core: Add Client enum

This enum can be used in const assertions to ensure that all required
crate features have been selected, for example in backends.

core/src/client.rs

@@ -39,6 +39,43 @@ pub use management::ManagementClient;
 #[cfg(feature = "ui-client")]
 pub use ui::UiClient;
 
+/// Available client traits.
+///
+/// This enum does not provide access to the trait features.  It is only intended for backends to
+/// use in constant assertions to ensure that the correct features are enabled.
+#[non_exhaustive]
+pub enum Client {
+    AttestClient,
+    CertificateClient,
+    CounterClient,
+    CryptoClient,
+    FilesystemClient,
+    ManagementClient,
+    UiClient,
+}
+
+impl Client {
+    /// All enabled clients.
+    ///
+    /// The contents of this constant depends on the enabled features.
+    pub const ENABLED: &[Self] = &[
+        #[cfg(feature = "attest-client")]
+        Self::AttestClient,
+        #[cfg(feature = "certificate-client")]
+        Self::CertificateClient,
+        #[cfg(feature = "counter-client")]
+        Self::CounterClient,
+        #[cfg(feature = "crypto-client")]
+        Self::CryptoClient,
+        #[cfg(feature = "filesystem-client")]
+        Self::FilesystemClient,
+        #[cfg(feature = "management-client")]
+        Self::ManagementClient,
+        #[cfg(feature = "ui-client")]
+        Self::UiClient,
+    ];
+}
+
 // to be fair, this is a programmer error,
 // and could also just panic
 #[derive(Copy, Clone, Debug)]