Relax Logos::Error bounds

book/src/attributes/logos.md

@@ -32,9 +32,9 @@ For more details about extras, read the [eponym section](../extras.md).
 
 By default, **Logos** uses `()` as the error type, which means that it
 doesn't store any information about the error.
-This can be changed by using `#[logos(error = ErrorType)]` attribute on the enum.
-The type `ErrorType` can be any type that implements `Clone`, `PartialEq`,
-`Default` and `From<E>` for each callback's error type.
+This can be changed by using the `#[logos(error = ErrorType)]` attribute on the enum.
+The type `ErrorType` can be any type that implements `From<E>` for each callback's error type.
+If no error callback is provided, `ErrorType` must implement `Default` as well.
 
 Here is an example using a custom error type:
 
@@ -46,10 +46,7 @@ You can add error variants to `LexingError`,
 and implement `From<E>` for each error type `E` that could
 be returned by a callback. See [callbacks](../callbacks.md).
 
-`ErrorType` must implement the `Default` trait because invalid tokens, i.e.,
-literals that do not match any variant, will produce `Err(ErrorType::default())`.
-
-Alternatively, you can provide a callback with the alternate syntax
+You can provide a callback with the syntax
 `#[logos(error(ErrorType, callback = ...))]`, which allows you to include information
 from the lexer such as the span where the error occurred:
 
@@ -64,6 +61,10 @@ enum Token {
 }
 ```
 
+If you do not provide a callback, then invalid tokens, i.e.,
+literals that do not match any variant, will produce `Err(ErrorType::default())`.
+This requires `ErrorType` to implement the `Default` trait.
+
 ## Specifying path to logos
 
 You can force the derive macro to use a different path to `Logos`' crate

examples/array_language.rs

@@ -20,16 +20,10 @@ use std::path::Path;
 
 /* ANCHOR: error_type */
 /// Token error type, tied to the lifetime of the source.
-#[derive(Default, Debug, Clone, PartialEq)]
 enum LexingError<'s> {
     UnknownSymbol(&'s str),
-    InvalidInteger {
-        err: ParseIntError,
-        source: &'s str,
-    },
+    InvalidInteger { err: ParseIntError, source: &'s str },
     UnknownVariable(&'s str),
-    #[default]
-    Other,
 }
 /* ANCHOR_END: error_type */
 
@@ -41,7 +35,6 @@ impl Display for LexingError<'_> {
                 write!(f, "int error in source `{source}`: {err}")
             }
             Self::UnknownVariable(s) => write!(f, "unknown variable `{s}`"),
-            Self::Other => write!(f, "unknown error"),
         }
     }
 }
@@ -51,7 +44,7 @@ type Environment = HashMap<String, Vec<i128>>;
 /* ANCHOR_END: environment */
 
 /* ANCHOR: callbacks */
-/// Parse lexer slice as an i128
+/// Parse lexer slice as an `i128`
 fn number_callback<'s>(lex: &mut Lexer<'s, Token>) -> Result<i128, LexingError<'s>> {
     let source = lex.slice();
     let res = source.parse();

examples/custom_error.rs

@@ -11,12 +11,10 @@ use logos::Logos;
 
 use std::num::ParseIntError;
 
-#[derive(Default, Debug, Clone, PartialEq)]
+#[derive(Debug, PartialEq)]
 enum LexingError {
     InvalidInteger(String),
     NonAsciiCharacter(char),
-    #[default]
-    Other,
 }
 
 /// Error type returned by calling `lex.slice().parse()` to u8.

src/lib.rs

@@ -26,7 +26,6 @@
 
 extern crate core;
 
-use core::fmt::Debug;
 #[cfg(feature = "export_derive")]
 pub use logos_derive::Logos;
 
@@ -53,7 +52,7 @@ pub trait Logos<'source>: Sized {
 
     /// Error type returned by the lexer. This can be set using
     /// `#[logos(error = MyError)]`. Defaults to `()` if not set.
-    type Error: Default + Clone + PartialEq + Debug + 'source;
+    type Error: 'source;
 
     /// The heart of Logos. Called by the `Lexer`. The implementation for this function
     /// is generated by the `logos-derive` crate.

tests/src/lib.rs

@@ -15,6 +15,7 @@ pub fn assert_lex<'a, Token>(
 ) where
     Token: Logos<'a> + fmt::Debug + PartialEq,
     Token::Extras: Default,
+    Token::Error: fmt::Debug + PartialEq,
 {
     let mut lex = Token::lexer(source);