rm unused code

[tersec]

To start with, it doesn't work right now:

import ssz_serialization
echo SSZ.supports(uint64)   # is false

in Nim 2.0.8 prints false, for example.

status-im/nimbus-eth2#6573 (comment) has more context -- essentially compiles( ... declval ...) behaves a bit oddly at times.

Nothing either in nim-ssz-serialization or nimbus-eth2 uses it, either.

If people truly want this functionality, it can be reintroduced and reviewed as new code in a separate PR, but it's more important to remove the broken dependent on questionable assumptions which vary depending on all three of (NimMajor, NimMinor, NimPatch).

[tersec]

Ah, it is used, by the test suite (which apparently doesn't catch this, or runs in other ways) via nim-serialization:

NimScript] exec: nim c  --styleCheck:usages --styleCheck:error --verbosity:0 --hints:off --skipParentCfg --skipUserCfg --outdir:build '--nimcache:build/nimcache/$projectName'  --mm:refc --threads:on -d:PREFER_BLST_SHA256=false -d:PREFER_HASHTREE_SHA256=false --mm:refc -r tests/test_all
/Users/runner/work/nim-ssz-serialization/nim-ssz-serialization/tests/test_ssz_roundtrip.nim(14, 1) template/generic instantiation of `executeRoundtripTests` from here
/Users/runner/.nimble/pkgs2/serialization-0.2.2-7d65be9aa3c3249957ee779435e7093367ad719d/serialization/testing/generic_suite.nim(197, 8) template/generic instantiation of `suite` from here
/Users/runner/.nimble/pkgs2/serialization-0.2.2-7d65be9aa3c3249957ee779435e7093367ad719d/serialization/testing/generic_suite.nim(198, 5) template/generic instantiation of `test` from here
/Users/runner/.nimble/pkgs2/unittest2-0.2.2-6ac7f7ea74df812a25be5d6d36a59d23789eb508/unittest2.nim(1127, 24) template/generic instantiation of `failingOnExceptions` from here
/Users/runner/.nimble/pkgs2/unittest2-0.2.2-6ac7f7ea74df812a25be5d6d36a59d23789eb508/unittest2.nim(1131, 26) template/generic instantiation of `failingOnExceptions` from here
/Users/runner/.nimble/pkgs2/serialization-0.2.2-7d65be9aa3c3249957ee779435e7093367ad719d/serialization/testing/generic_suite.nim(205, 7) template/generic instantiation of `intTests` from here
/Users/runner/.nimble/pkgs2/serialization-0.2.2-7d65be9aa3c3249957ee779435e7093367ad719d/serialization/testing/generic_suite.nim(200, 9) template/generic instantiation of `roundtrip` from here
/Users/runner/.nimble/pkgs2/serialization-0.2.2-7d65be9aa3c3249957ee779435e7093367ad719d/serialization/testing/generic_suite.nim(194, 18) Error: type mismatch
Expression: supports(Format, typeof(low(int8)))
  [1] Format: typedesc[SSZ]
  [2] typeof(low(int8)): typedesc[int8]

Expected one of (first mismatch at [position]):
[1] template supports(_: type Json; T: type): bool

[tersec]

Well, it doesn't actually work anyway, because when this error hits, that supports(Format, type(val)) is returning false, so it's never executing these checks:

diff --git a/serialization/testing/generic_suite.nim b/serialization/testing/generic_suite.nim
index 5f6df0a..3206f64 100644
--- a/serialization/testing/generic_suite.nim
+++ b/serialization/testing/generic_suite.nim
@@ -192,6 +192,7 @@ proc executeRoundtripTests*(Format: type) =
     # If this doesn't work reliably, it will fail too silently.
     # We need to report the number of checks passed.
     when supports(Format, type(val)):
+      static: doAssert false
       roundtripChecks(Format, val)
 
   suite(name(Format) & " generic roundtrip tests"):

SSZ generic roundtrip tests ....... (0.00s)
[Summary] 7 tests run (0.00s): 7 OK, 0 FAILED, 0 SKIPPED

so it silently looks like it works, but doesn't, because of this failing supports

For nim-json-serialization, it does work, in a manner of speaking:

template supports*(_: type Json, T: type): bool =
  # The JSON format should support every type
  true

But it's also a no-op of a call.

And that appears to be 100% of the codebase usage of this particular mixin. One broken, one vacuous.