Add support for for-in looping/iterating over the keys of dictionaries

[turbolent]

Description

Implementation of onflow/flips#360

We already support iterating over the elements of arrays. Add support for iterating over the keys of dictionaries.

The diff for DictionaryKeyIterator might look odd, but it now follows ArrayIterator, so the diff is viewed best side-by-side for that portion.

---

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[github-actions]

Benchstat comparison

• Base branch: onflow:master
• Base commit: 3f3470e

Results

	old.txt	new.txt
	time/op		delta
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ByteArrayValueToByteSlice-4	81.9ns ± 0%	81.2ns ± 0%	~	(p=1.000 n=1+1)
ByteSliceToByteArrayValue-4	924ns ± 0%	921ns ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ContractFunctionInvocation-4	422µs ± 0%	409µs ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
Emit-4	4.58ms ± 0%	4.55ms ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ExportType/composite_type-4	270ns ± 0%	261ns ± 0%	~	(p=1.000 n=1+1)
ExportType/simple_type-4	77.8ns ± 0%	77.7ns ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
FlowTokenContract-4	616µs ± 0%	627µs ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ImperativeFib-4	23.2µs ± 0%	24.4µs ± 0%	~	(p=1.000 n=1+1)
InterpretRecursionFib-4	2.27ms ± 0%	2.31ms ± 0%	~	(p=1.000 n=1+1)
NewInterpreter/new_interpreter-4	851ns ± 0%	856ns ± 0%	~	(p=1.000 n=1+1)
NewInterpreter/new_sub-interpreter-4	329ns ± 0%	322ns ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	1.87ns ± 0%	2.51ns ± 0%	~	(p=1.000 n=1+1)
QualifiedIdentifierCreation/Three_levels-4	85.0ns ± 0%	85.3ns ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
RuntimeFungibleTokenTransferInterpreter-4	647µs ± 0%	640µs ± 0%	~	(p=1.000 n=1+1)
RuntimeFungibleTokenTransferVM-4	703µs ± 0%	721µs ± 0%	~	(p=1.000 n=1+1)
RuntimeResourceDictionaryValues-4	2.72ms ± 0%	2.69ms ± 0%	~	(p=1.000 n=1+1)
RuntimeResourceTracking-4	11.4ms ± 0%	11.5ms ± 0%	~	(p=1.000 n=1+1)
RuntimeScriptNoop-4	15.4µs ± 0%	15.2µs ± 0%	~	(p=1.000 n=1+1)
RuntimeVMInvokeContractImperativeFib-4	40.6µs ± 0%	41.1µs ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
SuperTypeInference/arrays-4	220ns ± 0%	231ns ± 0%	~	(p=1.000 n=1+1)
SuperTypeInference/composites-4	98.1ns ± 0%	89.2ns ± 0%	~	(p=1.000 n=1+1)
SuperTypeInference/integers-4	313ns ± 0%	318ns ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ValueIsSubtypeOfSemaType-4	60.5ns ± 0%	65.9ns ± 0%	~	(p=1.000 n=1+1)
	alloc/op		delta
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ByteArrayValueToByteSlice-4	32.0B ± 0%	32.0B ± 0%	~	(all equal)
ByteSliceToByteArrayValue-4	856B ± 0%	845B ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ContractFunctionInvocation-4	153kB ± 0%	152kB ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
Emit-4	1.50MB ± 0%	1.51MB ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ExportType/composite_type-4	120B ± 0%	120B ± 0%	~	(all equal)
ExportType/simple_type-4	0.00B	0.00B	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
FlowTokenContract-4	223kB ± 0%	223kB ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ImperativeFib-4	8.30kB ± 0%	8.30kB ± 0%	~	(all equal)
InterpretRecursionFib-4	1.19MB ± 0%	1.19MB ± 0%	~	(p=1.000 n=1+1)
NewInterpreter/new_interpreter-4	976B ± 0%	976B ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-4	232B ± 0%	232B ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	0.00B	0.00B	~	(all equal)
QualifiedIdentifierCreation/Three_levels-4	64.0B ± 0%	64.0B ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
RuntimeFungibleTokenTransferInterpreter-4	171kB ± 0%	170kB ± 0%	~	(p=1.000 n=1+1)
RuntimeFungibleTokenTransferVM-4	192kB ± 0%	193kB ± 0%	~	(p=1.000 n=1+1)
RuntimeResourceDictionaryValues-4	1.77MB ± 0%	1.77MB ± 0%	~	(p=1.000 n=1+1)
RuntimeResourceTracking-4	8.59MB ± 0%	8.59MB ± 0%	~	(p=1.000 n=1+1)
RuntimeScriptNoop-4	8.06kB ± 0%	8.05kB ± 0%	~	(p=1.000 n=1+1)
RuntimeVMInvokeContractImperativeFib-4	13.3kB ± 0%	13.3kB ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
SuperTypeInference/arrays-4	72.0B ± 0%	72.0B ± 0%	~	(all equal)
SuperTypeInference/composites-4	0.00B	0.00B	~	(all equal)
SuperTypeInference/integers-4	0.00B	0.00B	~	(all equal)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ValueIsSubtypeOfSemaType-4	32.0B ± 0%	32.0B ± 0%	~	(all equal)
	allocs/op		delta
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ByteArrayValueToByteSlice-4	1.00 ± 0%	1.00 ± 0%	~	(all equal)
ByteSliceToByteArrayValue-4	5.00 ± 0%	5.00 ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ContractFunctionInvocation-4	2.47k ± 0%	2.47k ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
Emit-4	40.0k ± 0%	40.0k ± 0%	~	(p=1.000 n=1+1)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
ExportType/composite_type-4	3.00 ± 0%	3.00 ± 0%	~	(all equal)
ExportType/simple_type-4	0.00	0.00	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
FlowTokenContract-4	3.58k ± 0%	3.58k ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ImperativeFib-4	176 ± 0%	176 ± 0%	~	(all equal)
InterpretRecursionFib-4	17.7k ± 0%	17.7k ± 0%	~	(all equal)
NewInterpreter/new_interpreter-4	15.0 ± 0%	15.0 ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-4	4.00 ± 0%	4.00 ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
QualifiedIdentifierCreation/One_level-4	0.00	0.00	~	(all equal)
QualifiedIdentifierCreation/Three_levels-4	2.00 ± 0%	2.00 ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/runtime goos:linux goarch:amd64
RuntimeFungibleTokenTransferInterpreter-4	3.27k ± 0%	3.27k ± 0%	~	(all equal)
RuntimeFungibleTokenTransferVM-4	3.73k ± 0%	3.73k ± 0%	~	(all equal)
RuntimeResourceDictionaryValues-4	36.8k ± 0%	36.8k ± 0%	~	(all equal)
RuntimeResourceTracking-4	144k ± 0%	144k ± 0%	~	(all equal)
RuntimeScriptNoop-4	114 ± 0%	114 ± 0%	~	(all equal)
RuntimeVMInvokeContractImperativeFib-4	424 ± 0%	424 ± 0%	~	(all equal)
pkg:github.com/onflow/cadence/sema goos:linux goarch:amd64
SuperTypeInference/arrays-4	3.00 ± 0%	3.00 ± 0%	~	(all equal)
SuperTypeInference/composites-4	0.00	0.00	~	(all equal)
SuperTypeInference/integers-4	0.00	0.00	~	(all equal)
pkg:github.com/onflow/cadence/interpreter goos:linux goarch:amd64
ValueIsSubtypeOfSemaType-4	1.00 ± 0%	1.00 ± 0%	~	(all equal)

[SupunS]

Definitely a useful feature to have!

[SupunS]

Nice!