feat: trait_variant::make supports rewriting of the original trait.

Author: sargarass

trait-variant/examples/variant.rs

@@ -43,4 +43,18 @@ where
     fn build<T: Display>(&self, items: impl Iterator<Item = T>) -> Self::B<T>;
 }
 
+#[trait_variant::make(Send + Sync)]
+pub trait GenericTraitWithBounds<'x, S: Sync, Y, const X: usize>
+where
+    Y: Sync,
+{
+    const CONST: usize = 3;
+    type F;
+    type A<const ANOTHER_CONST: u8>;
+    type B<T: Display>: FromIterator<T>;
+
+    async fn take(&self, s: S);
+    fn build<T: Display>(&self, items: impl Iterator<Item = T>) -> Self::B<T>;
+}
+
 fn main() {}

trait-variant/src/lib.rs

@@ -14,15 +14,15 @@ mod variant;
 /// fn` and/or `-> impl Trait` return types.
 ///
 /// ```
-/// #[trait_variant::make(IntFactory: Send)]
-/// trait LocalIntFactory {
+/// #[trait_variant::make(Send)]
+/// trait IntFactory {
 ///     async fn make(&self) -> i32;
 ///     fn stream(&self) -> impl Iterator<Item = i32>;
 ///     fn call(&self) -> u32;
 /// }
 /// ```
 ///
-/// The above example causes a second trait called `IntFactory` to be created:
+/// The above example causes the trait to be rewritten as:
 ///
 /// ```
 /// # use core::future::Future;
@@ -35,6 +35,19 @@ mod variant;
 ///
 /// Note that ordinary methods such as `call` are not affected.
 ///
+/// If you want to preserve an original trait untouched, `make` can be used to create a new trait with bounds on `async
+/// fn` and/or `-> impl Trait` return types.
+///
+/// ```
+/// #[trait_variant::make(IntFactory: Send)]
+/// trait LocalIntFactory {
+///     async fn make(&self) -> i32;
+///     fn stream(&self) -> impl Iterator<Item = i32>;
+///     fn call(&self) -> u32;
+/// }
+/// ```
+///
+/// The example causes a second trait called `IntFactory` to be created.
 /// 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-variant/src/variant.rs

@@ -32,20 +32,33 @@ impl Parse for Attrs {
     }
 }
 
-struct MakeVariant {
-    name: Ident,
-    #[allow(unused)]
-    colon: Token![:],
-    bounds: Punctuated<TraitBound, Plus>,
+enum MakeVariant {
+    // Creates a variant of a trait under a new name with additional bounds while preserving the original trait.
+    Create {
+        name: Ident,
+        _colon: Token![:],
+        bounds: Punctuated<TraitBound, Plus>,
+    },
+    // Rewrites the original trait into a new trait with additional bounds.
+    Rewrite {
+        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![+])?,
-        })
+        let variant = if input.peek(Ident) && input.peek2(Token![:]) {
+            MakeVariant::Create {
+                name: input.parse()?,
+                _colon: input.parse()?,
+                bounds: input.parse_terminated(TraitBound::parse, Token![+])?,
+            }
+        } else {
+            MakeVariant::Rewrite {
+                bounds: input.parse_terminated(TraitBound::parse, Token![+])?,
+            }
+        };
+        Ok(variant)
     }
 }
 
@@ -56,43 +69,51 @@ pub fn make(
     let attrs = parse_macro_input!(attr as Attrs);
     let item = parse_macro_input!(item as ItemTrait);
 
-    let maybe_allow_async_lint = if attrs
-        .variant
-        .bounds
-        .iter()
-        .any(|b| b.path.segments.last().unwrap().ident == "Send")
-    {
-        quote! { #[allow(async_fn_in_trait)] }
-    } else {
-        quote! {}
-    };
+    match attrs.variant {
+        MakeVariant::Create { name, bounds, .. } => {
+            let maybe_allow_async_lint = if bounds
+                .iter()
+                .any(|b| b.path.segments.last().unwrap().ident == "Send")
+            {
+                quote! { #[allow(async_fn_in_trait)] }
+            } else {
+                quote! {}
+            };
 
-    let variant = mk_variant(&attrs, &item);
-    let blanket_impl = mk_blanket_impl(&attrs, &item);
+            let variant = mk_variant(&name, bounds, &item);
+            let blanket_impl = mk_blanket_impl(&name, &item);
 
-    quote! {
-        #maybe_allow_async_lint
-        #item
+            quote! {
+                #maybe_allow_async_lint
+                #item
 
-        #variant
+                #variant
 
-        #blanket_impl
+                #blanket_impl
+            }
+            .into()
+        }
+        MakeVariant::Rewrite { bounds, .. } => {
+            let variant = mk_variant(&item.ident, bounds, &item);
+            quote! {
+                #variant
+            }
+            .into()
+        }
     }
-    .into()
 }
 
-fn mk_variant(attrs: &Attrs, tr: &ItemTrait) -> TokenStream {
-    let MakeVariant {
-        ref name,
-        colon: _,
-        ref bounds,
-    } = attrs.variant;
-    let bounds: Vec<_> = bounds
+fn mk_variant(
+    variant: &Ident,
+    with_bounds: Punctuated<TraitBound, Plus>,
+    tr: &ItemTrait,
+) -> TokenStream {
+    let bounds: Vec<_> = with_bounds
         .into_iter()
         .map(|b| TypeParamBound::Trait(b.clone()))
         .collect();
     let variant = ItemTrait {
-        ident: name.clone(),
+        ident: variant.clone(),
         supertraits: tr.supertraits.iter().chain(&bounds).cloned().collect(),
         items: tr
             .items
@@ -104,21 +125,8 @@ fn mk_variant(attrs: &Attrs, tr: &ItemTrait) -> TokenStream {
     quote! { #variant }
 }
 
+// Transforms a one item declaration within the definition if it has `async fn` and/or `-> impl Trait` return types by adding new bounds.
 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();
     };
@@ -160,9 +168,8 @@ fn transform_item(item: &TraitItem, bounds: &Vec<TypeParamBound>) -> TraitItem {
     })
 }
 
-fn mk_blanket_impl(attrs: &Attrs, tr: &ItemTrait) -> TokenStream {
+fn mk_blanket_impl(variant: &Ident, tr: &ItemTrait) -> TokenStream {
     let orig = &tr.ident;
-    let variant = &attrs.variant.name;
     let (_impl, orig_ty_generics, _where) = &tr.generics.split_for_impl();
     let items = tr
         .items