testing: compare Rule types and ignore struct fields

[bendrucker]

As observed in #738, AssertIssues was not comparing the rule in an issue. Purely removing IgnoreFields is not an option because it causes panics due to unexported fields:

Panic output

--- FAIL: Test_TerraformModulePinnedSource (0.00s)
panic: cannot handle unexported field at {tflint.Issues}[0].Rule.(*terraformrules.TerraformModulePinnedSourceRule).attributeName:
	"github.com/terraform-linters/tflint/rules/terraformrules".TerraformModulePinnedSourceRule
consider using a custom Comparer; if you control the implementation of type, you can also consider using an Exporter, AllowUnexported, or cmpopts.IgnoreUnexported [recovered]
	panic: cannot handle unexported field at {tflint.Issues}[0].Rule.(*terraformrules.TerraformModulePinnedSourceRule).attributeName:
	"github.com/terraform-linters/tflint/rules/terraformrules".TerraformModulePinnedSourceRule
consider using a custom Comparer; if you control the implementation of type, you can also consider using an Exporter, AllowUnexported, or cmpopts.IgnoreUnexported

goroutine 14 [running]:
testing.tRunner.func1.1(0x2029340, 0xc0001bbfd0)
	/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:941 +0x3d0
testing.tRunner.func1(0xc0002237a0)
	/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:944 +0x3f9
panic(0x2029340, 0xc0001bbfd0)
	/usr/local/Cellar/go/1.14/libexec/src/runtime/panic.go:967 +0x15d
github.com/google/go-cmp/cmp.validator.apply(0xc000240780, 0x2029340, 0xc0001bbca0, 0x1b8, 0x2029340, 0xc0001bbdc0, 0x1b8)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/options.go:241 +0x5ac
github.com/google/go-cmp/cmp.(*state).tryOptions(0xc000240780, 0x2800380, 0x2029340, 0x2029340, 0xc0001bbca0, 0x1b8, 0x2029340, 0xc0001bbdc0, 0x1b8, 0x4)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:279 +0x132
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27dea00, 0xc0000a0f00)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:234 +0x2d9
github.com/google/go-cmp/cmp.(*state).compareStruct(0xc000240780, 0x2800380, 0x21f7c00, 0x21f7c00, 0xc0001bbca0, 0x199, 0x21f7c00, 0xc0001bbdc0, 0x199)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:400 +0x61c
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27de940, 0xc000217640)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:262 +0xdb5
github.com/google/go-cmp/cmp.(*state).comparePtr(0xc000240780, 0x2800380, 0x235f1c0, 0x235f1c0, 0xc0001bbca0, 0x16, 0x235f1c0, 0xc0001bbdc0, 0x16)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:552 +0x31d
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27dea80, 0xc000217600)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:268 +0xc52
github.com/google/go-cmp/cmp.(*state).compareInterface(0xc000240780, 0x2800380, 0x21f7d00, 0x21f7d00, 0xc00023e800, 0x194, 0x21f7d00, 0xc00023f180, 0x194)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:565 +0x2a1
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27dea00, 0xc0000a0e00)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:270 +0xb62
github.com/google/go-cmp/cmp.(*state).compareStruct(0xc000240780, 0x2800380, 0x22e56a0, 0x22e56a0, 0xc00023e800, 0x199, 0x22e56a0, 0xc00023f180, 0x199)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:400 +0x61c
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27de940, 0xc0002172c0)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:262 +0xdb5
github.com/google/go-cmp/cmp.(*state).comparePtr(0xc000240780, 0x2800380, 0x1fdaec0, 0x1fdaec0, 0xc0002102b8, 0x196, 0x1fdaec0, 0xc0002104b0, 0x196)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:552 +0x31d
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27de9c0, 0xc00008f380)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:268 +0xc52
github.com/google/go-cmp/cmp.(*state).statelessCompare(0xc000240780, 0x27de9c0, 0xc00008f380, 0x2027a00, 0x1)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:205 +0x97
github.com/google/go-cmp/cmp.(*state).compareSlice.func2(0x0, 0x0, 0x0, 0x340cc78)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:466 +0x85
github.com/google/go-cmp/cmp/internal/diff.Difference(0x1, 0x1, 0xc0001e4ec0, 0x1, 0x1, 0xc0004c7898)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/internal/diff/diff.go:221 +0x223
github.com/google/go-cmp/cmp.(*state).compareSlice(0xc000240780, 0x2800380, 0x2162fe0, 0x2162fe0, 0xc000470a60, 0x97, 0x2162fe0, 0xc000470a80, 0x97)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:465 +0x730
github.com/google/go-cmp/cmp.(*state).compareAny(0xc000240780, 0x27dd580, 0xc000217280)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:264 +0x8e9
github.com/google/go-cmp/cmp.Equal(0x2162fe0, 0xc000470a60, 0x2162fe0, 0xc000470a80, 0xc000470a40, 0x2, 0x2, 0x340c980)
	/Users/ben/src/go/pkg/mod/github.com/google/go-cmp@v0.4.0/cmp/compare.go:112 +0x315
github.com/terraform-linters/tflint/tflint.AssertIssues(0xc0002237a0, 0xc0002102b8, 0x1, 0x1, 0xc0002104b0, 0x1, 0x1)
	/Users/ben/src/terraform-linters/tflint/tflint/testing.go:55 +0x26d
github.com/terraform-linters/tflint/rules/terraformrules.Test_TerraformModulePinnedSource(0xc0002237a0)
	/Users/ben/src/terraform-linters/tflint/rules/terraformrules/terraform_module_pinned_source_test.go:514 +0x1675
testing.tRunner(0xc0002237a0, 0x24983a8)
	/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:992 +0xdc
created by testing.(*T).Run
	/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:1043 +0x357
FAIL	github.com/terraform-linters/tflint/rules/terraformrules	0.381s

IgnoreUnexported doesn't work because every rule is its own struct and we have to pass struct values. Not to mention it doesn't fit the intent here.

In usage, AssertIssues is just being used with matching rule types and not with equal values. This implements a custom comparer that performs that check, considering two Rule interfaces equal if they both have the same concrete type.

[wata727]

👍

Thank you!