Add a default case for impl_deserialize_for_internally_tagged_enum

[aesteve]

Attempt to implement #629 .

• I did not find any unit tests apart from the doctest, let me know if I missed something
• I tried to make the implementation backwards compatible with the existing one. If I understood correctly, it accepted a , at the end of the list of all variants so I replicated this behaviour. But I think in the current version, it'd accept 2 commas, not sure if what I wrote is the cleanest.
• I'd really encourage anyone to actually check this (backwards-compatibility especially) in very details, since that's the very first macro I'm writing
• let me know if the doc is unclear or if you want me to change anything, no problem

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.24%. Comparing base (5d76174) to head (421dc0f).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #634   +/-   ##
=======================================
  Coverage   61.24%   61.24%           
=======================================
  Files          39       39           
  Lines       16277    16277           
=======================================
  Hits         9969     9969           
  Misses       6308     6308           

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[Mingun]

Good, but small improvements are possible. Also, please update Changelog.md: add new entry under Features (links to issues are listed at the end of version section). Link it to the issue #629, not to this PR.

[Mingun]

You can use intermediate macro with two possible variants to select required behavior. General idea:

macro_rules! def {
  // variant 1
  (
    $($variant_tag:literal),*
    _ => Ok( $tokens:tt )
  ) => {
    _ => Ok($tokens),
  },
  // variant 2
  (
    $($variant_tag:literal),*
  ) => {
    _ => Err(A::Error::unknown_field(&tag, &[$($variant_tag),+])),
  },
}

// use 
def!(
  $($variant_tag),*
  $( _ => Ok(deserialize_variant!( de, $enum, $($default_variant)? )), )?
)

If $(...)? part exists, then the first alternative would be selected, otherwise the second. That way we would not need to use #[allow(unreachable)] attribute, which could be forbidden in the project via #[forbid(unreachable)]

[aesteve]

Thanks for the pointer, I had the conviction the allow_unreachable wasn't a clean solution.

I get the general idea here, but this code doesn't compile ("no rules expected the token ,") and I have no clue what I should change.

Thanks.

Closest I got:

#[macro_export]
#[doc(hidden)]
macro_rules! def {
  // variant 1
  (
    $($variant_tag:literal),*
    _ => Ok($($tokens:tt)*)
  ) => {
    _ => Ok($($tokens)*),
  };
  // variant 2
  (
    $($variant_tag:literal),*
  ) => {
    _ => Err(A::Error::unknown_field(&tag, &[$($variant_tag),+])),
  }
}

match tag.as_ref() {
  $(
    $variant_tag => Ok(deserialize_variant!( de, $enum, $($variant)+ )),
  )*
  quick_xml::def!(
    $($variant_tag),*
    $( _ => Ok(deserialize_variant!( de, $enum, $($default_variant)? )), )?
  )
}

[Mingun]

Thanks for your work! I implemented the helper macro myself and push it.