docs: contacts async_trait / boxed-future fix (whatsapp-rust#826)

api/contacts.mdx

@@ -245,6 +245,10 @@ The implementation automatically:
 - Includes tokens in profile picture requests
 - Skips tokens for groups and newsletters
 
+## Async compatibility
+
+`is_on_whatsapp` and `get_user_info` work correctly when called from `#[async_trait]` implementations or any context that boxes the returned future (`Box<dyn Future + Send>`). Earlier versions produced a compile error (`"implementation of FnOnce is not general enough"`) that could not be worked around in user code. Fixed in [#826](https://github.com/oxidezap/whatsapp-rust/pull/826) with no API changes.
+
 ## Error handling
 
 All methods return `Result<T, anyhow::Error>`. Common errors:

changelog/2026-06-10-hrtb-closures.mdx

@@ -0,0 +1,24 @@
+---
+title: "June 10, 2026 — Contacts: fix async_trait / boxed-future compilation error"
+description: "is_on_whatsapp() and get_user_info() now compile correctly when called from #[async_trait] implementations or any context that boxes the returned future."
+---
+
+## Bug Fix
+
+**Contacts: `is_on_whatsapp` and `get_user_info` compile in `#[async_trait]` contexts ([#826](https://github.com/oxidezap/whatsapp-rust/pull/826))**
+
+Calling either method from an `#[async_trait]` implementation — or any other context that boxes the returned future — previously produced a hard compiler error:
+
+```
+error: implementation of `FnOnce` is not general enough
+  = note: closure with signature `fn(&'0 IsOnWhatsAppResult) -> (&Jid, Option<&Jid>)`
+          must implement `FnOnce<(&'1 IsOnWhatsAppResult,)>`, for any two lifetimes `'0` and `'1`...
+```
+
+The root cause was inside the library: the internal `persist_lid_mappings` helper received closures that returned references tied to a concrete lifetime. Rust infers such closures at a single lifetime rather than the higher-ranked `for<'r> Fn(&'r _)` form. Because the closure types were embedded in the public methods' future types, the unprovable HRTB obligation leaked to every boxing consumer — nothing in user code could work around it.
+
+**Fix.** The three offending closures are now named `fn` items (`forward_lid_pair`, `reverse_lid_pair`, `user_info_lid_pair`), which implement `Fn` for every lifetime by construction. No API changes, no extra allocations, identical behavior.
+
+A compile-time regression guard (`tests/async_trait_boxed_future_compat.rs`) was added that reproduces the exact consumer shape from the original report — an `#[async_trait]` impl holding an `RwLock<Option<Arc<Client>>>` — and fails to compile if the issue is ever reintroduced.
+
+**No breaking changes.** Fixes [#825](https://github.com/oxidezap/whatsapp-rust/issues/825).

docs.json

@@ -138,6 +138,7 @@
                 "group": "Changelog",
                 "pages": [
                   "changelog/overview",
+                  "changelog/2026-06-10-hrtb-closures",
                   "changelog/2026-06-10-server-aware-lookup-probe",
                   "changelog/2026-06-10-phash-arena-sort",
                   "changelog/2026-06-10-attrs-inline-smallvec",