Tag the inserted lifetime bounds with the span of the other bounds

src/expand.rs

@@ -5,6 +5,7 @@ use proc_macro2::TokenStream;
 use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::collections::BTreeSet as Set;
 use syn::punctuated::Punctuated;
+use syn::spanned::Spanned;
 use syn::visit_mut::{self, VisitMut};
 use syn::{
     parse_quote, parse_quote_spanned, Attribute, Block, FnArg, GenericParam, Generics, Ident,
@@ -186,18 +187,30 @@ fn transform_sig(
     {
         match param {
             GenericParam::Type(param) => {
+                let span = if param.bounds.is_empty() {
+                    // This should use `Span::def_site()`, but that's not stable.
+                    param.ident.span()
+                } else {
+                    param.bounds.span()
+                };
                 let param = &param.ident;
-                let span = param.span();
+                let life = quote_spanned!(span=> : 'async_trait);
                 where_clause_or_default(&mut sig.generics.where_clause)
                     .predicates
-                    .push(parse_quote_spanned!(span=> #param: 'async_trait));
+                    .push(parse_quote!(#param #life));
             }
             GenericParam::Lifetime(param) => {
+                let span = if param.bounds.is_empty() {
+                    // This should use `Span::def_site()`, but that's not stable.
+                    param.lifetime.span()
+                } else {
+                    param.bounds.span()
+                };
                 let param = &param.lifetime;
-                let span = param.span();
+                let life = quote_spanned!(span=> : 'async_trait);
                 where_clause_or_default(&mut sig.generics.where_clause)
                     .predicates
-                    .push(parse_quote_spanned!(span=> #param: 'async_trait));
+                    .push(parse_quote!(#param #life));
             }
             GenericParam::Const(_) => {}
         }
@@ -385,7 +398,6 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
 }
 
 fn positional_arg(i: usize, pat: &Pat) -> Ident {
-    use syn::spanned::Spanned;
     format_ident!("__arg{}", i, span = pat.span())
 }
 

tests/ui/issue-93828.rs

@@ -0,0 +1,31 @@
+use async_trait::async_trait;
+
+struct Client;
+struct Client2;
+trait IntoUrl {}
+
+#[async_trait]
+pub trait ClientExt {
+    async fn publish<T: IntoUrl>(&self, url: T) -> String;
+}
+
+// https://github.com/rust-lang/rust/issues/93828
+#[async_trait]
+impl ClientExt for Client {
+    async fn publish<T: IntoUrl>(&self, url: T) -> String {
+        "Foo".to_string()
+    }
+}
+
+// Variant test case with no bounds at all.
+// This doesn't actually work correctly yet. It ought to insert the colon,
+// but getting it to do that would require a way to tell rustc that the bounds
+// don't actually exist in the source code, and it needs to insert them.
+#[async_trait]
+impl ClientExt for Client2 {
+    async fn publish<T>(&self, url: T) -> String {
+        "Foo".to_string()
+    }
+}
+
+fn main() {}

tests/ui/issue-93828.stderr

@@ -0,0 +1,39 @@
+error: future cannot be sent between threads safely
+  --> tests/ui/issue-93828.rs:15:59
+   |
+15 |       async fn publish<T: IntoUrl>(&self, url: T) -> String {
+   |  ___________________________________________________________^
+16 | |         "Foo".to_string()
+17 | |     }
+   | |_____^ future created by async block is not `Send`
+   |
+note: captured value is not `Send`
+  --> tests/ui/issue-93828.rs:15:41
+   |
+15 |     async fn publish<T: IntoUrl>(&self, url: T) -> String {
+   |                                         ^^^ has type `T` which is not `Send`
+   = note: required for the cast to the object type `dyn Future<Output = String> + Send`
+help: consider further restricting this bound
+   |
+15 |     async fn publish<T: IntoUrl + std::marker::Send>(&self, url: T) -> String {
+   |                                 +++++++++++++++++++
+
+error: future cannot be sent between threads safely
+  --> tests/ui/issue-93828.rs:26:50
+   |
+26 |       async fn publish<T>(&self, url: T) -> String {
+   |  __________________________________________________^
+27 | |         "Foo".to_string()
+28 | |     }
+   | |_____^ future created by async block is not `Send`
+   |
+note: captured value is not `Send`
+  --> tests/ui/issue-93828.rs:26:32
+   |
+26 |     async fn publish<T>(&self, url: T) -> String {
+   |                                ^^^ has type `T` which is not `Send`
+   = note: required for the cast to the object type `dyn Future<Output = String> + Send`
+help: consider further restricting this bound
+   |
+26 |     async fn publish<T + std::marker::Send>(&self, url: T) -> String {
+   |                        +++++++++++++++++++