diff --git a/src/tokenizer/japanese_tokenizer.rs b/src/tokenizer/japanese_tokenizer.rs
index af56eb94f..c9981b201 100644
--- a/src/tokenizer/japanese_tokenizer.rs
+++ b/src/tokenizer/japanese_tokenizer.rs
@@ -21,7 +21,7 @@ pub struct JapaneseTokenizerStream {
 impl<'a> Tokenizer<'a> for JapaneseTokenizer {
     type TokenStreamImpl = JapaneseTokenizerStream;
 
-    fn token_stream(&mut self, text: &'a str) -> Self::TokenStreamImpl {
+    fn token_stream(&self, text: &'a str) -> Self::TokenStreamImpl {
         let mut tokens = vec![];
         let mut offset_from;
         let mut offset_to = 0;
diff --git a/src/tokenizer/mod.rs b/src/tokenizer/mod.rs
index 94431797f..634bc4810 100644
--- a/src/tokenizer/mod.rs
+++ b/src/tokenizer/mod.rs
@@ -159,7 +159,7 @@ mod test {
     #[test]
     fn test_raw_tokenizer() {
         let tokenizer_manager = TokenizerManager::default();
-        let mut en_tokenizer = tokenizer_manager.get("raw").unwrap();
+        let en_tokenizer = tokenizer_manager.get("raw").unwrap();
         let mut tokens: Vec<String> = vec![];
         {
             let mut add_token = |token: &Token| { tokens.push(token.text.clone()); };
@@ -174,7 +174,7 @@ mod test {
     fn test_en_tokenizer() {
         let tokenizer_manager = TokenizerManager::default();
         assert!(tokenizer_manager.get("en_doesnotexist").is_none());
-        let mut en_tokenizer = tokenizer_manager.get("en_stem").unwrap();
+        let en_tokenizer = tokenizer_manager.get("en_stem").unwrap();
         let mut tokens: Vec<String> = vec![];
         {
             let mut add_token = |token: &Token| { tokens.push(token.text.clone()); };
@@ -190,7 +190,7 @@ mod test {
     #[test]
     fn test_jp_tokenizer() {
         let tokenizer_manager = TokenizerManager::default();
-        let mut en_tokenizer = tokenizer_manager.get("ja").unwrap();
+        let en_tokenizer = tokenizer_manager.get("ja").unwrap();
         
         let mut tokens: Vec<String> = vec![];
         {
@@ -208,7 +208,7 @@ mod test {
     #[test]
     fn test_tokenizer_empty() {
         let tokenizer_manager = TokenizerManager::default();
-        let mut en_tokenizer = tokenizer_manager.get("en_stem").unwrap();
+        let en_tokenizer = tokenizer_manager.get("en_stem").unwrap();
         {
             let mut tokens: Vec<String> = vec![];
             {
diff --git a/src/tokenizer/raw_tokenizer.rs b/src/tokenizer/raw_tokenizer.rs
index 23bd6efe4..0d97103eb 100644
--- a/src/tokenizer/raw_tokenizer.rs
+++ b/src/tokenizer/raw_tokenizer.rs
@@ -13,7 +13,7 @@ pub struct RawTokenStream {
 impl<'a> Tokenizer<'a> for RawTokenizer {
     type TokenStreamImpl = RawTokenStream;
 
-    fn token_stream(&mut self, text: &'a str) -> Self::TokenStreamImpl {
+    fn token_stream(&self, text: &'a str) -> Self::TokenStreamImpl {
         let token = Token {
             offset_from: 0,
             offset_to: text.len(),
diff --git a/src/tokenizer/simple_tokenizer.rs b/src/tokenizer/simple_tokenizer.rs
index f6e223fc2..e9d93deee 100644
--- a/src/tokenizer/simple_tokenizer.rs
+++ b/src/tokenizer/simple_tokenizer.rs
@@ -16,7 +16,7 @@ pub struct SimpleTokenStream<'a> {
 impl<'a> Tokenizer<'a> for SimpleTokenizer {
     type TokenStreamImpl = SimpleTokenStream<'a>;
 
-    fn token_stream(&mut self, text: &'a str) -> Self::TokenStreamImpl {
+    fn token_stream(&self, text: &'a str) -> Self::TokenStreamImpl {
         SimpleTokenStream {
             text: text,
             chars: text.char_indices(),
diff --git a/src/tokenizer/token_stream_chain.rs b/src/tokenizer/token_stream_chain.rs
index 401e77bba..84d17fede 100644
--- a/src/tokenizer/token_stream_chain.rs
+++ b/src/tokenizer/token_stream_chain.rs
@@ -48,16 +48,18 @@ impl<'a, TTokenStream> TokenStream for TokenStreamChain<TTokenStream>
     }
 
     fn token(&self) -> &Token {
-        if self.stream_idx > self.token_streams.len() {
-            panic!("You called .token(), after the end of the token stream has been reached");
-        }
+        assert!(
+            self.stream_idx <= self.token_streams.len(),
+            "You called .token(), after the end of the token stream has been reached"
+        );
         &self.token
     }
 
     fn token_mut(&mut self) -> &mut Token {
-        if self.stream_idx > self.token_streams.len() {
-            panic!("You called .token(), after the end of the token stream has been reached");
-        }
+        assert!(
+            self.stream_idx <= self.token_streams.len(),
+            "You called .token(), after the end of the token stream has been reached"
+        );
         &mut self.token
     }
 }
diff --git a/src/tokenizer/tokenizer.rs b/src/tokenizer/tokenizer.rs
index 3b862f7ed..6297af0ca 100644
--- a/src/tokenizer/tokenizer.rs
+++ b/src/tokenizer/tokenizer.rs
@@ -48,7 +48,7 @@ pub trait Tokenizer<'a>: Sized + Clone {
     type TokenStreamImpl: TokenStream;
 
     /// Creates a token stream for a given `str`.
-    fn token_stream(&mut self, text: &'a str) -> Self::TokenStreamImpl;
+    fn token_stream(&self, text: &'a str) -> Self::TokenStreamImpl;
 
     /// Appends a token filter to the current tokenizer.
     ///
@@ -80,10 +80,9 @@ pub trait Tokenizer<'a>: Sized + Clone {
     }
 }
 
-
 pub trait BoxedTokenizer: Send + Sync {
-    fn token_stream<'a>(&mut self, text: &'a str) -> Box<TokenStream + 'a>;
-    fn token_stream_texts<'b>(&mut self, texts: &'b [&'b str]) -> Box<TokenStream + 'b>;
+    fn token_stream<'a>(&self, text: &'a str) -> Box<TokenStream + 'a>;
+    fn token_stream_texts<'b>(&self, texts: &'b [&'b str]) -> Box<TokenStream + 'b>;
     fn boxed_clone(&self) -> Box<BoxedTokenizer>;
 }
 
@@ -91,11 +90,11 @@ pub trait BoxedTokenizer: Send + Sync {
 struct BoxableTokenizer<A>(A) where A: for <'a> Tokenizer<'a> + Send + Sync;
 
 impl<A> BoxedTokenizer for BoxableTokenizer<A> where A: 'static + Send + Sync + for <'a> Tokenizer<'a> {
-    fn token_stream<'a>(&mut self, text: &'a str) -> Box<TokenStream + 'a> {
+    fn token_stream<'a>(&self, text: &'a str) -> Box<TokenStream + 'a> {
         box self.0.token_stream(text)
     }
 
-    fn token_stream_texts<'b>(&mut self, texts: &'b [&'b str]) -> Box<TokenStream + 'b> {
+    fn token_stream_texts<'b>(&self, texts: &'b [&'b str]) -> Box<TokenStream + 'b> {
         assert!(texts.len() > 0);
         if texts.len() == 1 {
             box self.0.token_stream(texts[0])
@@ -103,7 +102,7 @@ impl<A> BoxedTokenizer for BoxableTokenizer<A> where A: 'static + Send + Sync +
         else {
             let mut offsets = vec!();
             let mut total_offset = 0;
-            for text in texts {
+            for &text in texts {
                 offsets.push(total_offset);
                 total_offset += text.len();
             }
@@ -217,7 +216,7 @@ impl<'a, HeadTokenFilterFactory, TailTokenizer> Tokenizer<'a>
 {
     type TokenStreamImpl = HeadTokenFilterFactory::ResultTokenStream;
 
-    fn token_stream(&mut self, text: &'a str) -> Self::TokenStreamImpl {
+    fn token_stream(&self, text: &'a str) -> Self::TokenStreamImpl {
         let tail_token_stream = self.tail.token_stream(text );
         self.head.transform(tail_token_stream)
     }