemp::Client::connect(config))?;

src/lib.rs

@@ -21,10 +21,8 @@ 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) = syn::parse2(input.clone())? else {
-
+		return Err(syn::Error::new(Span::call_site(), "#[jni] is only supported on functions"));
-	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;
@@ -39,7 +37,7 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 					match i.to_string().as_ref() {
 						"package" => what_next = WhatNext::Package,
 						"class" => what_next = WhatNext::Class,
-						_ => panic!("unexpected keyword in attrs: {}", attr),
+						_ => return Err(syn::Error::new(Span::call_site(), "unexpected attribute on macro: {attr}")),
 					}
 				}
 			},
@@ -60,15 +58,15 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 		}
 	}
 
-	let package = package.expect("missing attribute 'package'");
+	let Some(package) = package else { return Err(syn::Error::new(Span::call_site(), "missing attribute 'package'")) };
-	let clazz = clazz.expect("missing attribute 'class'");
+	let Some(clazz) = clazz else { return Err(syn::Error::new(Span::call_site(), "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");
+					return Err(syn::Error::new(Span::call_site(), "empty Result type is not valid"));
 				};
 
 				// TODO this is terrible, macro returns a function and we call it?? there must be a
@@ -80,8 +78,8 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 
 				if last.ident == "Result" {
 					match &last.arguments {
-						syn::PathArguments::None => panic!("result without generics is not valid"),
+						syn::PathArguments::None => return Err(syn::Error::new(Span::call_site(), "Result without generics is not valid")),
-						syn::PathArguments::Parenthesized(_) => panic!("parenthesized result is not valid"),
+						syn::PathArguments::Parenthesized(_) => return Err(syn::Error::new(Span::call_site(), "Parenthesized Result is not valid")),
 						syn::PathArguments::AngleBracketed(ref generics) => for generic in generics.args.iter() {
 							match generic {
 								syn::GenericArgument::Lifetime(_) => continue,
@@ -89,7 +87,7 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 									out = (true, ReturnType::Type(syn::Token![->](Span::call_site()), Box::new(ty.clone())));
 									break;
 								},
-								_ => panic!("unexpected type in Result generic"),
+								_ => return Err(syn::Error::new(Span::call_site(), "unexpected type in Result")),
 							}
 						}
 					}
@@ -97,7 +95,7 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 
 				out
 			},
-			_ => panic!("unsupported return type"),
+			_ => return Err(syn::Error::new(Span::call_site(), "unsupported return type")),
 		},
 	};
 
@@ -107,7 +105,7 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 
 	for arg in fn_item.sig.inputs {
 		let FnArg::Typed(ty) = arg else {
-			panic!("jni macro doesn't work on methods");
+			return Err(syn::Error::new(Span::call_site(), "#[jni] macro doesn't work on methods"));
 		};
 		incoming.append_all(quote::quote!( #ty , ));
 		let pat = ty.pat;
@@ -119,13 +117,22 @@ fn generate_jni_wrapper(attrs: TokenStream, input: TokenStream) -> Result<TokenS
 	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 Some(env_ident) = forwarding.clone().into_iter().next() else {
+		return Err(syn::Error::new(Span::call_site(), "missing JNIEnv argument"));
+	};
+
 	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
+					Err(e) => {
+						#env_ident
+							.throw_new("java/lang/RuntimeException", format!("{e:?}"))
+							.expect("failed throwing Java exception from native call");
+						return 0;
+					}
 				}
 			}
 		}