`CStr::from_bytes_with_nul` returns non-actionable error result

[nyurik]

Proposal

Problem statement

One of CStr constructors, CStr::from_bytes_with_nul(bytes: &[u8]) handles 3 cases:

1. bytes has one NULL as the last value - creates CStr
2. bytes has no NULL - error
3. bytes has a NULL in some other position - error

The 3rd case is error that may require lossy conversion, but the 2nd case can easily be handled by the user code. Unfortunately, this function returns an opaque FromBytesWithNulError error in both 2nd and 3rd case, so the user cannot detect just the 2nd case - having to re-implement the entire function and bring in the memchr dependency.

Motivating examples or use cases

In this code, my FFI code needs to copy user's &[u8] into a C-allocated memory blob in a NUL-terminated CStr format. My code must first validate if &[u8] has a trailing NUL (case 1), no NUL (adds one on the fly - case 2), or NUL in the middle (3rd case - error). I had to re-implement from_bytes_with_nul and add memchrdependency just to handle the 2nd case.

Solution sketch

It may make sense to do one of these:

• stabilize the kind of the error this function returns - so it can be examined
• introduce a new function that returns validation result for all 3 cases
• something else?

CStr::validate_bytes(value: &[u8]) -> CStrValidation -- returns an enum with valid, NotNulTerminated, InteriorNul

[kennytm]

Exposing the kind() should be the most natural, as that kind private field is already there.

[Amanieu]

We discussed this in the libs-api meeting and we are in favor of adding this. However since there are no other possible ways for from_bytes_with_nul to fail, the error type itself should be turned into an enum rather than having a separate kind method.

[nyurik]

Thx @Amanieu , are you saying that the enum FromBytesWithNulErrorKind should be renamed into enum FromBytesWithNulError , and the struct FromBytesWithNulError deleted?

[Amanieu]

Yes, essentially.

[nyurik]

wouldn't this be a breaking change?

[cuviper]

It's mostly opaque, but one way it breaks is that pattern FromBytesWithNulError { .. } works now with a struct, and won't work with an enum.

[nyurik]

so i guess its no biggie. I can submit a PR in a sec

[nyurik]

PR here rust-lang/rust#134143