Add 'async_trait lifetime to impl Trait arguments

Author: dtolnay

Cargo.toml

@@ -17,7 +17,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.84", features = ["full", "visit-mut"] }
+syn = { version = "1.0.96", features = ["full", "visit-mut"] }
 
 [dev-dependencies]
 futures = "0.3"

src/expand.rs

@@ -1,4 +1,4 @@
-use crate::lifetime::CollectLifetimes;
+use crate::lifetime::{AddLifetimeToImplTrait, CollectLifetimes};
 use crate::parse::Item;
 use crate::receiver::{has_self_in_block, has_self_in_sig, mut_pat, ReplaceSelf};
 use proc_macro2::TokenStream;
@@ -283,6 +283,7 @@ fn transform_sig(
                     let m = mut_pat(&mut arg.pat);
                     arg.pat = parse_quote!(#m #positional);
                 }
+                AddLifetimeToImplTrait.visit_type_mut(&mut arg.ty);
             }
         }
     }

src/lifetime.rs

@@ -1,6 +1,8 @@
 use proc_macro2::Span;
 use syn::visit_mut::{self, VisitMut};
-use syn::{GenericArgument, Lifetime, Receiver, TypeReference};
+use syn::{
+    parse_quote_spanned, Expr, GenericArgument, Lifetime, Receiver, TypeImplTrait, TypeReference,
+};
 
 pub struct CollectLifetimes {
     pub elided: Vec<Lifetime>,
@@ -62,3 +64,22 @@ impl VisitMut for CollectLifetimes {
         visit_mut::visit_generic_argument_mut(self, gen);
     }
 }
+
+pub struct AddLifetimeToImplTrait;
+
+impl VisitMut for AddLifetimeToImplTrait {
+    fn visit_type_impl_trait_mut(&mut self, ty: &mut TypeImplTrait) {
+        let span = ty.impl_token.span;
+        let lifetime = parse_quote_spanned!(span=> 'async_trait);
+        ty.bounds.insert(0, lifetime);
+        if let Some(punct) = ty.bounds.pairs_mut().next().unwrap().punct_mut() {
+            punct.span = span;
+        }
+    }
+
+    fn visit_expr_mut(&mut self, _e: &mut Expr) {
+        // Do not recurse into impl Traits inside of an array length expression.
+        //
+        //    fn outer(arg: [u8; { fn inner(_: impl Trait) {}; 0 }]);
+    }
+}