diff --git a/dust-lang/src/lex.rs b/dust-lang/src/lex.rs
index 66538b5..530eb36 100644
--- a/dust-lang/src/lex.rs
+++ b/dust-lang/src/lex.rs
@@ -48,6 +48,8 @@ pub fn lex<'chars, 'src: 'chars>(input: &'src str) -> Result<Vec<(Token<'chars>,
 #[derive(Debug, Clone)]
 /// Low-level tool for lexing a single token at a time.
 ///
+/// **Note**: It is a logic error to call `next_token` with different inputs.
+///
 /// # Examples
 /// ```
 /// # use dust_lang::*;
@@ -89,6 +91,8 @@ impl Lexer {
     }
 
     /// Produce the next token.
+    ///
+    /// It is a logic error to call this method with different inputs.
     pub fn next_token<'src>(&mut self, source: &'src str) -> Result<(Token<'src>, Span), LexError> {
         self.skip_whitespace(source);
 
diff --git a/dust-lang/src/parse.rs b/dust-lang/src/parse.rs
index d15e8ca..a8a061b 100644
--- a/dust-lang/src/parse.rs
+++ b/dust-lang/src/parse.rs
@@ -230,7 +230,7 @@ impl<'src> Parser<'src> {
                     Ok(Node::new(node.statement, (left_span.0, right_span.1)))
                 } else {
                     Err(ParseError::ExpectedClosingParenthesis {
-                        actual: TokenOwned::from(self.current.0),
+                        actual: self.current.0.to_owned(),
                         span: self.current.1,
                     })
                 }
@@ -260,7 +260,7 @@ impl<'src> Parser<'src> {
                         nodes.push(instruction);
                     } else {
                         return Err(ParseError::ExpectedClosingSquareBrace {
-                            actual: TokenOwned::from(self.current.0),
+                            actual: self.current.0.to_owned(),
                             span: self.current.1,
                         });
                     }
@@ -285,7 +285,7 @@ impl<'src> Parser<'src> {
                     self.next_token()?;
                 } else {
                     return Err(ParseError::ExpectedOpeningParenthesis {
-                        actual: TokenOwned::from(self.current.0),
+                        actual: self.current.0.to_owned(),
                         span: self.current.1,
                     });
                 }
@@ -311,7 +311,7 @@ impl<'src> Parser<'src> {
                         }
                     } else {
                         return Err(ParseError::ExpectedClosingParenthesis {
-                            actual: TokenOwned::from(self.current.0),
+                            actual: self.current.0.to_owned(),
                             span: self.current.1,
                         });
                     }
@@ -326,9 +326,7 @@ impl<'src> Parser<'src> {
                     left_span,
                 ))
             }
-            _ => Err(ParseError::UnexpectedToken(TokenOwned::from(
-                self.current.0,
-            ))),
+            _ => Err(ParseError::UnexpectedToken(self.current.0.to_owned())),
         }
     }
 
diff --git a/dust-lang/src/token.rs b/dust-lang/src/token.rs
index 25ea17a..55849a2 100644
--- a/dust-lang/src/token.rs
+++ b/dust-lang/src/token.rs
@@ -2,7 +2,8 @@ use std::fmt::{self, Display, Formatter};
 
 use serde::{Deserialize, Serialize};
 
-#[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
+/// Source code token.
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
 pub enum Token<'src> {
     Eof,
 
@@ -33,6 +34,33 @@ pub enum Token<'src> {
     Star,
 }
 
+impl<'src> Token<'src> {
+    pub fn to_owned(&self) -> TokenOwned {
+        match self {
+            Token::Eof => TokenOwned::Eof,
+            Token::Identifier(text) => TokenOwned::Identifier(text.to_string()),
+            Token::Boolean(boolean) => TokenOwned::Boolean(*boolean),
+            Token::Float(float) => TokenOwned::Float(*float),
+            Token::Integer(integer) => TokenOwned::Integer(*integer),
+            Token::String(text) => TokenOwned::String(text.to_string()),
+            Token::IsEven => TokenOwned::IsEven,
+            Token::IsOdd => TokenOwned::IsOdd,
+            Token::Length => TokenOwned::Length,
+            Token::ReadLine => TokenOwned::ReadLine,
+            Token::WriteLine => TokenOwned::WriteLine,
+            Token::Comma => TokenOwned::Comma,
+            Token::Dot => TokenOwned::Dot,
+            Token::Equal => TokenOwned::Equal,
+            Token::Plus => TokenOwned::Plus,
+            Token::Star => TokenOwned::Star,
+            Token::LeftParenthesis => TokenOwned::LeftParenthesis,
+            Token::RightParenthesis => TokenOwned::RightParenthesis,
+            Token::LeftSquareBrace => TokenOwned::LeftSquareBrace,
+            Token::RightSquareBrace => TokenOwned::RightSquareBrace,
+        }
+    }
+}
+
 impl<'src> Display for Token<'src> {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         match self {
@@ -60,6 +88,9 @@ impl<'src> Display for Token<'src> {
     }
 }
 
+/// Owned version of `Token`, which owns all the strings.
+///
+/// This is used for errors.
 #[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
 pub enum TokenOwned {
     Eof,
@@ -90,30 +121,3 @@ pub enum TokenOwned {
     RightSquareBrace,
     Star,
 }
-
-impl<'str> From<Token<'str>> for TokenOwned {
-    fn from(token: Token<'str>) -> Self {
-        match token {
-            Token::Eof => TokenOwned::Eof,
-            Token::Identifier(text) => TokenOwned::Identifier(text.to_string()),
-            Token::Boolean(boolean) => TokenOwned::Boolean(boolean),
-            Token::Float(float) => TokenOwned::Float(float),
-            Token::Integer(integer) => TokenOwned::Integer(integer),
-            Token::String(text) => TokenOwned::String(text.to_string()),
-            Token::IsEven => TokenOwned::IsEven,
-            Token::IsOdd => TokenOwned::IsOdd,
-            Token::Length => TokenOwned::Length,
-            Token::ReadLine => TokenOwned::ReadLine,
-            Token::WriteLine => TokenOwned::WriteLine,
-            Token::Comma => TokenOwned::Comma,
-            Token::Dot => TokenOwned::Dot,
-            Token::Equal => TokenOwned::Equal,
-            Token::Plus => TokenOwned::Plus,
-            Token::Star => TokenOwned::Star,
-            Token::LeftParenthesis => TokenOwned::LeftParenthesis,
-            Token::RightParenthesis => TokenOwned::RightParenthesis,
-            Token::LeftSquareBrace => TokenOwned::LeftSquareBrace,
-            Token::RightSquareBrace => TokenOwned::RightSquareBrace,
-        }
-    }
-}