Repository: dtolnay/async-trait (rust)

Issue:
Weird interaction with `single_use_lifetimes`
[playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a3f95d64d1d61fac41ec315a644ae353)

```
warning: lifetime parameter `'impl0` only used once
 --> src/lib.rs:7:19
  |
7 | impl<T> Trait for &T {}
  |                   ^
  |                   |
  |                   this lifetime...
  |                   ...is used only here
  |
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(single_use_lifetimes)]
  |         ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
  |
7 - impl<T> Trait for &T {}
7 + impl<T {}
  |
```
The suggestion is particularly helpful :laughing: 

Pull request description:
Delete replacement of elided lifetimes in impl heading
This reverts PR #107. The code from that PR is no longer necessary for 2 reasons.

1. As of https://github.com/rust-lang/rust/pull/85499 in Rust 1.56.0, the problem described in #106 involving `FnOnce(&Self::ThreadPool)` vs `FnOnce(&<&P as ProcessPool>::ThreadPool)` does not occur. These two trait bounds mean the same thing to sufficiently new versions of Rust.

2. As of #143 in async-trait 0.1.43 we no longer replace `FnOnce(&Self::ThreadPool)` to `FnOnce(&<&P as ProcessPool>::ThreadPool)` in generated code, so even with old rustc, the old problem does not occur.

Fixes #238.

Patch:
diff --git a/src/expand.rs b/src/expand.rs
index 88338db..2499177 100644
--- a/src/expand.rs
+++ b/src/expand.rs
@@ -80,13 +80,6 @@ pub fn expand(input: &mut Item, is_local: bool) {
             }
         }
         Item::Impl(input) => {
-            let mut lifetimes = CollectLifetimes::new("'impl");
-            lifetimes.visit_type_mut(&mut *input.self_ty);
-            lifetimes.visit_path_mut(&mut input.trait_.as_mut().unwrap().1);
-            let params = &input.generics.params;
-            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 {
@@ -167,7 +160,7 @@ fn transform_sig(
         ReturnType::Type(arrow, ret) => (*arrow, quote!(#ret)),
     };
 
-    let mut lifetimes = CollectLifetimes::new("'life");
+    let mut lifetimes = CollectLifetimes::new();
     for arg in sig.inputs.iter_mut() {
         match arg {
             FnArg::Receiver(arg) => lifetimes.visit_receiver_mut(arg),
diff --git a/src/lifetime.rs b/src/lifetime.rs
index 86eac24..1e5a658 100644
--- a/src/lifetime.rs
+++ b/src/lifetime.rs
@@ -9,15 +9,13 @@ use syn::{
 pub struct CollectLifetimes {
     pub elided: Vec<Lifetime>,
     pub explicit: Vec<Lifetime>,
-    pub name: &'static str,
 }
 
 impl CollectLifetimes {
-    pub fn new(name: &'static str) -> Self {
+    pub fn new() -> Self {
         CollectLifetimes {
             elided: Vec::new(),
             explicit: Vec::new(),
-            name,
         }
     }
 
@@ -37,7 +35,7 @@ impl CollectLifetimes {
     }
 
     fn next_lifetime(&mut self, span: Span) -> Lifetime {
-        let name = format!("{}{}", self.name, self.elided.len());
+        let name = format!("'life{}", self.elided.len());
         let life = Lifetime::new(&name, span);
         self.elided.push(life.clone());
         life


Test patch:
diff --git a/tests/test.rs b/tests/test.rs
index 102f97e..4fc46f0 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -1585,3 +1585,22 @@ pub mod issue236 {
         }
     }
 }
+
+// https://github.com/dtolnay/async-trait/issues/238
+pub mod issue238 {
+    #![deny(single_use_lifetimes)]
+
+    use async_trait::async_trait;
+
+    #[async_trait]
+    pub trait Trait {
+        async fn f();
+    }
+
+    pub struct Struct;
+
+    #[async_trait]
+    impl Trait for &Struct {
+        async fn f() {}
+    }
+}