panic when formatting `de::Error` (with feature `custom-error-messages`)

[t-moe]

I'm getting a panic in impl de::Error for Error when the feature custom-error-messages is enabled

serde-json-core/src/de/mod.rs

Lines 765 to 784 in ca67457

	impl de::Error for Error {
	#[cfg_attr(not(feature = "custom-error-messages"), allow(unused_variables))]
	fn custom<T>(msg: T) -> Self
	where
	T: fmt::Display,
	{
	#[cfg(not(feature = "custom-error-messages"))]
	{
	Error::CustomError
	}
	#[cfg(feature = "custom-error-messages")]
	{
	use core::fmt::Write;
	let mut string = heapless::String::new();
	write!(string, "{:.64}", msg).unwrap();
	Error::CustomErrorWithMessage(string)
	}
	}
	}

The panic occurs on the line: write!(string, "{:.64}", msg).unwrap()
when it tries to format an error message like

unknown variant a1, expected one of b2, b3, b4, b5, b6, b7

The panic occurs because serde uses format_args!

https://github.com/serde-rs/serde/blob/31000e1874ff01362f91e7b53794e402fab4fc78/serde/src/de/mod.rs#L255-L259

  Error::custom(format_args!(
                "unknown variant `{}`, expected {}",
                variant,
                OneOf { names: expected }
            ))

and format_args! cannot be be truncated using specified precision.

See the following simplified reproducer, generates the same panic:

let x = serde_json_core::de::Error::custom(format_args!(
          "unknown variant `{}`, expected {}",
          "a",
          "one of bbbbbbbbbbbbbbbbbbbbbbbbb,ccccccccccccccccccc,ddddddddddddddddddddddddd,eeeeeeeeee"
  ));
  error!("{:?}", x);