def-mod 0.3.0

/*!
`def_mod!` provides a familiar syntax to the standard module declarations, but with the added benefit of being able
to easily define implementation routes and to statically verify module exports.

---
```rust
extern crate def_mod;

use def_mod::def_mod;

def_mod! {
	// This has the exact same behaviour as Rust's.
	mod my_mod;

	// Much like the one above, it also has the same effect as Rust's.
	mod my_first_mod {
		// It also checks to see if a method with the type `fn(u32) -> u8` was exported.
    	// It will fail to compile if it finds none.
		fn method(_: u32) -> u8;

		// Much like the method declaration from above, this will check to see if a type was exported.
		type MyStruct;

		// This will also check to see if the type `MyOtherStruct` was export.
		type MyOtherStruct {
			// This will check if this method exists on this type. (MyOtherStruct::method)
			fn method(_: u32) -> u8;
		}
	}

	// Attributes are declared like normal.
	#[cfg(windows)]
	mod my_second_mod;

	// When declaring an attribute, you can optionally add a path.
	// This path is then used as the path attribute for the module file.
	// All attributes declared with a path are treated as _mutually exclusive_.
	// So a `mod` declaration is generated for each.
	// This makes it a lot easier to manage cross-platform code.
	// Note: attributes that don't have a path are copied to each module declaration.

	#[cfg(windows)] = "sys/win/mod.rs"
	#[cfg(not(windows))] = "sys/nix/mod.rs"
	mod sys;

	// Expands to:

	#[cfg(windows)]
	#[path = "sys/win/mod.rs"]
	mod sys;

	#[cfg(not(windows))]
	#[path = "sys/nix/mod.rs"]
	mod sys;

	// You can also declare attributes on methods or types themselves, and they will be used when verifying the type.
	// This module itself will be verified when not on a windows system.
	#[cfg(not(windows))]
	mod my_third_mod {
		// This method will only be verified when on linux.
		#[cfg(linux)]
		fn interop() -> u8;
		// Same with this type. It will only be verified when on a linux system
		#[cfg(linux)]
		type SomeStruct {
			fn interop() -> u8;
		}
	}
}

fn main() {
}
```

NOTE: that `def_mod` uses syntax tricks to assert for types.  
So that means when you use the path shorthand, it will still only check the module that is loaded, not all potential modules.    

Eg, if I've got a module that has two possible impls, one for windows and one for unix.  
If I compile on windows, `def_mod` can't check if the unix module has the correct symbols declared, because it's not the module that's compiled.  

You would need to explicitly enable compilation of the modules you'd want to check, because you can declare  
arbitrary #[cfg] attributes, there's no way in `def_mod` to do this.  
It's entirely up to you.  

---

In case you're curious as to what the macro generates:

A method assertion is transformed to something like:

```rust
const _VALUE: fn(u32) -> u8 = my_mod::method;
```

A type assertion is transformed into a use decl, inside their own scope:

```rust
{
	use self::my_mod::Test;
	// Any method assertions for the type will also be placed inside the same scope.
}
```

All of those are shoved into a method, generated by the macro.

For example, given something like:

```rust
def_mod! {
	mod my_mod {
		fn plus_one(value: u8) -> u8;

		type MyStruct {
			fn new() -> MyStruct;
		}
	}
}
```

It'll turn into something like this:

```rust
mod my_mod;

fn __load_my_mod() {
	const _ASSERT_METHOD_0: fn(u8) -> u8 = my_mod::method;
	{
		use self::my_mod::MyStruct;
		const _ASSERT_METHOD_1: fn() -> MyStruct = MyStruct::new;
	}
}
```
*/

#![feature(proc_macro_diagnostic)]

extern crate proc_macro;
extern crate proc_macro2;
extern crate quote;
extern crate syn;

use proc_macro::TokenStream as TStream;

use proc_macro2::{Span, TokenStream};
use quote::{quote, quote_spanned, ToTokens};
use syn::*;
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::synom::{Parser, Synom};

/// 
/// ASd `asd`
/// 
/// 
#[proc_macro]
pub fn def_mod(tokens: TStream) -> TStream {
	let t = ModuleDecl::parse_all;
	let declarations: Vec<ModuleDecl> = t.parse(tokens).unwrap();

	let mut output = TokenStream::new();

	for module in declarations {
		let mut pathed_attrs = vec![];
		let mut custom_attrs = vec![];
		// Group the attributes that were declared with a path value.
		for attr in module.attrs {
			if let (attr, Some(path)) = attr {
				pathed_attrs.push((attr, path));
			} else {
				custom_attrs.push(attr.0);
			};
		}
		// Ghost the attr vectors, so no one can change them...
		let pathed_attrs = &pathed_attrs;
		let custom_attrs = &custom_attrs;

		let vis = &module.vis;
		let mod_token = &module.mod_token;
		let module_name = &module.ident;

		if pathed_attrs.is_empty() {
			let t = quote_spanned! { module_name.span() =>
				#(#custom_attrs)*
				#vis #mod_token #module_name;
			};
			t.to_tokens(&mut output);
		} else {
			for (attr, path) in pathed_attrs {
				let t = quote_spanned! { module_name.span() =>
					#attr
					#[path=#path]
					#(#custom_attrs)*
					#vis #mod_token #module_name;
				};
				t.to_tokens(&mut output);
			}
		}

		// Generate a load function, if the module was declared with some items.
		if let ModuleBody::Content((_brace, body)) = module.body {
			let mut index: u32 = 0;
			let items: Vec<TokenStream> = body.into_iter()
				.map(|item| {
					// Transform each item into the corresponding check.
					match item {
						DeclItem::Method(method_item) => {
							let t = gen_method_assertion(index, module_name, method_item);
							index += 1;
							t
						}
						DeclItem::Type(type_item) => {
							let attrs = &type_item.attrs;
							let type_name = &type_item.ident;

							let method_items = if let TypeDeclBody::Content((_brace, body)) = type_item.body {
								body.into_iter()
									.map(|method_item| {
										let t = gen_method_assertion(index, type_name, method_item);
										index += 1;
										t
									})
									.collect()
							} else {
								vec![]
							};

							// We use the actual use declaration here to test for the type itself, as it'll fail if it doesn't exist or not exported.
							// It also makes the codegen easier, because we don't have to qualify the full name type.
							quote! {
								#(#attrs)*
								{
									use self::#module_name::#type_name;
									#(#method_items)*
								}
							}
						}
					}
				})
				.collect();

			let function_name = format!("_load_{}", module_name);
			let function_name = Ident::new(&function_name, Span::call_site());
			let t = quote! {
				#[allow(dead_code)]
				fn #function_name() {
					#(#items)*
				}
			};
			t.to_tokens(&mut output);
		}
	}
	output.into()
}

///
/// A module declaration: `mod my_mod`
/// 
/// The only difference between this and a normal mod is
/// the ability to add a path literal to attributes:
/// ```rust
/// #[cfg(target_os = "windows")] = "my_mod/win/mod.rs"
/// mod my_mod;
/// ```
/// 
#[derive(Debug)]
struct ModuleDecl {
	attrs: Vec<(Attribute, Option<LitStr>)>,
	vis: Visibility,
	mod_token: Token![mod],
	ident: Ident,
	body: ModuleBody,
}

#[derive(Debug)]
enum ModuleBody {
	Content((token::Brace, Vec<DeclItem>)),
	Terminated(Token![;]),
}

impl ModuleDecl {
	named!(parse_all -> Vec<ModuleDecl>, do_parse!(
		decls: many0!(syn!(ModuleDecl)) >>
		(decls)
	));
}

impl Synom for ModuleDecl {
	named!(parse -> Self, do_parse!(
		attrs: many0!(do_parse!(
			attr: call!(Attribute::parse_outer) >>
			eq: option!(punct!(=)) >>
			path: cond!(eq.is_some(), syn!(LitStr))>>
			(attr, path)
		)) >>
		vis: syn!(Visibility) >>
		mod_token: keyword!(mod) >>
		ident: syn!(Ident) >>
		body: alt! (
			punct!(;) => { ModuleBody::Terminated }
			|
			braces!(many0!(DeclItem::parse)) => { ModuleBody::Content }
		) >>
		(ModuleDecl {
			attrs,
			vis,
			mod_token,
			ident,
			body,
		})
	));
}

impl ToTokens for ModuleDecl {
	fn to_tokens(&self, tokens: &mut TokenStream) {
		self.vis.to_tokens(tokens);
	}
}

#[derive(Debug)]
enum DeclItem {
	Method(TraitItemMethod),
	Type(TypeDecl),
}

#[derive(Debug)]
struct TypeDecl {
	attrs: Vec<Attribute>,
	ident: Ident,
	body: TypeDeclBody,
}

#[derive(Debug)]
enum TypeDeclBody {
	Content((token::Brace, Vec<TraitItemMethod>)),
	Terminated(Token![;]),
}

impl DeclItem {
	named!(parse -> Self, alt!(
		syn!(TraitItemMethod) => { DeclItem::Method }
		|
		syn!(TypeDecl) => { DeclItem::Type }
	));
}

impl Synom for TypeDecl {
	named!(parse -> Self, do_parse!(
			attrs: many0!(Attribute::parse_outer) >>
			_type: keyword!(type) >>
			ident: syn!(Ident) >>
			body: alt!(
				punct!(;) => { TypeDeclBody::Terminated }
				|
				braces!(many0!(TraitItemMethod::parse)) => { TypeDeclBody::Content }
			) >>
			(TypeDecl {
				attrs,
				ident,
				body,
			})
		)
	);
}

fn gen_method_assertion(index: u32, context: &Ident, method_item: TraitItemMethod) -> TokenStream {
	if let Some(body) = method_item.default {
		body.span()
			.unstable()
			.error("A body isn't valid here.")
			.emit();
		return TokenStream::new();
	}
	let attrs = &method_item.attrs;
	let load_name = format!("_ASSERT_METHOD_{}", index);
	let load_ident = Ident::new(&load_name, Span::call_site());
	let (ty, path) = convert(context, method_item.sig);
	quote! {
		#(#attrs)*
		const #load_ident: #ty = #path;
	}
}

fn convert(context: &Ident, sig: MethodSig) -> (TypeBareFn, ExprPath) {
	// @TODO Jezza - 19 Dec. 2018: Generic path attributes?

//	println!("Context: {}", context);
//	println!("Sig: {:?}", sig);
//	pub struct MethodSig {
//		pub constness: Option<Token![const]>,
//		pub unsafety: Option<Token![unsafe]>,
//		pub abi: Option<Abi>,
//		pub ident: Ident,
//		pub decl: FnDecl,
//	}
//	pub struct FnDecl {
//		pub fn_token: Token![fn],
//		pub generics: Generics,
//		pub paren_token: token::Paren,
//		pub inputs: Punctuated<FnArg, Token![,]>,
//		pub variadic: Option<Token![...]>,
//		pub output: ReturnType,
//	}
//	pub enum FnArg {
//		/// Self captured by reference in a function signature: `&self` or `&mut
//		/// self`.
//		///
//		/// *This type is available if Syn is built with the `"full"` feature.*
//		pub SelfRef(ArgSelfRef {
//			pub and_token: Token![&],
//			pub lifetime: Option<Lifetime>,
//			pub mutability: Option<Token![mut]>,
//			pub self_token: Token![self],
//		}),
//	
//		/// Self captured by value in a function signature: `self` or `mut
//		/// self`.
//		///
//		/// *This type is available if Syn is built with the `"full"` feature.*
//		pub SelfValue(ArgSelf {
//			pub mutability: Option<Token![mut]>,
//			pub self_token: Token![self],
//		}),
//	
//		/// An explicitly typed pattern captured by a function signature.
//		///
//		/// *This type is available if Syn is built with the `"full"` feature.*
//		pub Captured(ArgCaptured {
//			pub pat: Pat,
//			pub colon_token: Token![:],
//			pub ty: Type,
//		}),
//	
//		/// A pattern whose type is inferred captured by a function signature.
//		pub Inferred(Pat),
//		/// A type not bound to any pattern in a function signature.
//		pub Ignored(Type),
//	}
//	pub struct Generics {
//		pub lt_token: Option<Token![<]>,
//		pub params: Punctuated<GenericParam, Token![,]>,
//		pub gt_token: Option<Token![>]>,
//		pub where_clause: Option<WhereClause>,
//	}
	let MethodSig {
		constness,
		unsafety,
		abi,
		ident,
		decl,
	} = sig;

	let FnDecl {
		fn_token,
		generics,
		paren_token,
		inputs,
		variadic,
		output,
	} = decl;

	let inputs = {
		let mut values = Punctuated::new();
		for arg in inputs {
			let bare_fn_arg = match arg {
				FnArg::SelfRef(_v) => {
					continue;
				}
				FnArg::SelfValue(_v) => {
					continue;
				}
				FnArg::Captured(ArgCaptured {
					pat,
					colon_token,
					ty,
				}) => {
					BareFnArg {
						name: None,
						ty,
					}
				}
				FnArg::Inferred(_v) => {
					continue;
				}
				FnArg::Ignored(_v) => {
					continue;
				}
			};
			values.push(bare_fn_arg);
		}
		values
	};

	let type_bare_fn = TypeBareFn {
		unsafety,
		abi,
		fn_token,
		lifetimes: None,
		paren_token,
		inputs,
		variadic,
		output,
	};

	let mut segments = Punctuated::new();
	segments.push(PathSegment {
		ident: (*context).clone(),
		arguments: PathArguments::None,
	});
	segments.push(PathSegment {
		ident: ident.clone(),
		arguments: PathArguments::None,
	});
	let path = Path {
		leading_colon: None,
		segments,
	};
	let path = ExprPath {
		attrs: Vec::new(),
		qself: None,
		path,
	};

//	TypeBareFn {
//		=pub unsafety: Option<Token![unsafe]>,
//		=pub abi: Option<Abi>,
//		=pub fn_token: Token![fn],
//		pub lifetimes: Option<BoundLifetimes>,
//		=pub paren_token: token::Paren,
//		pub inputs: Punctuated<BareFnArg, Token![,]>,
//		=pub variadic: Option<Token![...]>,
//		=pub output: ReturnType,
//	}
//	pub struct BareFnArg {
//		pub name: Option<(BareFnArgName, Token![:])>,
//		pub ty: Type,
//	}
//	pub enum BareFnArgName {
//		/// Argument given a name.
//		Named(Ident),
//		/// Argument not given a name, matched with `_`.
//		Wild(Token![_]),
//	}

//	ExprPath {
//		pub attrs: Vec<Attribute>,
//		pub qself: Option<QSelf>,
//		pub path: Path,
//	}
//	pub struct Path {
//		pub leading_colon: Option<Token![::]>,
//		pub segments: Punctuated<PathSegment, Token![::]>,
//	}
//	pub struct PathSegment {
//		pub ident: Ident,
//		pub arguments: PathArguments,
//	}
	(type_bare_fn, path)
}