core: Detect and reject self-referencing local values

We already catch indirect cycles through the normal cycle detector, but
we never create self-edges in the graph so we need to handle a direct
self-reference separately here.

The prior behavior was simply to produce an incorrect result (since the
local value wasn't assigned a new value yet).

This fixes #18503.

terraform/eval_local.go

@@ -4,8 +4,11 @@ import (
 	"fmt"
 
 	"github.com/hashicorp/hcl2/hcl"
-	"github.com/hashicorp/terraform/addrs"
 	"github.com/zclconf/go-cty/cty"
+
+	"github.com/hashicorp/terraform/addrs"
+	"github.com/hashicorp/terraform/lang"
+	"github.com/hashicorp/terraform/tfdiags"
 )
 
 // EvalLocal is an EvalNode implementation that evaluates the
@@ -17,11 +20,32 @@ type EvalLocal struct {
 }
 
 func (n *EvalLocal) Eval(ctx EvalContext) (interface{}, error) {
-	val, diags := ctx.EvaluateExpr(n.Expr, cty.DynamicPseudoType, nil)
+	var diags tfdiags.Diagnostics
+
+	// We ignore diags here because any problems we might find will be found
+	// again in EvaluateExpr below.
+	refs, _ := lang.ReferencesInExpr(n.Expr)
+	for _, ref := range refs {
+		if ref.Subject == n.Addr {
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Self-referencing local value",
+				Detail:   fmt.Sprintf("Local value %s cannot use its own result as part of its expression.", n.Addr),
+				Subject:  ref.SourceRange.ToHCL().Ptr(),
+				Context:  n.Expr.Range().Ptr(),
+			})
+		}
+	}
 	if diags.HasErrors() {
 		return nil, diags.Err()
 	}
 
+	val, moreDiags := ctx.EvaluateExpr(n.Expr, cty.DynamicPseudoType, nil)
+	diags = diags.Append(moreDiags)
+	if moreDiags.HasErrors() {
+		return nil, diags.Err()
+	}
+
 	state := ctx.State()
 	if state == nil {
 		return nil, fmt.Errorf("cannot write local value to nil state")

terraform/eval_local_test.go

@@ -34,6 +34,11 @@ func TestEvalLocal(t *testing.T) {
 			"",
 			false,
 		},
+		{
+			"Hello, ${local.foo}",
+			nil,
+			true, // self-referencing
+		},
 	}
 
 	for _, test := range tests {
@@ -64,8 +69,9 @@ func TestEvalLocal(t *testing.T) {
 
 			ms := ctx.StateState.Module(addrs.RootModuleInstance)
 			gotLocals := ms.LocalValues
-			wantLocals := map[string]cty.Value{
-				"foo": hcl2shim.HCL2ValueFromConfigValue(test.Want),
+			wantLocals := map[string]cty.Value{}
+			if test.Want != nil {
+				wantLocals["foo"] = hcl2shim.HCL2ValueFromConfigValue(test.Want)
 			}
 
 			if !reflect.DeepEqual(gotLocals, wantLocals) {