feat(buffa): add MessageFullName trait

CHANGELOG.md

@@ -8,6 +8,36 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ### Added
 
+- **`buffa::MessageName` trait exposes a generated message's protobuf
+  identifiers as compile-time `&'static str` constants.** Codegen emits
+  `impl MessageName for #Msg` (and `for #MsgView<'a>`) with four consts:
+  `PACKAGE` (`"my.pkg"`, empty for the unnamed root package), `NAME`
+  (`"Outer.Inner"` — unqualified, with `.` between nesting levels),
+  `FULL_NAME` (`"my.pkg.Outer.Inner"`), and `TYPE_URL`
+  (`"type.googleapis.com/my.pkg.Outer.Inner"` — the
+  `google.protobuf.Any.type_url` form). All four are computed at codegen
+  time as string literals, so there's no runtime allocation or
+  concatenation — unlike `prost::Name`, whose `full_name()` and
+  `type_url()` are runtime `format!` calls. `PACKAGE` and `NAME` are
+  separate consts because the dotted `FULL_NAME` cannot be split
+  unambiguously (`foo.Bar.Baz` could be package `foo.Bar` + message `Baz`
+  or package `foo` + nested `Bar.Baz`).
+
+  The trait has no supertrait — it doesn't reach into the wire codec —
+  so view types implement it too: a generic event-sourcing registry can
+  bound on `T: MessageName` and dispatch zero-copy views and owned
+  messages identically. Useful for type-erased registries, logging, and
+  any code that needs the protobuf name without the descriptor machinery.
+  The inherent `Foo::TYPE_URL` const generated since 0.4.0 is unchanged
+  and equal to `<Foo as MessageName>::TYPE_URL`; for messages that also
+  implement `ExtensionSet`, `FULL_NAME` is equal to
+  `ExtensionSet::PROTO_FQN` (all derive from the same codegen source).
+  `MessageName` is **not** object-safe (associated `const` only) — use it
+  as a bound, not `dyn MessageName`. Migrating from `prost::Name`: rename
+  the bound and replace runtime `M::full_name()` / `M::type_url()` calls
+  with the consts. ([#108](https://github.com/anthropics/buffa/pull/108),
+  by @yordis)
+
 - **`buf.build/anthropics/buffa` is published to the public Buf Schema
   Registry.** `buf generate` can now reference `protoc-gen-buffa` as a
   `remote:` plugin with no local install: `remote: buf.build/anthropics/buffa`

buffa-codegen/src/impl_message.rs

@@ -322,6 +322,54 @@ fn message_uses_size_cache(
 
 /// Generate `impl DefaultInstance` and `impl Message` for a message.
 ///
+/// Emit `impl #generics ::buffa::MessageName for #ty { … }`.
+///
+/// `generics` is the impl-side generic parameter list (`<'a>` for the
+/// view type, empty for the owned message). `ty` is the implementing
+/// type *with* any generics applied (`Foo` or `FooView<'a>`).
+///
+/// All four consts are computed at codegen time as string literals so
+/// `T::FULL_NAME` etc. are zero-cost at runtime — no `format!`,
+/// `concat!`, or lazy static. `PACKAGE` and `NAME` are split here rather
+/// than left to the consumer because the dotted `FULL_NAME` cannot be
+/// re-split unambiguously: `foo.Bar.Baz` could be package `foo.Bar` +
+/// message `Baz`, or package `foo` + nested `Bar.Baz`. Codegen knows
+/// which.
+pub(crate) fn message_name_impl(
+    current_package: &str,
+    proto_fqn: &str,
+    generics: &TokenStream,
+    ty: &TokenStream,
+) -> TokenStream {
+    let name = if current_package.is_empty() {
+        proto_fqn.to_string()
+    } else {
+        // Strip `"<package>."` atomically — a two-step
+        // `strip_prefix(package)` then `strip_prefix(".")` would
+        // partial-match a prefix-overlapping package (`package = "foo"`
+        // against `proto_fqn = "food.Bar"`) and silently violate the
+        // documented `PACKAGE + "." + NAME == FULL_NAME` invariant.
+        //
+        // `proto_fqn` is always `"<package>.<rest>"` for a non-empty
+        // package (it's built by joining message segments onto the
+        // package), so the strip should never fail. Fall back
+        // defensively rather than panic on a malformed descriptor.
+        proto_fqn
+            .strip_prefix(&format!("{current_package}."))
+            .unwrap_or(proto_fqn)
+            .to_string()
+    };
+    let type_url = format!("type.googleapis.com/{proto_fqn}");
+    quote! {
+        impl #generics ::buffa::MessageName for #ty {
+            const PACKAGE: &'static str = #current_package;
+            const NAME: &'static str = #name;
+            const FULL_NAME: &'static str = #proto_fqn;
+            const TYPE_URL: &'static str = #type_url;
+        }
+    }
+}
+
 /// `preserve_unknown_fields`: when `true`, the generated merge collects
 /// unknown fields into `self.__buffa_unknown_fields` and both `compute_size` and
 /// `write_to` include them.
@@ -543,6 +591,13 @@ pub fn generate_message_impl(
         quote! {}
     };
 
+    let message_name_impl = message_name_impl(
+        current_package,
+        proto_fqn,
+        &quote! {},
+        &quote! { #name_ident },
+    );
+
     Ok(quote! {
         impl ::buffa::DefaultInstance for #name_ident {
             fn default_instance() -> &'static Self {
@@ -552,6 +607,8 @@ pub fn generate_message_impl(
             }
         }
 
+        #message_name_impl
+
         impl ::buffa::Message for #name_ident {
             /// Returns the total encoded size in bytes.
             ///

buffa-codegen/src/tests/feature_gating.rs

@@ -198,6 +198,45 @@ fn gated_view_module_is_cfg_blocked() {
         squashed.contains(r#"#[cfg(feature = "views")] pub mod view"#),
         "the views cfg must precede `pub mod view`: {content}"
     );
+    // The view's `impl MessageName for FooView<'a>` lives in the
+    // `<stem>.__view.rs` content file, which the stitcher `include!`s
+    // *inside* the `#[cfg(feature = "views")] pub mod view` block, so
+    // it disappears with the feature. The owned-message impl is in the
+    // `<stem>.rs` content file (unconditional). Pin both: a refactor
+    // that emits the view impl into a non-View content file would ship
+    // a name-resolution error against a cfg'd-out type.
+    let cfg = CodeGenConfig {
+        generate_json: true,
+        generate_views: true,
+        generate_text: false,
+        preserve_unknown_fields: true,
+        gate_impls_on_crate_features: true,
+        ..CodeGenConfig::default()
+    };
+    let files =
+        generate(&[fixture()], &["gated.proto".to_string()], &cfg).expect("should generate");
+    for f in &files {
+        let has_view_impl = f.content.contains("MessageName for OuterView");
+        let has_owned_impl = squash(&f.content).contains("impl ::buffa::MessageName for Outer ");
+        match f.kind {
+            GeneratedFileKind::View => assert!(
+                has_view_impl,
+                "view MessageName impl must be in the view content file ({})",
+                f.name
+            ),
+            GeneratedFileKind::Owned => assert!(
+                has_owned_impl && !has_view_impl,
+                "owned content file must have the owned (not view) MessageName impl ({})",
+                f.name
+            ),
+            _ => assert!(
+                !has_view_impl && !has_owned_impl,
+                "MessageName impl leaked into a {:?} file ({})",
+                f.kind,
+                f.name
+            ),
+        }
+    }
 }
 
 #[test]

buffa-codegen/src/tests/generation.rs

@@ -1241,6 +1241,77 @@ fn test_type_url_doubly_nested() {
     );
 }
 
+#[test]
+fn test_message_name_consts() {
+    // The four `MessageName` consts must hold the documented invariant
+    // `PACKAGE + "." + NAME == FULL_NAME` (joining dot omitted when
+    // `PACKAGE` is empty), and `TYPE_URL == "type.googleapis.com/" +
+    // FULL_NAME`. The atomic-prefix-strip in `message_name_impl` makes
+    // a partial-match (`package = "foo"` against `proto_fqn =
+    // "food.Bar"`) impossible to slip through silently — pin the shape
+    // here so a refactor that re-introduces a two-step strip fails this
+    // test instead of shipping a broken `NAME`.
+    let mut file = proto3_file("named.proto");
+    file.package = Some("my.pkg".to_string());
+    file.message_type.push(DescriptorProto {
+        name: Some("Outer".to_string()),
+        nested_type: vec![DescriptorProto {
+            name: Some("Inner".to_string()),
+            ..Default::default()
+        }],
+        ..Default::default()
+    });
+    let files = generate(
+        &[file],
+        &["named.proto".to_string()],
+        &CodeGenConfig::default(),
+    )
+    .expect("should generate");
+    let content = joined(&files);
+    // Top-level: PACKAGE + "." + NAME == FULL_NAME.
+    for snippet in [
+        r#"const PACKAGE: &'static str = "my.pkg""#,
+        r#"const NAME: &'static str = "Outer""#,
+        r#"const FULL_NAME: &'static str = "my.pkg.Outer""#,
+        r#"const TYPE_URL: &'static str = "type.googleapis.com/my.pkg.Outer""#,
+        // Nested: NAME carries the dotted nesting path; PACKAGE stays
+        // at the proto package — NOT `DescriptorProto.name` (which is
+        // just `"Inner"`).
+        r#"const NAME: &'static str = "Outer.Inner""#,
+        r#"const FULL_NAME: &'static str = "my.pkg.Outer.Inner""#,
+    ] {
+        assert!(content.contains(snippet), "missing `{snippet}`: {content}");
+    }
+
+    // Empty package: PACKAGE is "", NAME == FULL_NAME, no joining dot.
+    let mut root = proto3_file("root.proto");
+    root.message_type.push(DescriptorProto {
+        name: Some("Root".to_string()),
+        nested_type: vec![DescriptorProto {
+            name: Some("Leaf".to_string()),
+            ..Default::default()
+        }],
+        ..Default::default()
+    });
+    let files = generate(
+        &[root],
+        &["root.proto".to_string()],
+        &CodeGenConfig::default(),
+    )
+    .expect("should generate");
+    let content = joined(&files);
+    for snippet in [
+        r#"const PACKAGE: &'static str = """#,
+        r#"const NAME: &'static str = "Root""#,
+        r#"const FULL_NAME: &'static str = "Root""#,
+        r#"const NAME: &'static str = "Root.Leaf""#,
+        r#"const FULL_NAME: &'static str = "Root.Leaf""#,
+        r#"const TYPE_URL: &'static str = "type.googleapis.com/Root.Leaf""#,
+    ] {
+        assert!(content.contains(snippet), "missing `{snippet}`: {content}");
+    }
+}
+
 #[test]
 fn test_message_scalar_fields() {
     let mut file = proto3_file("scalars.proto");

buffa-codegen/src/view.rs

@@ -166,6 +166,17 @@ pub(crate) fn generate_view_with_nesting(
         }
     };
 
+    // The view participates in the same name-keyed registries as the
+    // owned message — a generic event-sourcing dispatch should not have
+    // to round-trip a zero-copy view through `to_owned_message()` just
+    // to read its FQN. Same consts, different `Self`.
+    let message_name_impl = crate::impl_message::message_name_impl(
+        current_package,
+        proto_fqn,
+        &quote! { <'a> },
+        &quote! { #view_ident<'a> },
+    );
+
     let serialize_impl = if ctx.config.generate_json {
         crate::feature_gates::cfg_block(
             generate_view_serialize(
@@ -337,6 +348,8 @@ pub(crate) fn generate_view_with_nesting(
 
         #serialize_impl
 
+        #message_name_impl
+
         impl<'v> ::buffa::DefaultViewInstance for #view_ident<'v> {
             fn default_view_instance<'a>() -> &'a Self
             where

buffa-descriptor/src/generated/google.protobuf.compiler.plugin.__view.rs

@@ -257,6 +257,12 @@ impl<'__a> ::serde::Serialize for VersionView<'__a> {
         __map.end()
     }
 }
+impl<'a> ::buffa::MessageName for VersionView<'a> {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "Version";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.Version";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.Version";
+}
 impl<'v> ::buffa::DefaultViewInstance for VersionView<'v> {
     fn default_view_instance<'a>() -> &'a Self
     where
@@ -655,6 +661,12 @@ impl<'__a> ::serde::Serialize for CodeGeneratorRequestView<'__a> {
         __map.end()
     }
 }
+impl<'a> ::buffa::MessageName for CodeGeneratorRequestView<'a> {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "CodeGeneratorRequest";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorRequest";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorRequest";
+}
 impl<'v> ::buffa::DefaultViewInstance for CodeGeneratorRequestView<'v> {
     fn default_view_instance<'a>() -> &'a Self
     where
@@ -1000,6 +1012,12 @@ impl<'__a> ::serde::Serialize for CodeGeneratorResponseView<'__a> {
         __map.end()
     }
 }
+impl<'a> ::buffa::MessageName for CodeGeneratorResponseView<'a> {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "CodeGeneratorResponse";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorResponse";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorResponse";
+}
 impl<'v> ::buffa::DefaultViewInstance for CodeGeneratorResponseView<'v> {
     fn default_view_instance<'a>() -> &'a Self
     where
@@ -1360,6 +1378,12 @@ pub mod code_generator_response {
             __map.end()
         }
     }
+    impl<'a> ::buffa::MessageName for FileView<'a> {
+        const PACKAGE: &'static str = "google.protobuf.compiler";
+        const NAME: &'static str = "CodeGeneratorResponse.File";
+        const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorResponse.File";
+        const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorResponse.File";
+    }
     impl<'v> ::buffa::DefaultViewInstance for FileView<'v> {
         fn default_view_instance<'a>() -> &'a Self
         where

buffa-descriptor/src/generated/google.protobuf.compiler.plugin.rs

@@ -73,6 +73,12 @@ impl ::buffa::DefaultInstance for Version {
         VALUE.get_or_init(|| ::buffa::alloc::boxed::Box::new(Self::default()))
     }
 }
+impl ::buffa::MessageName for Version {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "Version";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.Version";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.Version";
+}
 impl ::buffa::Message for Version {
     /// Returns the total encoded size in bytes.
     ///
@@ -411,6 +417,12 @@ impl ::buffa::DefaultInstance for CodeGeneratorRequest {
         VALUE.get_or_init(|| ::buffa::alloc::boxed::Box::new(Self::default()))
     }
 }
+impl ::buffa::MessageName for CodeGeneratorRequest {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "CodeGeneratorRequest";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorRequest";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorRequest";
+}
 impl ::buffa::Message for CodeGeneratorRequest {
     /// Returns the total encoded size in bytes.
     ///
@@ -819,6 +831,12 @@ impl ::buffa::DefaultInstance for CodeGeneratorResponse {
         VALUE.get_or_init(|| ::buffa::alloc::boxed::Box::new(Self::default()))
     }
 }
+impl ::buffa::MessageName for CodeGeneratorResponse {
+    const PACKAGE: &'static str = "google.protobuf.compiler";
+    const NAME: &'static str = "CodeGeneratorResponse";
+    const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorResponse";
+    const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorResponse";
+}
 impl ::buffa::Message for CodeGeneratorResponse {
     /// Returns the total encoded size in bytes.
     ///
@@ -1367,6 +1385,12 @@ pub mod code_generator_response {
             VALUE.get_or_init(|| ::buffa::alloc::boxed::Box::new(Self::default()))
         }
     }
+    impl ::buffa::MessageName for File {
+        const PACKAGE: &'static str = "google.protobuf.compiler";
+        const NAME: &'static str = "CodeGeneratorResponse.File";
+        const FULL_NAME: &'static str = "google.protobuf.compiler.CodeGeneratorResponse.File";
+        const TYPE_URL: &'static str = "type.googleapis.com/google.protobuf.compiler.CodeGeneratorResponse.File";
+    }
     impl ::buffa::Message for File {
         /// Returns the total encoded size in bytes.
         ///