An Error likely has a message, it might have a cause, and someday, it may have a trace/frame. How should they be formatted? What is a good default, and how should a user configure to their needs?

Output options:

• Top message only

  ship exploded
  

• Top message + message of source chain

  ship exploded: cat hair in generator
  

• Top message + message of source chain + trace/frame

  ship exploded
      at main.rs:55
      at ship.rs:89
  Caused by: cat hair in generator
      at ship::parts::generator.rs:33
      at ship::parts::engine.rs:789
      at ship.rs:89
      at main.rs:55
  

User format flags:

The user can specify some flags when trying to format a value:

• Default: {}
• Alternative: {:#}
• Plus: {:+}
  • Currently only used by numerics, so Errors could interpret this differently.
• Precision: {:.3}
  • Currently defined for floats and strings. For strings, implies a "maximum width" of a string.
  • We could hijack this for Errors to mean "maximum iterators down source chain"

Proposed usage

• Default ({}): Print only the top-level message. This is inline with the recommendation for Error
  • Example: println!("top only = {}", err) outputs top only = ship exploded.
  • Alternative: This could also be achieved and possibly clearer by setting the "precision" flag to 0, such as println!("top only: {:.0}", err).
• Message chain ({:+}): Prints the message, and the message of each source, joined by ": ".
  • Example: println!("chain = {:+}", err) outputs chain = ship exploded: cat hair in generator.
• With trace/frame ({:#}): Prints the message and stack trace/frame
  • Example: println!("top trace = {:#}", err) outputs top trace = ship exploded\n at ship.rs:89.
• Message chain with trace/frame ({:+#}): Prints the message and stack trace/frame, and message and trace for each source, joined by \nCaused by:.
• Message chain maximum ({:+.2}): Sets the maximum messages that should be printed down the source chain.