Support count and count.index completion in blocks

This provides completion hints inside blocks for `count.index` references within resource, data and module blocks anywhere the `count` meta-argument is supported. It detects if count is used already and does not suggest duplicates.

context/context.go

@@ -0,0 +1,28 @@
+package context
+
+import (
+	"context"
+
+	"github.com/hashicorp/hcl-lang/schema"
+)
+
+type bodyExtCtxKey struct{}
+
+type bodyActiveCountCtxKey struct{}
+
+func WithExtensions(ctx context.Context, ext *schema.BodyExtensions) context.Context {
+	return context.WithValue(ctx, bodyExtCtxKey{}, ext)
+}
+
+func WithActiveCount(ctx context.Context) context.Context {
+	return context.WithValue(ctx, bodyActiveCountCtxKey{}, true)
+}
+
+func ExtensionsFromContext(ctx context.Context) (*schema.BodyExtensions, bool) {
+	ext, ok := ctx.Value(bodyExtCtxKey{}).(*schema.BodyExtensions)
+	return ext, ok
+}
+
+func ActiveCountFromContext(ctx context.Context) bool {
+	return ctx.Value(bodyActiveCountCtxKey{}) != nil
+}

decoder/body_candidates.go

@@ -17,6 +17,17 @@ func (d *PathDecoder) bodySchemaCandidates(body *hclsyntax.Body, schema *schema.
 	candidates := lang.NewCandidates()
 	count := 0
 
+	if schema.Extensions != nil {
+		// check if this schema supports Count attribute
+		if schema.Extensions.Count {
+			// check if Count is already used inside this body, so we don't
+			// suggest a duplicate
+			if _, ok := body.Attributes["count"]; !ok {
+				candidates.List = append(candidates.List, countAttributeCandidate(editRng))
+			}
+		}
+	}
+
 	if len(schema.Attributes) > 0 {
 		attrNames := sortedAttributeNames(schema.Attributes)
 		for _, name := range attrNames {
@@ -135,3 +146,17 @@ func isBlockDeclarable(body *hclsyntax.Body, blockType string, bSchema *schema.B
 	}
 	return true
 }
+
+func countAttributeCandidate(editRng hcl.Range) lang.Candidate {
+	return lang.Candidate{
+		Label:       "count",
+		Detail:      "optional, number",
+		Description: lang.PlainText("The distinct index number (starting with 0) corresponding to the instance"),
+		Kind:        lang.AttributeCandidateKind,
+		TextEdit: lang.TextEdit{
+			NewText: "count",
+			Snippet: "count = ${1:1}",
+			Range:   editRng,
+		},
+	}
+}