Create variant macro

README.md

@@ -1,13 +1,13 @@
-# `impl_trait_utils`
+# impl-trait-utils
 
 Utilities for working with impl traits in Rust.
 
-## `trait_transformer`
+## `make_variant`
 
-Trait transformer is an experimental crate that generates specialized versions of a base trait. For example, if you want a `Send`able version of your trait, you'd write:
+`make_variant` generates a specialized version of a base trait that uses `async fn` and/or `-> impl Trait`. For example, if you want a `Send`able version of your trait, you'd write:
 
 ```rust
-#[trait_transformer(SendIntFactory: Send)]
+#[trait_transformer::make_variant(SendIntFactory: Send)]
 trait IntFactory {
     async fn make(&self) -> i32;
     // ..or..
@@ -16,7 +16,13 @@ trait IntFactory {
 }
 ```
 
-Which creates a new `SendIntFactory: IntFactory + Send` trait and additionally bounds `SendIntFactory::make(): Send` and `SendIntFactory::stream(): Send`. The generated sytax is still experimental, as it relies on the nightly and unstable `async_fn_in_trait`, `return_position_impl_trait_in_trait`, and `return_type_notation` features.
+Which creates a new `SendIntFactory: IntFactory + Send` trait and additionally bounds `SendIntFactory::make(): Send` and `SendIntFactory::stream(): Send`. Ordinary methods are not affected.
+
+Implementers of the trait can choose to implement the variant instead of the original trait. The macro creates a blanket impl which ensures that any type which implements the variant also implements the original trait.
+
+## `trait_transformer`
+
+`trait_transformer` does the same thing as `make_variant`, but using experimental nightly-only syntax that depends on the `return_type_notation` feature. It may be used to experiment with new kinds of trait transformations in the future.
 
 #### License and usage notes
 

trait_transformer/examples/variant.rs

@@ -0,0 +1,27 @@
+// Copyright (c) 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::future::Future;
+
+use trait_transformer::make_variant;
+
+#[make_variant(SendIntFactory: Send)]
+trait IntFactory {
+    const NAME: &'static str;
+
+    type MyFut<'a>: Future
+    where
+        Self: 'a;
+
+    async fn make(&self, x: u32, y: &str) -> i32;
+    fn stream(&self) -> impl Iterator<Item = i32>;
+    fn call(&self) -> u32;
+    fn another_async(&self, input: Result<(), &str>) -> Self::MyFut<'_>;
+}
+
+fn main() {}

trait_transformer/src/lib.rs

@@ -6,7 +6,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![doc = include_str!("../../README.md")]
+
 mod transformer;
+mod variant;
 
 #[proc_macro_attribute]
 pub fn trait_transformer(
@@ -15,3 +18,11 @@ pub fn trait_transformer(
 ) -> proc_macro::TokenStream {
     transformer::trait_transformer(attr, item)
 }
+
+#[proc_macro_attribute]
+pub fn make_variant(
+    attr: proc_macro::TokenStream,
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    variant::make_variant(attr, item)
+}

trait_transformer/src/variant.rs

@@ -0,0 +1,216 @@
+// Copyright (c) 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::iter;
+
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{
+    parse::{Parse, ParseStream},
+    parse_macro_input,
+    punctuated::Punctuated,
+    token::Plus,
+    Error, FnArg, Generics, Ident, ItemTrait, Pat, PatType, Result, ReturnType, Signature, Token,
+    TraitBound, TraitItem, TraitItemConst, TraitItemFn, TraitItemType, Type, TypeImplTrait,
+    TypeParamBound,
+};
+
+struct Attrs {
+    variant: MakeVariant,
+}
+
+impl Parse for Attrs {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Ok(Self {
+            variant: MakeVariant::parse(input)?,
+        })
+    }
+}
+
+struct MakeVariant {
+    name: Ident,
+    #[allow(unused)]
+    colon: Token![:],
+    bounds: Punctuated<TraitBound, Plus>,
+}
+
+impl Parse for MakeVariant {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Ok(Self {
+            name: input.parse()?,
+            colon: input.parse()?,
+            bounds: input.parse_terminated(TraitBound::parse, Token![+])?,
+        })
+    }
+}
+
+pub fn make_variant(
+    attr: proc_macro::TokenStream,
+    item: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    let attrs = parse_macro_input!(attr as Attrs);
+    let item = parse_macro_input!(item as ItemTrait);
+
+    let variant = mk_variant(&attrs, &item);
+    let blanket_impl = mk_blanket_impl(&attrs, &item);
+    let output = quote! {
+        #item
+        #variant
+        #blanket_impl
+    };
+
+    output.into()
+}
+
+fn mk_variant(attrs: &Attrs, tr: &ItemTrait) -> TokenStream {
+    let MakeVariant {
+        ref name,
+        colon: _,
+        ref bounds,
+    } = attrs.variant;
+    let bounds: Vec<_> = bounds
+        .into_iter()
+        .map(|b| TypeParamBound::Trait(b.clone()))
+        .collect();
+    let variant = ItemTrait {
+        ident: name.clone(),
+        supertraits: tr.supertraits.iter().chain(&bounds).cloned().collect(),
+        items: tr
+            .items
+            .iter()
+            .map(|item| transform_item(item, &bounds))
+            .collect(),
+        ..tr.clone()
+    };
+    quote! { #variant }
+}
+
+fn transform_item(item: &TraitItem, bounds: &Vec<TypeParamBound>) -> TraitItem {
+    // #[make_variant(SendIntFactory: Send)]
+    // trait IntFactory {
+    //     async fn make(&self, x: u32, y: &str) -> i32;
+    //     fn stream(&self) -> impl Iterator<Item = i32>;
+    //     fn call(&self) -> u32;
+    // }
+    //
+    // becomes:
+    //
+    // trait SendIntFactory: Send {
+    //     fn make(&self, x: u32, y: &str) -> impl ::core::future::Future<Output = i32> + Send;
+    //     fn stream(&self) -> impl Iterator<Item = i32> + Send;
+    //     fn call(&self) -> u32;
+    // }
+    let TraitItem::Fn(fn_item @ TraitItemFn { sig, .. }) = item else {
+        return item.clone();
+    };
+    let (arrow, output) = if sig.asyncness.is_some() {
+        let orig = match &sig.output {
+            ReturnType::Default => quote! { () },
+            ReturnType::Type(_, ty) => quote! { #ty },
+        };
+        let future = syn::parse2(quote! { ::core::future::Future<Output = #orig> }).unwrap();
+        let ty = Type::ImplTrait(TypeImplTrait {
+            impl_token: syn::parse2(quote! { impl }).unwrap(),
+            bounds: iter::once(TypeParamBound::Trait(future))
+                .chain(bounds.iter().cloned())
+                .collect(),
+        });
+        (syn::parse2(quote! { -> }).unwrap(), ty)
+    } else {
+        match &sig.output {
+            ReturnType::Type(arrow, ty) => match &**ty {
+                Type::ImplTrait(it) => {
+                    let ty = Type::ImplTrait(TypeImplTrait {
+                        impl_token: it.impl_token,
+                        bounds: it.bounds.iter().chain(bounds).cloned().collect(),
+                    });
+                    (*arrow, ty)
+                }
+                _ => return item.clone(),
+            },
+            ReturnType::Default => return item.clone(),
+        }
+    };
+    TraitItem::Fn(TraitItemFn {
+        sig: Signature {
+            asyncness: None,
+            output: ReturnType::Type(arrow, Box::new(output)),
+            ..sig.clone()
+        },
+        ..fn_item.clone()
+    })
+}
+
+fn mk_blanket_impl(attrs: &Attrs, tr: &ItemTrait) -> TokenStream {
+    let orig = &tr.ident;
+    let variant = &attrs.variant.name;
+    let items = tr.items.iter().map(|item| blanket_impl_item(item, variant));
+    quote! {
+        impl<T> #orig for T where T: #variant {
+            #(#items)*
+        }
+    }
+}
+
+fn blanket_impl_item(item: &TraitItem, variant: &Ident) -> TokenStream {
+    // impl<T> IntFactory for T where T: SendIntFactory {
+    //     const NAME: &'static str = <Self as SendIntFactory>::NAME;
+    //     type MyFut<'a> = <Self as SendIntFactory>::MyFut<'a> where Self: 'a;
+    //     async fn make(&self, x: u32, y: &str) -> i32 {
+    //         <Self as SendIntFactory>::make(self, x, y).await
+    //     }
+    // }
+    match item {
+        TraitItem::Const(TraitItemConst {
+            ident,
+            generics,
+            ty,
+            ..
+        }) => {
+            quote! {
+                const #ident #generics: #ty = <Self as #variant>::#ident;
+            }
+        }
+        TraitItem::Fn(TraitItemFn { sig, .. }) => {
+            let ident = &sig.ident;
+            let args = sig.inputs.iter().map(|arg| match arg {
+                FnArg::Receiver(_) => quote! { self },
+                FnArg::Typed(PatType { pat, .. }) => match &**pat {
+                    Pat::Ident(arg) => quote! { #arg },
+                    _ => Error::new_spanned(pat, "patterns are not supported in arguments")
+                        .to_compile_error(),
+                },
+            });
+            let maybe_await = if sig.asyncness.is_some() {
+                quote! { .await }
+            } else {
+                quote! {}
+            };
+            quote! {
+                #sig {
+                    <Self as #variant>::#ident(#(#args),*)#maybe_await
+                }
+            }
+        }
+        TraitItem::Type(TraitItemType {
+            ident,
+            generics:
+                Generics {
+                    params,
+                    where_clause,
+                    ..
+                },
+            ..
+        }) => {
+            quote! {
+                type #ident<#params> = <Self as #variant>::#ident<#params> #where_clause;
+            }
+        }
+        _ => Error::new_spanned(item, "unsupported item type").into_compile_error(),
+    }
+}