Support for impl Trait in associated type (type_alias_impl_trait)

src/expand.rs

@@ -3,12 +3,13 @@ use crate::parse::Item;
 use crate::receiver::{has_self_in_block, has_self_in_sig, mut_pat, ReplaceSelf};
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned, ToTokens};
+use std::collections::BTreeSet as Set;
 use syn::punctuated::Punctuated;
-use syn::visit_mut::VisitMut;
+use syn::visit_mut::{self, VisitMut};
 use syn::{
     parse_quote, Attribute, Block, FnArg, GenericParam, Generics, Ident, ImplItem, Lifetime, Pat,
     PatIdent, Receiver, ReturnType, Signature, Stmt, Token, TraitItem, Type, TypeParamBound,
-    WhereClause,
+    TypePath, WhereClause,
 };
 
 macro_rules! parse_quote_spanned {
@@ -34,6 +35,7 @@ enum Context<'a> {
     },
     Impl {
         impl_generics: &'a Generics,
+        associated_type_impl_traits: &'a Set<Ident>,
     },
 }
 
@@ -71,7 +73,7 @@ pub fn expand(input: &mut Item, is_local: bool) {
                         method.attrs.push(parse_quote!(#[must_use]));
                         if let Some(block) = block {
                             has_self |= has_self_in_block(block);
-                            transform_block(sig, block);
+                            transform_block(context, sig, block);
                             method.attrs.push(lint_suppress_with_body());
                         } else {
                             method.attrs.push(lint_suppress_without_body());
@@ -90,16 +92,26 @@ pub fn expand(input: &mut Item, is_local: bool) {
             let elided = lifetimes.elided;
             input.generics.params = parse_quote!(#(#elided,)* #params);
 
+            let mut associated_type_impl_traits = Set::new();
+            for inner in &input.items {
+                if let ImplItem::Type(assoc) = inner {
+                    if let Type::ImplTrait(_) = assoc.ty {
+                        associated_type_impl_traits.insert(assoc.ident.clone());
+                    }
+                }
+            }
+
             let context = Context::Impl {
                 impl_generics: &input.generics,
+                associated_type_impl_traits: &associated_type_impl_traits,
             };
             for inner in &mut input.items {
                 if let ImplItem::Method(method) = inner {
                     let sig = &mut method.sig;
                     if sig.asyncness.is_some() {
                         let block = &mut method.block;
                         let has_self = has_self_in_sig(sig) || has_self_in_block(block);
-                        transform_block(sig, block);
+                        transform_block(context, sig, block);
                         transform_sig(context, sig, has_self, false, is_local);
                         method.attrs.push(lint_suppress_with_body());
                     }
@@ -296,7 +308,7 @@ fn transform_sig(
 //
 //         ___ret
 //     })
-fn transform_block(sig: &mut Signature, block: &mut Block) {
+fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
     if let Some(Stmt::Item(syn::Item::Verbatim(item))) = block.stmts.first() {
         if block.stmts.len() == 1 && item.to_string() == ";" {
             return;
@@ -345,18 +357,24 @@ fn transform_block(sig: &mut Signature, block: &mut Block) {
     }
 
     let stmts = &block.stmts;
-    let let_ret = match &sig.output {
+    let let_ret = match &mut sig.output {
         ReturnType::Default => quote_spanned! {block.brace_token.span=>
             let _: () = { #(#decls)* #(#stmts)* };
         },
-        ReturnType::Type(_, ret) => quote_spanned! {block.brace_token.span=>
-            if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<#ret> {
-                return __ret;
+        ReturnType::Type(_, ret) => {
+            if contains_associated_type_impl_trait(context, ret) {
+                quote!(#(#decls)* #(#stmts)*)
+            } else {
+                quote_spanned! {block.brace_token.span=>
+                    if let ::core::option::Option::Some(__ret) = ::core::option::Option::None::<#ret> {
+                        return __ret;
+                    }
+                    let __ret: #ret = { #(#decls)* #(#stmts)* };
+                    #[allow(unreachable_code)]
+                    __ret
+                }
             }
-            let __ret: #ret = { #(#decls)* #(#stmts)* };
-            #[allow(unreachable_code)]
-            __ret
-        },
+        }
     };
     let box_pin = quote_spanned!(block.brace_token.span=>
         Box::pin(async move { #let_ret })
@@ -380,6 +398,41 @@ fn has_bound(supertraits: &Supertraits, marker: &Ident) -> bool {
     false
 }
 
+fn contains_associated_type_impl_trait(context: Context, ret: &mut Type) -> bool {
+    struct AssociatedTypeImplTraits<'a> {
+        set: &'a Set<Ident>,
+        contains: bool,
+    }
+
+    impl<'a> VisitMut for AssociatedTypeImplTraits<'a> {
+        fn visit_type_path_mut(&mut self, ty: &mut TypePath) {
+            if ty.qself.is_none()
+                && ty.path.segments.len() == 2
+                && ty.path.segments[0].ident == "Self"
+                && self.set.contains(&ty.path.segments[1].ident)
+            {
+                self.contains = true;
+            }
+            visit_mut::visit_type_path_mut(self, ty);
+        }
+    }
+
+    match context {
+        Context::Trait { .. } => false,
+        Context::Impl {
+            associated_type_impl_traits,
+            ..
+        } => {
+            let mut visit = AssociatedTypeImplTraits {
+                set: associated_type_impl_traits,
+                contains: false,
+            };
+            visit.visit_type_mut(ret);
+            visit.contains
+        }
+    }
+}
+
 fn where_clause_or_default(clause: &mut Option<WhereClause>) -> &mut WhereClause {
     clause.get_or_insert_with(|| WhereClause {
         where_token: Default::default(),

tests/test.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(
     async_trait_nightly_testing,
-    feature(min_specialization, min_const_generics)
+    feature(min_specialization, min_const_generics, type_alias_impl_trait)
 )]
 #![allow(
     clippy::let_underscore_drop,
@@ -1278,3 +1278,25 @@ pub mod issue149 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/152
+#[cfg(async_trait_nightly_testing)]
+pub mod issue152 {
+    use async_trait::async_trait;
+
+    #[async_trait]
+    trait Trait {
+        type Assoc;
+
+        async fn f(&self) -> Self::Assoc;
+    }
+
+    struct Struct;
+
+    #[async_trait]
+    impl Trait for Struct {
+        type Assoc = impl Sized;
+
+        async fn f(&self) -> Self::Assoc {}
+    }
+}