convert: Fix MismatchMessage for object attributes

[alisdair]

When determining which object attributes cause a type mismatch, we must disregard equal attribute types. Previously we would attempt to find a conversion between them, deciding that the attribute type was a mismatch if no conversion exists. However, this is invalid if the two types are equal, because GetConversion returns nil for equal primitive types.

Because of non-deterministic map iteration order, the test case added in this commit failed approximately 2/3 of the time before the change. It now passes consistently.

Terraform issue report: hashicorp/terraform#27010

[codecov]

Codecov Report

Merging #78 (0d0da20) into main (72c7e3e) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   70.35%   70.27%   -0.09%     
==========================================
  Files          79       79              
  Lines        6548     6550       +2     
==========================================
- Hits         4607     4603       -4     
- Misses       1497     1503       +6     
  Partials      444      444              

Impacted Files	Coverage Δ
cty/convert/mismatch_msg.go	71.42% <100.00%> (+0.59%)	⬆️
cty/convert/compare_types.go	92.59% <0.00%> (-7.41%)	⬇️

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[apparentlymart]

Thanks! This makes sense to me, and indeed this (equal types returning a nil conversion) is a gotcha that's caught me out a few times now.

I remember when I first built this that there was a good reason why returning nil for equal types was important, but I don't remember now what it was. Perhaps when I have some spare time I'll try making equal types return a non-op conversion instead and see what breaks to see if it reminds me why it's designed this way. 😀