Error Messages do not give hints, what element caused the error.

[mohs8421]

I am currently working in the context of SUPL, where the rasn compiler generated these pieces:

    #[derive(AsnType, Debug, Clone, Decode, Encode, PartialEq, Eq, Hash)]
    #[rasn(delegate, identifier = "MCC-MNC-Digit", value("0..=9"))]
    pub struct MCCMNCDigit(pub u8);
    #[derive(AsnType, Debug, Clone, Decode, Encode, PartialEq, Eq, Hash)]
    #[rasn(delegate, size("2..=3"))]
    pub struct MNC(pub SequenceOf<MCCMNCDigit>);

This was used in a larger context, and I had a failure on an attemt to use rasn::uper::encode.
The error I get was looking like this:

EncodeError { kind: InvalidLength { length: 0, expected: Range { start: Some(2), end: Some(3) } }, codec: Uper, backtrace: Backtrace(()) }

For the error itself this information is certainly correct, but in using this it is insufficient. It took me quite some time to get the information, that the MNC declaration was behind this length constraint.

I traced it back to encode_with_tag_and_constraints in which I found the required information. I suggest to enrich the error information at this point to improve it's usability.

[mohs8421]

the decoder provides more details like this:

DecodeError { kind: FieldError { name: "ULPPDU.message", nested: DecodeError { kind: FieldError { name: "SUPLINIT.pos_method", nested: DecodeError { kind: Incomplete { needed: Size(1) }, codec: Uper, backtrace: Backtrace(()) } }, codec: Uper, backtrace: Backtrace(()) } }, codec: Uper, backtrace: Backtrace(()) }

this implies that the general idea of enriching the error with further context is already present, it's just not implemented on the encoding side.