core: Add helpers for checking mechanisms at compile time

To be able to compare mechanism variants in const contexts, we use
add a const_eq function.  And to be able to indicate which mechanism
is potentially missing, we add a const panic method that includes the
mechanism variant in the panic message.

To reduce the boilerplate code, we use a macro to generate the enum.

Author: robin-nitrokey

core/src/types.rs

@@ -452,163 +452,149 @@ impl Client {
     ];
 }
 
-#[derive(Copy, Clone, Eq, PartialEq, Debug, Serialize, Deserialize)]
-#[non_exhaustive]
-pub enum Mechanism {
-    #[cfg(feature = "aes256-cbc")]
-    Aes256Cbc,
-    #[cfg(feature = "chacha8-poly1305")]
-    Chacha8Poly1305,
-    #[cfg(feature = "ed255")]
-    Ed255,
-    #[cfg(feature = "hmac-blake2s")]
-    HmacBlake2s,
-    #[cfg(feature = "hmac-sha1")]
-    HmacSha1,
-    #[cfg(feature = "hmac-sha256")]
-    HmacSha256,
-    #[cfg(feature = "hmac-sha512")]
-    HmacSha512,
-    // P256XSha256,
-    #[cfg(feature = "p256")]
-    P256,
-    #[cfg(feature = "p256")]
-    P256Prehashed,
-    #[cfg(feature = "p384")]
-    P384,
-    #[cfg(feature = "p384")]
-    P384Prehashed,
-    #[cfg(feature = "p521")]
-    P521,
-    #[cfg(feature = "p521")]
-    P521Prehashed,
-    #[cfg(feature = "brainpoolp256r1")]
-    BrainpoolP256R1,
-    #[cfg(feature = "brainpoolp256r1")]
-    BrainpoolP256R1Prehashed,
-    #[cfg(feature = "brainpoolp384r1")]
-    BrainpoolP384R1,
-    #[cfg(feature = "brainpoolp384r1")]
-    BrainpoolP384R1Prehashed,
-    #[cfg(feature = "brainpoolp512r1")]
-    BrainpoolP512R1,
-    #[cfg(feature = "brainpoolp512r1")]
-    BrainpoolP512R1Prehashed,
-    #[cfg(feature = "secp256k1")]
-    Secp256k1,
-    #[cfg(feature = "secp256k1")]
-    Secp256k1Prehashed,
-    // clients can also do hashing by themselves
-    #[cfg(feature = "sha256")]
-    Sha256,
-    #[cfg(feature = "tdes")]
-    Tdes,
-    #[cfg(feature = "totp")]
-    Totp,
-    #[cfg(feature = "trng")]
-    Trng,
-    #[cfg(feature = "x255")]
-    X255,
-    /// Used to serialize the output of a diffie-hellman
-    #[cfg(feature = "shared-secret")]
-    SharedSecret,
-
-    /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
-    /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
-    /// It should only be used if absolutely necessary.
-    #[cfg(feature = "rsa2048")]
-    Rsa2048Raw,
-    /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
-    /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
-    /// It should only be used if absolutely necessary.
-    #[cfg(feature = "rsa3072")]
-    Rsa3072Raw,
-    /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
-    /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
-    /// It should only be used if absolutely necessary.
-    #[cfg(feature = "rsa4096")]
-    Rsa4096Raw,
-
-    #[cfg(feature = "rsa2048")]
-    Rsa2048Pkcs1v15,
-    #[cfg(feature = "rsa3072")]
-    Rsa3072Pkcs1v15,
-    #[cfg(feature = "rsa4096")]
-    Rsa4096Pkcs1v15,
+macro_rules! generate_mechanism {
+    (
+        $(#[$outer:meta])*
+        $vis:vis enum $name:ident {
+            $(
+                #[cfg($($cfg_cond:tt)*)]
+                $(#[$inner:meta])*
+                $variant:ident,
+            )*
+        }
+    ) => {
+        $(#[$outer])*
+        $vis enum $name {
+            $(
+                #[cfg($($cfg_cond)*)]
+                $(#[$inner])*
+                $variant,
+            )*
+        }
+
+        impl $name {
+            /// All enabled mechanisms.
+            ///
+            /// The contents of this constant depends on the enabled features.
+            pub const ENABLED: &[Self] = &[
+                $(
+                    #[cfg($($cfg_cond)*)]
+                    Self::$variant,
+                )*
+            ];
+
+            /// Check equality in a const-friendly way.
+            pub const fn const_eq(&self, other: Self) -> bool {
+                let _ = other;
+                match *self {
+                    $(
+                        #[cfg($($cfg_cond)*)]
+                        Self::$variant => matches!(other, Self::$variant),
+                    )*
+                }
+            }
+
+            /// Panic with a message contain this mechanism.
+            ///
+            /// This is intended for compile time checks that don’t have a direct way to indicate
+            /// a missing mechanism in a panic message.
+            pub const fn panic(&self) -> ! {
+                match *self {
+                    $(
+                        #[cfg($($cfg_cond)*)]
+                        Self::$variant => panic!(concat!("panic for mechanism: ", stringify!($variant))),
+                    )*
+                }
+            }
+        }
+    }
 }
 
-impl Mechanism {
-    /// All enabled mechanisms.
-    ///
-    /// The contents of this constant depends on the enabled features.
-    pub const ENABLED: &[Self] = &[
+generate_mechanism! {
+    #[derive(Copy, Clone, Eq, PartialEq, Debug, Serialize, Deserialize)]
+    #[non_exhaustive]
+    pub enum Mechanism {
         #[cfg(feature = "aes256-cbc")]
-        Self::Aes256Cbc,
+        Aes256Cbc,
         #[cfg(feature = "chacha8-poly1305")]
-        Self::Chacha8Poly1305,
+        Chacha8Poly1305,
         #[cfg(feature = "ed255")]
-        Self::Ed255,
+        Ed255,
         #[cfg(feature = "hmac-blake2s")]
-        Self::HmacBlake2s,
+        HmacBlake2s,
         #[cfg(feature = "hmac-sha1")]
-        Self::HmacSha1,
+        HmacSha1,
         #[cfg(feature = "hmac-sha256")]
-        Self::HmacSha256,
+        HmacSha256,
         #[cfg(feature = "hmac-sha512")]
-        Self::HmacSha512,
+        HmacSha512,
+        // P256XSha256,
         #[cfg(feature = "p256")]
-        Self::P256,
+        P256,
         #[cfg(feature = "p256")]
-        Self::P256Prehashed,
+        P256Prehashed,
         #[cfg(feature = "p384")]
-        Self::P384,
+        P384,
         #[cfg(feature = "p384")]
-        Self::P384Prehashed,
+        P384Prehashed,
         #[cfg(feature = "p521")]
-        Self::P521,
+        P521,
         #[cfg(feature = "p521")]
-        Self::P521Prehashed,
+        P521Prehashed,
         #[cfg(feature = "brainpoolp256r1")]
-        Self::BrainpoolP256R1,
+        BrainpoolP256R1,
         #[cfg(feature = "brainpoolp256r1")]
-        Self::BrainpoolP256R1Prehashed,
+        BrainpoolP256R1Prehashed,
         #[cfg(feature = "brainpoolp384r1")]
-        Self::BrainpoolP384R1,
+        BrainpoolP384R1,
         #[cfg(feature = "brainpoolp384r1")]
-        Self::BrainpoolP384R1Prehashed,
+        BrainpoolP384R1Prehashed,
         #[cfg(feature = "brainpoolp512r1")]
-        Self::BrainpoolP512R1,
+        BrainpoolP512R1,
         #[cfg(feature = "brainpoolp512r1")]
-        Self::BrainpoolP512R1Prehashed,
+        BrainpoolP512R1Prehashed,
         #[cfg(feature = "secp256k1")]
-        Self::Secp256k1,
+        Secp256k1,
         #[cfg(feature = "secp256k1")]
-        Self::Secp256k1Prehashed,
+        Secp256k1Prehashed,
+        // clients can also do hashing by themselves
         #[cfg(feature = "sha256")]
-        Self::Sha256,
+        Sha256,
         #[cfg(feature = "tdes")]
-        Self::Tdes,
+        Tdes,
         #[cfg(feature = "totp")]
-        Self::Totp,
+        Totp,
         #[cfg(feature = "trng")]
-        Self::Trng,
+        Trng,
         #[cfg(feature = "x255")]
-        Self::X255,
+        X255,
         #[cfg(feature = "shared-secret")]
-        Self::SharedSecret,
+        /// Used to serialize the output of a diffie-hellman
+        SharedSecret,
+
         #[cfg(feature = "rsa2048")]
-        Self::Rsa2048Raw,
+        /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
+        /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
+        /// It should only be used if absolutely necessary.
+        Rsa2048Raw,
         #[cfg(feature = "rsa3072")]
-        Self::Rsa3072Raw,
+        /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
+        /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
+        /// It should only be used if absolutely necessary.
+        Rsa3072Raw,
         #[cfg(feature = "rsa4096")]
-        Self::Rsa4096Raw,
+        /// Exposes the Raw RSA encryption/decryption primitive. Be aware this is dangerous.
+        /// Not having any padding can allow an attacker to obtain plaintexts and forge signatures.
+        /// It should only be used if absolutely necessary.
+        Rsa4096Raw,
+
         #[cfg(feature = "rsa2048")]
-        Self::Rsa2048Pkcs1v15,
+        Rsa2048Pkcs1v15,
         #[cfg(feature = "rsa3072")]
-        Self::Rsa3072Pkcs1v15,
+        Rsa3072Pkcs1v15,
         #[cfg(feature = "rsa4096")]
-        Self::Rsa4096Pkcs1v15,
-    ];
+        Rsa4096Pkcs1v15,
+    }
 }
 
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]