add benchmark used for DecoderSpec

Not a great benchmark, but record it here for posterity. Practical
testing with the AWS provider gives us a 98% speedup for simple plans.

lang/blocktoattr/fixup_bench_test.go

@@ -0,0 +1,97 @@
+package blocktoattr
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hcldec"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/hashicorp/terraform/configs/configschema"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func ambiguousNestedBlock(nesting int) *configschema.NestedBlock {
+	ret := &configschema.NestedBlock{
+		Nesting: configschema.NestingList,
+		Block: configschema.Block{
+			Attributes: map[string]*configschema.Attribute{
+				"a": {Type: cty.String, Required: true},
+				"b": {Type: cty.String, Optional: true},
+			},
+		},
+	}
+	if nesting > 0 {
+		ret.BlockTypes = map[string]*configschema.NestedBlock{
+			"nested0": ambiguousNestedBlock(nesting - 1),
+			"nested1": ambiguousNestedBlock(nesting - 1),
+			"nested2": ambiguousNestedBlock(nesting - 1),
+			"nested3": ambiguousNestedBlock(nesting - 1),
+			"nested4": ambiguousNestedBlock(nesting - 1),
+			"nested5": ambiguousNestedBlock(nesting - 1),
+			"nested6": ambiguousNestedBlock(nesting - 1),
+			"nested7": ambiguousNestedBlock(nesting - 1),
+			"nested8": ambiguousNestedBlock(nesting - 1),
+			"nested9": ambiguousNestedBlock(nesting - 1),
+		}
+	}
+	return ret
+}
+
+func schemaWithAmbiguousNestedBlock(nesting int) *configschema.Block {
+	return &configschema.Block{
+		BlockTypes: map[string]*configschema.NestedBlock{
+			"maybe_block": ambiguousNestedBlock(nesting),
+		},
+	}
+}
+
+const configForFixupBlockAttrsBenchmark = `
+maybe_block {
+  a = "hello"
+  b = "world"
+  nested0 {
+    a = "the"
+    nested1 {
+	  a = "deeper"
+      nested2 {
+        a = "we"
+        nested3 {
+          a = "go"
+          b = "inside"
+        }
+      }
+    }
+  }
+}
+`
+
+func configBodyForFixupBlockAttrsBenchmark() hcl.Body {
+	f, diags := hclsyntax.ParseConfig([]byte(configForFixupBlockAttrsBenchmark), "", hcl.Pos{Line: 1, Column: 1})
+	if diags.HasErrors() {
+		panic("test configuration is invalid")
+	}
+	return f.Body
+}
+
+func BenchmarkFixUpBlockAttrs(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		body := configBodyForFixupBlockAttrsBenchmark()
+		schema := schemaWithAmbiguousNestedBlock(5)
+		b.StartTimer()
+
+		spec := schema.DecoderSpec()
+		fixedBody := FixUpBlockAttrs(body, schema)
+		val, diags := hcldec.Decode(fixedBody, spec, nil)
+		if diags.HasErrors() {
+			b.Fatal("diagnostics during decoding", diags)
+		}
+		if !val.Type().IsObjectType() {
+			b.Fatal("result is not an object")
+		}
+		blockVal := val.GetAttr("maybe_block")
+		if !blockVal.Type().IsListType() || blockVal.LengthInt() != 1 {
+			b.Fatal("result has wrong value for 'maybe_block'")
+		}
+	}
+}