Merge #983

983: Handle generics in NativeScript derive and `#[methods]` r=chitoyuu a=chitoyuu

The derive macros are now aware of both lifetime and type parameters. `'static` bounds are added to both kinds, which improves the error message shown when a user tries to put a lifetime into a NativeClass.

This behavior is chosen because unlike type parameters, lifetime parameters aren't very useful in this context: only `'static` would be supported anyway, and any attempted usage is hightly likely to have originated from user error.

Also added the `#[skip]` attribute for skipping fields in `FromVarargs`. This is useful mainly for skipping marker fields in generic code.

Close #601

## Error message

The error message that appears when lifetime parameters are declared for `NativeClass` types now clearly indicate where the problem lies and what the expectations are:

```
error[E0478]: lifetime bound not satisfied
 --> tests/ui/derive_fail_lifetime.rs:3:10
  |
3 | #[derive(NativeClass)]
  |          ^^^^^^^^^^^
  |
note: lifetime parameter instantiated with the lifetime `'a` as defined here
 --> tests/ui/derive_fail_lifetime.rs:4:12
  |
4 | struct Foo<'a> {
  |            ^^
  = note: but lifetime parameter must outlive the static lifetime
  = note: this error originates in the derive macro `NativeClass` (in Nightly builds, run with -Z macro-backtrace for more info)
```

Compares this with the old error message:

```
error[E0726]: implicit elided lifetime not allowed here
 --> tests/ui/derive_fail_lifetime.rs:4:8
  |
4 | struct Foo<'a> {
  |        ^^^ expected lifetime parameter
  |
  = note: assuming a `'static` lifetime...
help: indicate the anonymous lifetime
  |
4 | struct Foo<'_><'a> {
  |           ++++

error[E0106]: missing lifetime specifier
 --> tests/ui/derive_fail_lifetime.rs:4:8
  |
4 | struct Foo<'a> {
  |        ^^^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
3 ~ #[derive(NativeClass<'a>)]
4 ~ struct Foo<'a><'a> {
  |

error[E0277]: the trait bound `&gdnative::prelude::Node: OwnerArg<'_, Reference, Shared
>` is not satisfied
 --> tests/ui/derive_fail_lifetime.rs:3:10
  |
3 | #[derive(NativeClass)]
  |          ^^^^^^^^^^^ the trait `OwnerArg<'_, Reference, Shared>` is not implemented
 for `&gdnative::prelude::Node`
  |
  = help: the following other types implement trait `OwnerArg<'a, T, Own>`:
            &'a T
            TRef<'a, T, Own>
  = note: this error originates in the derive macro `NativeClass` (in Nightly builds, r
un with -Z macro-backtrace for more info)
```

Close #174

Co-authored-by: Chitose Yuuzaki <chitoyuu@potatoes.gay>

Author: bors[bot]

gdnative-derive/src/extend_bounds.rs

@@ -1,5 +1,6 @@
 use std::collections::HashSet;
 
+use syn::spanned::Spanned;
 use syn::visit::{self, Visit};
 use syn::{Generics, Ident, Type, TypePath};
 
@@ -46,7 +47,12 @@ impl<'ast> BoundsVisitor<'ast> {
     }
 }
 
-pub fn with_visitor<'ast, F>(generics: Generics, bound: &syn::Path, op: F) -> Generics
+pub fn with_visitor<'ast, F>(
+    generics: Generics,
+    bound: Option<&syn::Path>,
+    lifetime: Option<&str>,
+    op: F,
+) -> Generics
 where
     F: FnOnce(&mut BoundsVisitor<'ast>),
 {
@@ -55,28 +61,57 @@ where
     op(&mut visitor);
 
     // where thing: is_trait
-    fn where_predicate(thing: Type, is_trait: syn::Path) -> syn::WherePredicate {
+    fn where_predicate(
+        thing: Type,
+        bound: Option<&syn::Path>,
+        lifetime: Option<&str>,
+    ) -> syn::WherePredicate {
+        let mut bounds = vec![];
+
+        if let Some(bound) = bound {
+            bounds.push(syn::TypeParamBound::Trait(syn::TraitBound {
+                paren_token: None,
+                modifier: syn::TraitBoundModifier::None,
+                lifetimes: None,
+                path: bound.clone(),
+            }));
+        }
+
+        if let Some(lifetime) = lifetime {
+            bounds.push(syn::TypeParamBound::Lifetime(syn::Lifetime::new(
+                lifetime,
+                thing.span(),
+            )));
+        }
+
         syn::WherePredicate::Type(syn::PredicateType {
             lifetimes: None,
             bounded_ty: thing,
             colon_token: <Token![:]>::default(),
-            bounds: vec![syn::TypeParamBound::Trait(syn::TraitBound {
-                paren_token: None,
-                modifier: syn::TraitBoundModifier::None,
-                lifetimes: None,
-                path: is_trait,
-            })]
-            .into_iter()
-            .collect(),
+            bounds: bounds.into_iter().collect(),
         })
     }
 
     // place bounds on all used type parameters and associated types
-    let new_predicates = visitor
+    let mut new_predicates = visitor
         .used
         .into_iter()
         .cloned()
-        .map(|bounded_ty| where_predicate(syn::Type::Path(bounded_ty), bound.clone()));
+        .map(|bounded_ty| where_predicate(syn::Type::Path(bounded_ty), bound, lifetime))
+        .collect::<Vec<_>>();
+
+    // Add lifetime bounds to all type parameters, regardless of usage, due to how
+    // lifetimes for generic types are determined.
+    new_predicates.extend(generics.type_params().map(|param| {
+        where_predicate(
+            syn::Type::Path(syn::TypePath {
+                qself: None,
+                path: param.ident.clone().into(),
+            }),
+            None,
+            lifetime,
+        )
+    }));
 
     let mut generics = generics;
     generics

gdnative-derive/src/lib.rs

@@ -8,7 +8,7 @@ extern crate quote;
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::ToTokens;
-use syn::{AttributeArgs, DeriveInput, ItemFn, ItemImpl};
+use syn::{AttributeArgs, DeriveInput, ItemFn, ItemImpl, ItemType};
 
 mod extend_bounds;
 mod methods;
@@ -455,6 +455,41 @@ pub fn derive_native_class(input: TokenStream) -> TokenStream {
     TokenStream::from(derived)
 }
 
+/// Wires up necessary internals for a concrete monomorphization of a generic `NativeClass`,
+/// represented as a type alias, so it can be registered.
+///
+/// The monomorphized type will be available to Godot under the name of the type alias,
+/// once registered.
+///
+/// For more context, please refer to [gdnative::derive::NativeClass](NativeClass).
+///
+/// # Examples
+///
+/// ```ignore
+/// #[derive(NativeClass)]
+/// struct Pair<T> {
+///     a: T,
+///     b: T,
+/// }
+///
+/// #[gdnative::derive::monomorphize]
+/// type IntPair = Pair<i32>;
+///
+/// fn init(handle: InitHandle) {
+///     handle.add_class::<IntPair>();
+/// }
+/// ```
+#[proc_macro_attribute]
+pub fn monomorphize(meta: TokenStream, input: TokenStream) -> TokenStream {
+    let args = parse_macro_input!(meta as AttributeArgs);
+    let item_type = parse_macro_input!(input as ItemType);
+
+    match native_script::derive_monomorphize(args, item_type) {
+        Ok(tokens) => tokens.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}
+
 #[proc_macro_derive(ToVariant, attributes(variant))]
 pub fn derive_to_variant(input: TokenStream) -> TokenStream {
     match variant::derive_to_variant(variant::ToVariantTrait::ToVariant, input) {
@@ -493,7 +528,21 @@ pub fn derive_from_variant(input: TokenStream) -> TokenStream {
 ///     #[opt] baz: Option<Ref<Node>>,
 /// }
 /// ```
-#[proc_macro_derive(FromVarargs, attributes(opt))]
+///
+/// ## Field attributes
+///
+/// Attributes can be used to customize behavior of certain fields. All attributes are optional.
+///
+/// ### `#[opt]`
+///
+/// Marks an argument as optional. Required arguments must precede all optional arguments.
+/// Default values are obtained through `Default::default`.
+///
+/// ### `#[skip]`
+///
+/// Instructs the macro to skip a field. Skipped fields do not affect the signature of the
+/// argument list. They may be located anywhere. Values are obtained through `Default::default`.
+#[proc_macro_derive(FromVarargs, attributes(opt, skip))]
 pub fn derive_from_varargs(input: TokenStream) -> TokenStream {
     let derive_input = syn::parse_macro_input!(input as syn::DeriveInput);
     match varargs::derive_from_varargs(derive_input) {

gdnative-derive/src/methods.rs

@@ -1,4 +1,4 @@
-use syn::{spanned::Spanned, FnArg, ImplItem, ItemImpl, Pat, PatIdent, Signature, Type};
+use syn::{spanned::Spanned, FnArg, Generics, ImplItem, ItemImpl, Pat, PatIdent, Signature, Type};
 
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{quote, ToTokens};
@@ -330,6 +330,7 @@ pub(crate) struct ExportArgs {
 pub(crate) fn derive_methods(item_impl: ItemImpl) -> TokenStream2 {
     let derived = crate::automatically_derived();
     let (impl_block, export) = impl_gdnative_expose(item_impl);
+    let (impl_generics, _, where_clause) = impl_block.generics.split_for_impl();
 
     let class_name = export.class_ty;
 
@@ -390,7 +391,7 @@ pub(crate) fn derive_methods(item_impl: ItemImpl) -> TokenStream2 {
                 quote_spanned!(ret_span=>)
             };
 
-            let method = wrap_method(&class_name, &export_method)
+            let method = wrap_method(&class_name, &impl_block.generics, &export_method)
                 .unwrap_or_else(|err| err.to_compile_error());
 
             quote_spanned!( sig_span=>
@@ -410,7 +411,7 @@ pub(crate) fn derive_methods(item_impl: ItemImpl) -> TokenStream2 {
         #impl_block
 
         #derived
-        impl gdnative::export::NativeClassMethods for #class_name {
+        impl #impl_generics gdnative::export::NativeClassMethods for #class_name #where_clause {
             fn nativeclass_register(#builder: &::gdnative::export::ClassBuilder<Self>) {
                 use gdnative::export::*;
 
@@ -767,11 +768,17 @@ pub(crate) fn expand_godot_wrap_method(
         return Err(errors);
     }
 
-    wrap_method(&class_name, &export_method.expect("ExportMethod is valid")).map_err(|e| vec![e])
+    wrap_method(
+        &class_name,
+        &Generics::default(),
+        &export_method.expect("ExportMethod is valid"),
+    )
+    .map_err(|e| vec![e])
 }
 
 fn wrap_method(
     class_name: &Type,
+    generics: &Generics,
     export_method: &ExportMethod,
 ) -> Result<TokenStream2, syn::Error> {
     let ExportMethod {
@@ -783,6 +790,21 @@ fn wrap_method(
     let gdnative_core = crate::crate_gdnative_core();
     let automatically_derived = crate::automatically_derived();
 
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+    let turbofish_ty_generics = ty_generics.as_turbofish();
+
+    let generic_marker_decl = if generics.params.is_empty() {
+        quote!(())
+    } else {
+        quote!(core::marker::PhantomData #ty_generics)
+    };
+
+    let generic_marker_ctor = if generics.params.is_empty() {
+        quote!(())
+    } else {
+        quote!(core::marker::PhantomData)
+    };
+
     let sig_span = sig.ident.span();
     let ret_span = sig.output.span();
 
@@ -875,8 +897,8 @@ fn wrap_method(
 
         quote_spanned! { sig_span =>
             #automatically_derived
-            impl #gdnative_async::StaticArgsAsyncMethod<#class_name> for ThisMethod {
-                type Args = Args;
+            impl #impl_generics #gdnative_async::StaticArgsAsyncMethod<#class_name> for ThisMethod #ty_generics #where_clause {
+                type Args = Args #ty_generics;
 
                 fn spawn_with(
                     &self,
@@ -885,7 +907,7 @@ fn wrap_method(
                     __spawner.spawn(move |__ctx, __this, __args| {
                         let __future = __this
                             .#map_method(move |__rust_val, __base| {
-                                let Args { #(#destructure_arg_list,)* } = __args;
+                                let Args { #(#destructure_arg_list,)* __generic_marker } = __args;
 
                                 #[allow(unused_unsafe)]
                                 unsafe {
@@ -916,17 +938,19 @@ fn wrap_method(
                 }
             }
 
-            #gdnative_async::Async::new(#gdnative_async::StaticArgs::new(ThisMethod))
+            #gdnative_async::Async::new(#gdnative_async::StaticArgs::new(ThisMethod #turbofish_ty_generics {
+                _marker: #generic_marker_ctor,
+            }))
         }
     } else {
         quote_spanned! { sig_span =>
             #automatically_derived
-            impl #gdnative_core::export::StaticArgsMethod<#class_name> for ThisMethod {
-                type Args = Args;
+            impl #impl_generics #gdnative_core::export::StaticArgsMethod<#class_name> for ThisMethod #ty_generics #where_clause {
+                type Args = Args #ty_generics;
                 fn call(
                     &self,
                     __this: TInstance<'_, #class_name, #gdnative_core::object::ownership::Shared>,
-                    Args { #(#destructure_arg_list,)* }: Args,
+                    Args { #(#destructure_arg_list,)* __generic_marker }: Self::Args,
                 ) -> #gdnative_core::core_types::Variant {
                     __this
                         .#map_method(|__rust_val, __base| {
@@ -950,23 +974,48 @@ fn wrap_method(
                 }
             }
 
-            #gdnative_core::export::StaticArgs::new(ThisMethod)
+            #gdnative_core::export::StaticArgs::new(ThisMethod #turbofish_ty_generics {
+                _marker: #generic_marker_ctor,
+            })
         }
     };
 
+    // Necessary standard traits have to be implemented manually because the default derive isn't smart enough.
     let output = quote_spanned! { sig_span =>
         {
-            #[derive(Copy, Clone, Default)]
-            struct ThisMethod;
+            struct ThisMethod #ty_generics #where_clause {
+                _marker: #generic_marker_decl,
+            }
+
+            impl #impl_generics Copy for ThisMethod #ty_generics #where_clause {}
+            impl #impl_generics Clone for ThisMethod #ty_generics #where_clause {
+                fn clone(&self) -> Self {
+                    *self
+                }
+            }
+
+            impl #impl_generics Default for ThisMethod #ty_generics #where_clause {
+                fn default() -> Self {
+                    Self {
+                        _marker: #generic_marker_ctor,
+                    }
+                }
+            }
+
+            unsafe impl #impl_generics Send for ThisMethod #ty_generics #where_clause {}
+            unsafe impl #impl_generics Sync for ThisMethod #ty_generics #where_clause {}
 
             use #gdnative_core::export::{NativeClass, OwnerArg};
             use #gdnative_core::object::{Instance, TInstance};
             use #gdnative_core::derive::FromVarargs;
 
             #[derive(FromVarargs)]
             #automatically_derived
-            struct Args {
+            struct Args #ty_generics #where_clause {
                 #(#declare_arg_list,)*
+
+                #[skip]
+                __generic_marker: #generic_marker_decl,
             }
 
             #impl_body