#![allow(unused)]
use quote::{quote, quote_spanned};
use syn::{parse::Parse, parse::ParseStream, spanned::Spanned};

#[derive(Debug)]
pub(crate) struct ExportedFn {
    entire_span: proc_macro2::Span,
    signature: syn::Signature,
    is_public: bool,
    mut_receiver: bool,
}

impl Parse for ExportedFn {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let fn_all: syn::ItemFn = input.parse()?;
        let entire_span = fn_all.span();
        let str_type_path = syn::parse2::<syn::Path>(quote! { str }).unwrap();

        // Determine if the function is public.
        let is_public = match fn_all.vis {
            syn::Visibility::Public(_) => true,
            _ => false
        };
        // Determine whether function generates a special calling convention for a mutable
        // reciever.
        let mut_receiver = {
            if let Some(first_arg) = fn_all.sig.inputs.first() {
                match first_arg {
                    syn::FnArg::Receiver(syn::Receiver { reference: Some(_), ..}) => true,
                    syn::FnArg::Typed(syn::PatType { ref ty, .. }) => {
                        match ty.as_ref() {
                            &syn::Type::Reference(syn::TypeReference {
                                mutability: Some(_), ..  }) => true,
                            &syn::Type::Reference(syn::TypeReference { mutability: None,
                                                                       ref elem, .. }) => {
                                match elem.as_ref() {
                                    &syn::Type::Path(ref p) if p.path == str_type_path => false,
                                    _ => return Err(syn::Error::new(ty.span(),
                                           "references from Rhai in this position \
                                            must be mutable")),
                                }
                            },
                            _ => false,
                        }
                    },
                    _ => false,
                }
            } else {
                false
            }
        };

        // All arguments after the first must be moved except for &str.
        for arg in fn_all.sig.inputs.iter().skip(1) {
            let ty = match arg {
                syn::FnArg::Typed(syn::PatType { ref ty, .. }) => ty,
                _ => panic!("internal error: receiver argument outside of first position!?"),
            };
            let is_ok = match ty.as_ref() {
                &syn::Type::Reference(syn::TypeReference { mutability: Some(_), .. }) => false,
                &syn::Type::Reference(syn::TypeReference { mutability: None,
                                                           ref elem, .. }) => {
                    match elem.as_ref() {
                        &syn::Type::Path(ref p) if p.path == str_type_path => true,
                        _ => false,
                    }
                },
                &syn::Type::Verbatim(_) => false,
                _ => true,
            };
            if !is_ok {
                return Err(syn::Error::new(ty.span(), "this type in this position passes from \
                                                        Rhai by value"));
            }
        }

        // No returning references or pointers.
        if let syn::ReturnType::Type(_, ref rtype) = fn_all.sig.output {
            match rtype.as_ref() {
                &syn::Type::Ptr(_) => return Err(syn::Error::new(fn_all.sig.output.span(),
                                                 "cannot return a pointer to Rhai")),
                &syn::Type::Reference(_) => return Err(syn::Error::new(fn_all.sig.output.span(),
                                                       "cannot return a reference to Rhai")),
                _ => {},
            }
        }
        Ok(ExportedFn {
            entire_span,
            signature: fn_all.sig,
            is_public,
            mut_receiver,
        })
    }
}

impl ExportedFn {
    pub(crate) fn mutable_receiver(&self) -> bool {
        self.mut_receiver
    }

    pub(crate) fn is_public(&self) -> bool {
        self.is_public
    }

    pub(crate) fn span(&self) -> &proc_macro2::Span {
        &self.entire_span
    }

    pub(crate) fn name(&self) -> &syn::Ident {
        &self.signature.ident
    }

    pub(crate) fn arg_list(&self) -> impl Iterator<Item=&syn::FnArg> {
        self.signature.inputs.iter()
    }

    pub(crate) fn arg_count(&self) -> usize {
        self.signature.inputs.len()
    }

    pub(crate) fn return_type(&self) -> Option<&syn::Type> {
        if let syn::ReturnType::Type(_, ref rtype) = self.signature.output {
            Some(rtype)
        } else {
            None
        }
    }

    pub fn generate(self) -> proc_macro2::TokenStream {
        let name: syn::Ident = syn::Ident::new(&format!("rhai_fn__{}", self.name().to_string()),
                                               self.name().span());
        let impl_block = self.generate_impl("Token");
        quote! {
            #[allow(unused)]
            pub mod #name {
                use super::*;
                pub struct Token();
                #impl_block
            }
        }
    }

    pub fn generate_impl(&self, on_type_name: &str) -> proc_macro2::TokenStream {
        let name: syn::Ident = self.name().clone();
        let arg_count = self.arg_count();
        let is_method_call = self.mutable_receiver();

        let mut unpack_stmts: Vec<syn::Stmt> = Vec::new();
        let mut unpack_exprs: Vec<syn::Expr> = Vec::new();
        let mut input_type_exprs: Vec<syn::Expr> = Vec::new();
        let skip_first_arg;

        // Handle the first argument separately if the function has a "method like" receiver
        if is_method_call {
            skip_first_arg = true;
            let first_arg = self.arg_list().next().unwrap();
            let var = syn::Ident::new("arg0", proc_macro2::Span::call_site());
            match first_arg {
                syn::FnArg::Typed(pattern) =>  {
                    let arg_type: &syn::Type = {
                        match pattern.ty.as_ref() {
                            &syn::Type::Reference(
                                syn::TypeReference { ref elem, .. }) => elem.as_ref(),
                            ref p => p,
                        }
                    };
                    let downcast_span = quote_spanned!(
                        arg_type.span()=> args[0usize].downcast_mut::<#arg_type>().unwrap());
                    unpack_stmts.push(syn::parse2::<syn::Stmt>(quote! {
                        let #var: &mut _ = #downcast_span;
                    }).unwrap());
                    input_type_exprs.push(syn::parse2::<syn::Expr>(quote_spanned!(
                        arg_type.span()=> std::any::TypeId::of::<#arg_type>()
                    )).unwrap());
                },
                syn::FnArg::Receiver(_) => todo!("true self parameters not implemented yet"),
            }
            unpack_exprs.push(syn::parse2::<syn::Expr>(quote! { #var }).unwrap());
        } else {
            skip_first_arg = false;
        }

        // Handle the rest of the arguments, which all are passed by value.
        //
        // The only exception is strings, which need to be downcast to ImmutableString to enable a
        // zero-copy conversion to &str by reference.
        let str_type_path = syn::parse2::<syn::Path>(quote! { str }).unwrap();
        for (i, arg) in self.arg_list().enumerate().skip(skip_first_arg as usize) {
            let var = syn::Ident::new(&format!("arg{}", i), proc_macro2::Span::call_site());
            let is_str_ref;
            match arg {
                syn::FnArg::Typed(pattern) =>  {
                    let arg_type: &syn::Type = pattern.ty.as_ref();
                    let downcast_span = match pattern.ty.as_ref() {
                        &syn::Type::Reference(syn::TypeReference { mutability: None,
                                                                   ref elem, .. }) => {
                            match elem.as_ref() {
                                &syn::Type::Path(ref p) if p.path == str_type_path => {
                                    is_str_ref = true;
                                    quote_spanned!(arg_type.span()=>
                                                   args[#i]
                                                   .downcast_clone::<rhai::ImmutableString>()
                                                   .unwrap())
                                },
                                _ => panic!("internal error: why wasn't this found earlier!?"),
                            }
                        },
                        _ => {
                            is_str_ref = false;
                            quote_spanned!(arg_type.span()=>
                                            args[#i].downcast_clone::<#arg_type>().unwrap())
                        },
                    };

                    unpack_stmts.push(syn::parse2::<syn::Stmt>(quote! {
                        let #var = #downcast_span;
                    }).unwrap());
                    if !is_str_ref {
                        input_type_exprs.push(syn::parse2::<syn::Expr>(quote_spanned!(
                            arg_type.span()=> std::any::TypeId::of::<#arg_type>()
                        )).unwrap());
                    } else {
                        input_type_exprs.push(syn::parse2::<syn::Expr>(quote_spanned!(
                            arg_type.span()=> std::any::TypeId::of::<rhai::ImmutableString>()
                        )).unwrap());
                    }
                },
                syn::FnArg::Receiver(_) => panic!("internal error: how did this happen!?"),
            }
            if !is_str_ref {
                unpack_exprs.push(syn::parse2::<syn::Expr>(quote! { #var }).unwrap());
            } else {
                unpack_exprs.push(syn::parse2::<syn::Expr>(quote! { &#var }).unwrap());
            }
        }

        // In method calls, the first argument will need to be mutably borrowed. Because Rust marks
        // that as needing to borrow the entire array, all of the previous argument unpacking via
        // clone needs to happen first.
        if is_method_call {
            let arg0 = unpack_stmts.remove(0);
            unpack_stmts.push(arg0);
        }

        let type_name = syn::Ident::new(on_type_name, proc_macro2::Span::call_site());
        quote! {
            impl rhai::plugin::PluginFunction for #type_name {
                fn call(&self,
                        args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                    if args.len() != #arg_count {
                        return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                format!("wrong arg count: {} != {}",
                                        args.len(), #arg_count), rhai::Position::none())));
                    }
                    #(#unpack_stmts)*
                    Ok(rhai::Dynamic::from(#name(#(#unpack_exprs),*)))
                }

                fn is_method_call(&self) -> bool { #is_method_call }
                fn is_varadic(&self) -> bool { false }
                fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(#type_name()) }
                fn input_types(&self) -> Box<[std::any::TypeId]> {
                    vec![#(#input_type_exprs),*].into_boxed_slice()
                }
            }
        }
    }
}

#[cfg(test)]
mod function_tests {
    use super::ExportedFn;

    use proc_macro2::TokenStream;
    use quote::quote;

    #[test]
    fn minimal_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn do_nothing() { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "do_nothing");
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());
        assert_eq!(item_fn.arg_list().count(), 0);
    }

    #[test]
    fn one_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn do_something(x: usize) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "do_something");
        assert_eq!(item_fn.arg_list().count(), 1);
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());

        assert_eq!(item_fn.arg_list().next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { x: usize }).unwrap());
    }

    #[test]
    fn two_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn do_something(x: usize, y: f32) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "do_something");
        assert_eq!(item_fn.arg_list().count(), 2);
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());

        assert_eq!(item_fn.arg_list().next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { x: usize }).unwrap());
        assert_eq!(item_fn.arg_list().skip(1).next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { y: f32 }).unwrap());
    }

    #[test]
    fn usize_returning_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn get_magic_number() -> usize { 42 }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "get_magic_number");
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert_eq!(item_fn.arg_list().count(), 0);
        assert_eq!(item_fn.return_type().unwrap(),
                &syn::Type::Path(syn::TypePath {
                    qself: None,
                    path: syn::parse2::<syn::Path>(quote! { usize }).unwrap()
                })
        );
    }

    #[test]
    fn ref_returning_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn get_magic_phrase() -> &'static str { "open sesame" }
        };

        let err = syn::parse2::<ExportedFn>(input_tokens).unwrap_err();
        assert_eq!(format!("{}", err),
                   "cannot return a reference to Rhai");
    }

    #[test]
    fn ptr_returning_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn get_magic_phrase() -> *const str { "open sesame" }
        };

        let err = syn::parse2::<ExportedFn>(input_tokens).unwrap_err();
        assert_eq!(format!("{}", err),
                   "cannot return a pointer to Rhai");
    }

    #[test]
    fn ref_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn greet(who: &Person) { }
        };

        let err = syn::parse2::<ExportedFn>(input_tokens).unwrap_err();
        assert_eq!(format!("{}", err),
                   "references from Rhai in this position must be mutable");
    }

    #[test]
    fn ref_second_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn greet(count: usize, who: &Person) { }
        };

        let err = syn::parse2::<ExportedFn>(input_tokens).unwrap_err();
        assert_eq!(format!("{}", err),
                   "this type in this position passes from Rhai by value");
    }

    #[test]
    fn mut_ref_second_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn give(item_name: &str, who: &mut Person) { }
        };

        let err = syn::parse2::<ExportedFn>(input_tokens).unwrap_err();
        assert_eq!(format!("{}", err),
                   "this type in this position passes from Rhai by value");
    }

    #[test]
    fn str_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn log(message: &str) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "log");
        assert_eq!(item_fn.arg_list().count(), 1);
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());

        assert_eq!(item_fn.arg_list().next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { message: &str }).unwrap());
    }

    #[test]
    fn str_second_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn log(level: usize, message: &str) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "log");
        assert_eq!(item_fn.arg_list().count(), 2);
        assert!(!item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());

        assert_eq!(item_fn.arg_list().next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { level: usize }).unwrap());
        assert_eq!(item_fn.arg_list().skip(1).next().unwrap(),
                   &syn::parse2::<syn::FnArg>(quote! { message: &str }).unwrap());
    }

    #[test]
    fn private_fn() {
        let input_tokens: TokenStream = quote! {
            fn do_nothing() { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "do_nothing");
        assert!(!item_fn.mutable_receiver());
        assert!(!item_fn.is_public());
        assert!(item_fn.return_type().is_none());
        assert_eq!(item_fn.arg_list().count(), 0);
    }

    #[test]
    fn receiver_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn act_upon(&mut self) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "act_upon");
        assert!(item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());
        assert_eq!(item_fn.arg_list().count(), 1);
    }

    #[test]
    fn immutable_receiver_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn act_upon(&self) { }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_eq!(&item_fn.name().to_string(), "act_upon");
        assert!(item_fn.mutable_receiver());
        assert!(item_fn.is_public());
        assert!(item_fn.return_type().is_none());
        assert_eq!(item_fn.arg_list().count(), 1);
    }
}

#[cfg(test)]
mod generate_tests {
    use super::ExportedFn;

    use proc_macro2::TokenStream;
    use quote::quote;

    fn assert_streams_eq(actual: TokenStream, expected: TokenStream) {
        let actual = actual.to_string();
        let expected = expected.to_string();
        if &actual != &expected {
            let mut counter = 0;
            let iter = actual.chars().zip(expected.chars())
                .inspect(|_| counter += 1)
                .skip_while(|(a, e)| *a == *e);
            let (actual_diff, expected_diff) = {
                let mut actual_diff = String::new();
                let mut expected_diff = String::new();
                for (a, e) in iter.take(50) {
                    actual_diff.push(a);
                    expected_diff.push(e);
                }
                (actual_diff, expected_diff)
            };
            eprintln!("actual != expected, diverge at char {}", counter);
        }
        assert_eq!(actual, expected);
    }

    #[test]
    fn minimal_fn () {
        let input_tokens: TokenStream = quote! {
            pub fn do_nothing() { }
        };

        let expected_tokens = quote! {
            #[allow(unused)]
            pub mod rhai_fn__do_nothing {
                use super::*;
                pub struct Token();
                impl rhai::plugin::PluginFunction for Token {
                    fn call(&self,
                            args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                    ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                        if args.len() != 0usize {
                            return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                    format!("wrong arg count: {} != {}",
                                            args.len(), 0usize), rhai::Position::none())));
                        }
                        Ok(rhai::Dynamic::from(do_nothing()))
                    }

                    fn is_method_call(&self) -> bool { false }
                    fn is_varadic(&self) -> bool { false }
                    fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(Token()) }
                    fn input_types(&self) -> Box<[std::any::TypeId]> {
                        vec![].into_boxed_slice()
                    }
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_streams_eq(item_fn.generate(), expected_tokens);
    }

    #[test]
    fn one_arg_usize_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn do_something(x: usize) { }
        };

        let expected_tokens = quote! {
            #[allow(unused)]
            pub mod rhai_fn__do_something {
                use super::*;
                pub struct Token();
                impl rhai::plugin::PluginFunction for Token {
                    fn call(&self,
                            args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                    ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                        if args.len() != 1usize {
                            return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                    format!("wrong arg count: {} != {}",
                                            args.len(), 1usize), rhai::Position::none())));
                        }
                        let arg0 = args[0usize].downcast_clone::<usize>().unwrap();
                        Ok(rhai::Dynamic::from(do_something(arg0)))
                    }

                    fn is_method_call(&self) -> bool { false }
                    fn is_varadic(&self) -> bool { false }
                    fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(Token()) }
                    fn input_types(&self) -> Box<[std::any::TypeId]> {
                        vec![std::any::TypeId::of::<usize>()].into_boxed_slice()
                    }
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_streams_eq(item_fn.generate(), expected_tokens);
    }

    #[test]
    fn one_arg_usize_fn_impl() {
        let input_tokens: TokenStream = quote! {
            pub fn do_something(x: usize) { }
        };

        let expected_tokens = quote! {
            impl rhai::plugin::PluginFunction for MyType {
                fn call(&self,
                        args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                    if args.len() != 1usize {
                        return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                format!("wrong arg count: {} != {}",
                                        args.len(), 1usize), rhai::Position::none())));
                    }
                    let arg0 = args[0usize].downcast_clone::<usize>().unwrap();
                    Ok(rhai::Dynamic::from(do_something(arg0)))
                }

                fn is_method_call(&self) -> bool { false }
                fn is_varadic(&self) -> bool { false }
                fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(MyType()) }
                fn input_types(&self) -> Box<[std::any::TypeId]> {
                    vec![std::any::TypeId::of::<usize>()].into_boxed_slice()
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_streams_eq(item_fn.generate_impl("MyType"), expected_tokens);
    }

    #[test]
    fn two_arg_returning_usize_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn add_together(x: usize, y: usize) -> usize { x + y }
        };

        let expected_tokens = quote! {
            #[allow(unused)]
            pub mod rhai_fn__add_together {
                use super::*;
                pub struct Token();
                impl rhai::plugin::PluginFunction for Token {
                    fn call(&self,
                            args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                    ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                        if args.len() != 2usize {
                            return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                    format!("wrong arg count: {} != {}",
                                            args.len(), 2usize), rhai::Position::none())));
                        }
                        let arg0 = args[0usize].downcast_clone::<usize>().unwrap();
                        let arg1 = args[1usize].downcast_clone::<usize>().unwrap();
                        Ok(rhai::Dynamic::from(add_together(arg0, arg1)))
                    }

                    fn is_method_call(&self) -> bool { false }
                    fn is_varadic(&self) -> bool { false }
                    fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(Token()) }
                    fn input_types(&self) -> Box<[std::any::TypeId]> {
                        vec![std::any::TypeId::of::<usize>(),
                             std::any::TypeId::of::<usize>()].into_boxed_slice()
                    }
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert_streams_eq(item_fn.generate(), expected_tokens);
    }

    #[test]
    fn mut_arg_usize_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn increment(x: &mut usize, y: usize) { *x += y; }
        };

        let expected_tokens = quote! {
            #[allow(unused)]
            pub mod rhai_fn__increment {
                use super::*;
                pub struct Token();
                impl rhai::plugin::PluginFunction for Token {
                    fn call(&self,
                            args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                    ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                        if args.len() != 2usize {
                            return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                    format!("wrong arg count: {} != {}",
                                            args.len(), 2usize), rhai::Position::none())));
                        }
                        let arg1 = args[1usize].downcast_clone::<usize>().unwrap();
                        let arg0: &mut _ = args[0usize].downcast_mut::<usize>().unwrap();
                        Ok(rhai::Dynamic::from(increment(arg0, arg1)))
                    }

                    fn is_method_call(&self) -> bool { true }
                    fn is_varadic(&self) -> bool { false }
                    fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(Token()) }
                    fn input_types(&self) -> Box<[std::any::TypeId]> {
                        vec![std::any::TypeId::of::<usize>(),
                             std::any::TypeId::of::<usize>()].into_boxed_slice()
                    }
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert!(item_fn.mutable_receiver());
        assert_streams_eq(item_fn.generate(), expected_tokens);
    }

    #[test]
    fn str_arg_fn() {
        let input_tokens: TokenStream = quote! {
            pub fn special_print(message: &str) { eprintln!("----{}----", message); }
        };

        let expected_tokens = quote! {
            #[allow(unused)]
            pub mod rhai_fn__special_print {
                use super::*;
                pub struct Token();
                impl rhai::plugin::PluginFunction for Token {
                    fn call(&self,
                            args: &mut [&mut rhai::Dynamic], pos: rhai::Position
                    ) -> Result<rhai::Dynamic, Box<rhai::EvalAltResult>> {
                        if args.len() != 1usize {
                            return Err(Box::new(rhai::EvalAltResult::ErrorRuntime(
                                    format!("wrong arg count: {} != {}",
                                            args.len(), 1usize), rhai::Position::none())));
                        }
                        let arg0 = args[0usize].downcast_clone::<rhai::ImmutableString>().unwrap();
                        Ok(rhai::Dynamic::from(special_print(&arg0)))
                    }

                    fn is_method_call(&self) -> bool { false }
                    fn is_varadic(&self) -> bool { false }
                    fn clone_boxed(&self) -> Box<dyn rhai::plugin::PluginFunction> { Box::new(Token()) }
                    fn input_types(&self) -> Box<[std::any::TypeId]> {
                        vec![std::any::TypeId::of::<rhai::ImmutableString>()].into_boxed_slice()
                    }
                }
            }
        };

        let item_fn = syn::parse2::<ExportedFn>(input_tokens).unwrap();
        assert!(!item_fn.mutable_receiver());
        assert_streams_eq(item_fn.generate(), expected_tokens);
    }
}

#[cfg(test)]
mod ui_tests {
    #[test]
    fn all() {
        let t = trybuild::TestCases::new();
        t.compile_fail("ui_tests/*.rs");
    }
}