Standardize unimplemented API failures

2026-08-08 11:44:46 +02:00
parent 3179d7a57d
commit d3f5e27f52
41 changed files with 1875 additions and 1608 deletions
--- a/crates/libremetaverse-types/src/generated.rs
+++ b/crates/libremetaverse-types/src/generated.rs
@@ -3,7 +3,7 @@
 #![allow(non_camel_case_types)]

 /// `enum` shim for C# `LibreMetaverse.AssetType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct AssetType;
 impl AssetType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.AssetType";
@@ -11,7 +11,7 @@ impl AssetType {
 }

 /// `enum` shim for C# `LibreMetaverse.AttachmentPoint`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct AttachmentPoint;
 impl AttachmentPoint {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.AttachmentPoint";
@@ -19,7 +19,7 @@ impl AttachmentPoint {
 }

 /// `class` shim for C# `LibreMetaverse.CRC32`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct CRC32;
 impl CRC32 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.CRC32";
@@ -27,7 +27,7 @@ impl CRC32 {
 }

 /// `class` shim for C# `LibreMetaverse.CacheDictionary`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct CacheDictionary<T0 = (), T1 = ()>(std::marker::PhantomData<(T0, T1)>);
 impl<T0, T1> CacheDictionary<T0, T1> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.CacheDictionary";
@@ -35,7 +35,7 @@ impl<T0, T1> CacheDictionary<T0, T1> {
 }

 /// `class` shim for C# `LibreMetaverse.CacheDictionaryEnumerator`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct CacheDictionaryEnumerator;
 impl CacheDictionaryEnumerator {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.CacheDictionaryEnumerator";
@@ -43,7 +43,7 @@ impl CacheDictionaryEnumerator {
 }

 /// `enum` shim for C# `LibreMetaverse.ClickAction`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ClickAction;
 impl ClickAction {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.ClickAction";
@@ -51,7 +51,7 @@ impl ClickAction {
 }

 /// `struct` shim for C# `LibreMetaverse.Color4`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Color4;
 impl Color4 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Color4";
@@ -59,7 +59,7 @@ impl Color4 {
 }

 /// `class` shim for C# `LibreMetaverse.DoubleDictionary`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct DoubleDictionary<T0 = (), T1 = (), T2 = ()>(std::marker::PhantomData<(T0, T1, T2)>);
 impl<T0, T1, T2> DoubleDictionary<T0, T1, T2> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.DoubleDictionary";
@@ -67,7 +67,7 @@ impl<T0, T1, T2> DoubleDictionary<T0, T1, T2> {
 }

 /// `class` shim for C# `LibreMetaverse.EmptyRemovalStrategy`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct EmptyRemovalStrategy<T0 = ()>(std::marker::PhantomData<(T0,)>);
 impl<T0> EmptyRemovalStrategy<T0> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.EmptyRemovalStrategy";
@@ -75,7 +75,7 @@ impl<T0> EmptyRemovalStrategy<T0> {
 }

 /// `class` shim for C# `LibreMetaverse.EnumInfoAttribute`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct EnumInfoAttribute;
 impl EnumInfoAttribute {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.EnumInfoAttribute";
@@ -83,7 +83,7 @@ impl EnumInfoAttribute {
 }

 /// `class` shim for C# `LibreMetaverse.EnumInfoExtensions`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct EnumInfoExtensions;
 impl EnumInfoExtensions {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.EnumInfoExtensions";
@@ -91,7 +91,7 @@ impl EnumInfoExtensions {
 }

 /// `class` shim for C# `LibreMetaverse.ExpiringCache`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ExpiringCache<T0 = (), T1 = ()>(std::marker::PhantomData<(T0, T1)>);
 impl<T0, T1> ExpiringCache<T0, T1> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.ExpiringCache";
@@ -99,7 +99,7 @@ impl<T0, T1> ExpiringCache<T0, T1> {
 }

 /// `enum` shim for C# `LibreMetaverse.ExtraParamType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ExtraParamType;
 impl ExtraParamType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.ExtraParamType";
@@ -107,7 +107,7 @@ impl ExtraParamType {
 }

 /// `enum` shim for C# `LibreMetaverse.FaceType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct FaceType;
 impl FaceType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.FaceType";
@@ -115,7 +115,7 @@ impl FaceType {
 }

 /// `enum` shim for C# `LibreMetaverse.FolderType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct FolderType;
 impl FolderType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.FolderType";
@@ -123,7 +123,7 @@ impl FolderType {
 }

 /// `enum` shim for C# `LibreMetaverse.HoleType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct HoleType;
 impl HoleType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.HoleType";
@@ -136,7 +136,7 @@ pub trait ICacheDictionaryRemovalStrategy<T0 = ()> {
 }

 /// `enum` shim for C# `LibreMetaverse.InventoryType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct InventoryType;
 impl InventoryType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.InventoryType";
@@ -144,7 +144,7 @@ impl InventoryType {
 }

 /// `enum` shim for C# `LibreMetaverse.JointType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct JointType;
 impl JointType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.JointType";
@@ -152,7 +152,7 @@ impl JointType {
 }

 /// `class` shim for C# `LibreMetaverse.LruRemovalStrategy`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct LruRemovalStrategy<T0 = ()>(std::marker::PhantomData<(T0,)>);
 impl<T0> LruRemovalStrategy<T0> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.LruRemovalStrategy";
@@ -160,7 +160,7 @@ impl<T0> LruRemovalStrategy<T0> {
 }

 /// `enum` shim for C# `LibreMetaverse.Material`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Material;
 impl Material {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Material";
@@ -168,7 +168,7 @@ impl Material {
 }

 /// `struct` shim for C# `LibreMetaverse.Matrix4`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Matrix4;
 impl Matrix4 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Matrix4";
@@ -176,7 +176,7 @@ impl Matrix4 {
 }

 /// `class` shim for C# `LibreMetaverse.MruRemovalStrategy`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MruRemovalStrategy<T0 = ()>(std::marker::PhantomData<(T0,)>);
 impl<T0> MruRemovalStrategy<T0> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.MruRemovalStrategy";
@@ -184,7 +184,7 @@ impl<T0> MruRemovalStrategy<T0> {
 }

 /// `class` shim for C# `LibreMetaverse.MultiValueDictionary`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MultiValueDictionary<T0 = (), T1 = ()>(std::marker::PhantomData<(T0, T1)>);
 impl<T0, T1> MultiValueDictionary<T0, T1> {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.MultiValueDictionary";
@@ -192,7 +192,7 @@ impl<T0, T1> MultiValueDictionary<T0, T1> {
 }

 /// `enum` shim for C# `LibreMetaverse.ObjectCategory`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ObjectCategory;
 impl ObjectCategory {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.ObjectCategory";
@@ -200,7 +200,7 @@ impl ObjectCategory {
 }

 /// `enum` shim for C# `LibreMetaverse.PCode`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PCode;
 impl PCode {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.PCode";
@@ -208,7 +208,7 @@ impl PCode {
 }

 /// `enum` shim for C# `LibreMetaverse.PathCurve`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PathCurve;
 impl PathCurve {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.PathCurve";
@@ -216,7 +216,7 @@ impl PathCurve {
 }

 /// `enum` shim for C# `LibreMetaverse.PhysicsShapeType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PhysicsShapeType;
 impl PhysicsShapeType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.PhysicsShapeType";
@@ -224,7 +224,7 @@ impl PhysicsShapeType {
 }

 /// `enum` shim for C# `LibreMetaverse.Platform`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Platform;
 impl Platform {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Platform";
@@ -232,7 +232,7 @@ impl Platform {
 }

 /// `enum` shim for C# `LibreMetaverse.PrimFlags`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PrimFlags;
 impl PrimFlags {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.PrimFlags";
@@ -240,7 +240,7 @@ impl PrimFlags {
 }

 /// `enum` shim for C# `LibreMetaverse.PrimType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct PrimType;
 impl PrimType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.PrimType";
@@ -248,7 +248,7 @@ impl PrimType {
 }

 /// `enum` shim for C# `LibreMetaverse.ProfileCurve`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ProfileCurve;
 impl ProfileCurve {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.ProfileCurve";
@@ -256,7 +256,7 @@ impl ProfileCurve {
 }

 /// `struct` shim for C# `LibreMetaverse.Quaternion`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Quaternion;
 impl Quaternion {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Quaternion";
@@ -264,7 +264,7 @@ impl Quaternion {
 }

 /// `struct` shim for C# `LibreMetaverse.Ray`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Ray;
 impl Ray {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Ray";
@@ -272,7 +272,7 @@ impl Ray {
 }

 /// `enum` shim for C# `LibreMetaverse.SaleType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SaleType;
 impl SaleType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.SaleType";
@@ -280,7 +280,7 @@ impl SaleType {
 }

 /// `enum` shim for C# `LibreMetaverse.SculptType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SculptType;
 impl SculptType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.SculptType";
@@ -288,7 +288,7 @@ impl SculptType {
 }

 /// `enum` shim for C# `LibreMetaverse.SoundFlags`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct SoundFlags;
 impl SoundFlags {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.SoundFlags";
@@ -296,7 +296,7 @@ impl SoundFlags {
 }

 /// `class` shim for C# `LibreMetaverse.TokenBucket`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct TokenBucket;
 impl TokenBucket {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.TokenBucket";
@@ -304,7 +304,7 @@ impl TokenBucket {
 }

 /// `record struct` shim for C# `LibreMetaverse.UUID`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct UUID;
 impl UUID {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.UUID";
@@ -312,7 +312,7 @@ impl UUID {
 }

 /// `class` shim for C# `LibreMetaverse.Utils`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Utils;
 impl Utils {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Utils";
@@ -320,7 +320,7 @@ impl Utils {
 }

 /// `struct` shim for C# `LibreMetaverse.Vector2`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Vector2;
 impl Vector2 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Vector2";
@@ -328,7 +328,7 @@ impl Vector2 {
 }

 /// `struct` shim for C# `LibreMetaverse.Vector3`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Vector3;
 impl Vector3 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Vector3";
@@ -336,7 +336,7 @@ impl Vector3 {
 }

 /// `struct` shim for C# `LibreMetaverse.Vector3d`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Vector3d;
 impl Vector3d {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Vector3d";
@@ -344,7 +344,7 @@ impl Vector3d {
 }

 /// `struct` shim for C# `LibreMetaverse.Vector4`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Vector4;
 impl Vector4 {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.Vector4";
@@ -352,7 +352,7 @@ impl Vector4 {
 }

 /// `enum` shim for C# `LibreMetaverse.WearableType`; members remain intentionally unimplemented.
-#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct WearableType;
 impl WearableType {
    pub const CSHARP_NAME: &'static str = "LibreMetaverse.WearableType";
--- a/crates/libremetaverse-types/src/lib.rs
+++ b/crates/libremetaverse-types/src/lib.rs
@@ -1,5 +1,8 @@
 //! Core value types corresponding to `LibreMetaverse.Types`.

 mod generated;
+pub mod shim;

 pub use generated::*;
+pub use shim::NotImplemented as Error;
+pub use shim::{NotImplemented, not_implemented};
--- a/crates/libremetaverse-types/src/shim.rs
+++ b/crates/libremetaverse-types/src/shim.rs
@@ -0,0 +1,107 @@
+//! Failure-only infrastructure for callable API shims.
+//!
+//! Fallible synchronous and asynchronous members return [`NotImplemented`].
+//! Infallible members, trait methods, and constructors use
+//! [`unimplemented_api!`](crate::unimplemented_api). Stream and callback
+//! factories fail before returning a stream, registering a callback, or storing
+//! a trait object; an empty stream or no-op callback would be plausible behavior.
+
+use std::fmt;
+
+const PANIC_PREFIX: &str = "unimplemented C# API member: ";
+
+/// Typed failure returned by a fallible API shim.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct NotImplemented {
+    csharp_member: &'static str,
+}
+
+impl NotImplemented {
+    /// Creates a failure for one stable C# documentation ID.
+    #[must_use]
+    pub const fn new(csharp_member: &'static str) -> Self {
+        Self { csharp_member }
+    }
+
+    /// Returns the stable C# documentation ID of the unimplemented member.
+    #[must_use]
+    pub const fn csharp_member(self) -> &'static str {
+        self.csharp_member
+    }
+}
+
+impl fmt::Display for NotImplemented {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            formatter,
+            "C# API member {} is not implemented",
+            self.csharp_member
+        )
+    }
+}
+
+impl std::error::Error for NotImplemented {}
+
+/// Returns the standardized typed failure without constructing a fake value.
+///
+/// # Errors
+///
+/// Always returns [`NotImplemented`] with the supplied stable C# member ID.
+pub const fn not_implemented<T>(csharp_member: &'static str) -> Result<T, NotImplemented> {
+    Err(NotImplemented::new(csharp_member))
+}
+
+/// Panics for an infallible API whose signature cannot return [`NotImplemented`].
+///
+/// # Panics
+///
+/// Always, with a marker and the stable C# member ID.
+#[doc(hidden)]
+#[cold]
+#[track_caller]
+pub fn panic_unimplemented(csharp_member: &'static str) -> ! {
+    panic!("{PANIC_PREFIX}{csharp_member}")
+}
+
+/// Fails an infallible API shim with its stable C# documentation ID.
+///
+/// This is deliberately a panic rather than a default value. Translated tests
+/// expecting a domain error therefore cannot mistake an unimplemented API for
+/// the expected `Err` variant.
+///
+/// # Panics
+///
+/// Always, with the supplied stable C# member ID.
+#[macro_export]
+macro_rules! unimplemented_api {
+    ($csharp_member:expr $(,)?) => {{ $crate::shim::panic_unimplemented($csharp_member) }};
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{NotImplemented, PANIC_PREFIX, not_implemented};
+
+    const MEMBER: &str = "M:LibreMetaverse.GridClient.Login(System.String)";
+
+    #[test]
+    fn error_formats_and_preserves_member_id() {
+        let error = NotImplemented::new(MEMBER);
+        assert_eq!(error.csharp_member(), MEMBER);
+        assert_eq!(
+            error.to_string(),
+            format!("C# API member {MEMBER} is not implemented")
+        );
+        assert_eq!(not_implemented::<bool>(MEMBER), Err(error));
+    }
+
+    #[test]
+    fn panic_placeholder_is_not_an_expected_error_result() {
+        let panic = std::panic::catch_unwind(|| crate::unimplemented_api!(MEMBER))
+            .expect_err("the infallible placeholder must panic");
+        let message = panic
+            .downcast_ref::<String>()
+            .expect("placeholder panic must retain its visible marker");
+        assert_eq!(message, &format!("{PANIC_PREFIX}{MEMBER}"));
+        assert!(not_implemented::<()>(MEMBER).is_err());
+    }
+}