simplify the API regarding authentication

src/sasl/mechanisms/anonymous.rs

@@ -1,6 +1,6 @@
 //! Provides the SASL "ANONYMOUS" mechanism.
 
-use sasl::SaslMechanism;
+use sasl::{SaslMechanism, SaslCredentials, SaslSecret};
 
 pub struct Anonymous;
 
@@ -12,4 +12,13 @@ impl Anonymous {
 
 impl SaslMechanism for Anonymous {
     fn name(&self) -> &str { "ANONYMOUS" }
+
+    fn from_credentials(credentials: SaslCredentials) -> Result<Anonymous, String> {
+        if let SaslSecret::None = credentials.secret {
+            Ok(Anonymous)
+        }
+        else {
+            Err("the anonymous sasl mechanism requires no credentials".to_owned())
+        }
+    }
 }

src/sasl/mechanisms/plain.rs

@@ -1,6 +1,6 @@
 //! Provides the SASL "PLAIN" mechanism.
 
-use sasl::SaslMechanism;
+use sasl::{SaslMechanism, SaslCredentials, SaslSecret};
 
 pub struct Plain {
     username: String,
@@ -19,6 +19,15 @@ impl Plain {
 impl SaslMechanism for Plain {
     fn name(&self) -> &str { "PLAIN" }
 
+    fn from_credentials(credentials: SaslCredentials) -> Result<Plain, String> {
+        if let SaslSecret::Password(password) = credentials.secret {
+            Ok(Plain::new(credentials.username, password))
+        }
+        else {
+            Err("PLAIN requires a password".to_owned())
+        }
+    }
+
     fn initial(&mut self) -> Result<Vec<u8>, String> {
         let mut auth = Vec::new();
         auth.push(0);

src/sasl/mechanisms/scram.rs

@@ -2,7 +2,7 @@
 
 use base64;
 
-use sasl::SaslMechanism;
+use sasl::{SaslMechanism, SaslCredentials, SaslSecret};
 
 use error::Error;
 
@@ -172,6 +172,22 @@ impl<S: ScramProvider> SaslMechanism for Scram<S> {
         &self.name
     }
 
+    fn from_credentials(credentials: SaslCredentials) -> Result<Scram<S>, String> {
+        if let SaslSecret::Password(password) = credentials.secret {
+            if let Some(binding) = credentials.channel_binding {
+                Scram::new_with_channel_binding(credentials.username, password, binding)
+                      .map_err(|_| "can't generate nonce".to_owned())
+            }
+            else {
+                Scram::new(credentials.username, password)
+                      .map_err(|_| "can't generate nonce".to_owned())
+            }
+        }
+        else {
+            Err("SCRAM requires a password".to_owned())
+        }
+    }
+
     fn initial(&mut self) -> Result<Vec<u8>, String> {
         let mut gs2_header = Vec::new();
         if let Some(_) = self.channel_binding {

src/sasl/mod.rs

@@ -1,9 +1,23 @@
 //! Provides the `SaslMechanism` trait and some implementations.
 
+pub struct SaslCredentials {
+    pub username: String,
+    pub secret: SaslSecret,
+    pub channel_binding: Option<Vec<u8>>,
+}
+
+pub enum SaslSecret {
+    None,
+    Password(String),
+}
+
 pub trait SaslMechanism {
     /// The name of the mechanism.
     fn name(&self) -> &str;
 
+    /// Creates this mechanism from `SaslCredentials`.
+    fn from_credentials(credentials: SaslCredentials) -> Result<Self, String> where Self: Sized;
+
     /// Provides initial payload of the SASL mechanism.
     fn initial(&mut self) -> Result<Vec<u8>, String> {
         Ok(Vec::new())