From 857c40b22f90db6b3df7c49dd84f6c4caf0c5736 Mon Sep 17 00:00:00 2001
From: Iain McGinniss <309153+iainmcgin@users.noreply.github.com>
Date: Fri, 29 May 2026 16:27:54 +0000
Subject: [PATCH] view: add #[diagnostic::on_unimplemented] hint to
 HasMessageView

When a bound on HasMessageView fails - typically in downstream generated
code such as connect-rust service stubs, where the user cannot annotate
the call site - the compiler error now explains the cause and the fix:
regenerate the defining crate with buffa 0.7.0 or newer and views
enabled (generate_views(true) in buffa-build/buffa-codegen, views=true
for protoc-gen-buffa), or enable the crate's views feature when the
generated impls are feature-gated. Mirrors the existing ViewEncode
diagnostic in the same module.
---
 CHANGELOG.md      | 12 ++++++++++++
 buffa/src/view.rs |  9 +++++++++
 2 files changed, 21 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef4f636..8e30940 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,18 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ## [Unreleased]
 
+### Changed
+
+- **`HasMessageView` carries a `#[diagnostic::on_unimplemented]` hint.** When a
+  type is used where the generated view family is required but its crate was
+  generated without one (buffa older than 0.7.0, or views disabled) or has it
+  behind a disabled feature, the compile error now explains the cause and how
+  to fix it — regenerate the defining crate with buffa ≥ 0.7.0 and views
+  enabled (`generate_views(true)` / `views=true`), or enable the crate's views
+  feature — instead of only naming the missing trait bound. Downstream
+  consumers such as connect-rust rely on this trait for their request
+  wrappers, so the notes land directly in the consumer's build output.
+
 ## [0.7.0] - 2026-05-28
 
 This release is a minor bump under the
diff --git a/buffa/src/view.rs b/buffa/src/view.rs
index 045c0bc..7ecefa3 100644
--- a/buffa/src/view.rs
+++ b/buffa/src/view.rs
@@ -241,6 +241,15 @@ pub trait ViewReborrow: MessageView<'static> {
 /// Implementations are generated alongside the view and owned-view wrapper
 /// (and are therefore gated with them). Hand-written implementations are only
 /// needed for hand-written view types and must follow the same shape.
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` does not implement `HasMessageView` — its message-view family was not generated or is not enabled",
+    note = "the `HasMessageView` impl is emitted next to each message's view types: \
+            regenerate the crate that defines `{Self}` with buffa 0.7.0 or newer and \
+            views enabled — `generate_views(true)` (on by default) in a buffa-build / \
+            buffa-codegen config, or `views=true` for protoc-gen-buffa",
+    note = "if the defining crate feature-gates its generated impls, enabling its views \
+            feature is enough — no regeneration needed"
+)]
 pub trait HasMessageView: crate::Message + Sized {
     /// The zero-copy view of `Self`, borrowing from a buffer with lifetime
     /// `'a`.