don't plan data source reads during refresh-only

When executing a refresh-only plan, it is not valid to plan a data
source read. If the data source config is not known during planning, the
only valid update would be the prior state, if there is any.

internal/terraform/context_refresh_test.go

@@ -201,12 +201,12 @@ func TestContext2Refresh_dataComputedModuleVar(t *testing.T) {
 		},
 	})
 
-	s, diags := ctx.Refresh(m, states.NewState(), &PlanOpts{Mode: plans.NormalMode})
+	plan, diags := ctx.Plan(m, states.NewState(), &PlanOpts{Mode: plans.RefreshOnlyMode})
 	if diags.HasErrors() {
 		t.Fatalf("refresh errors: %s", diags.Err())
 	}
 
-	checkStateString(t, s, `
+	checkStateString(t, plan.PriorState, `
 <no state>
 `)
 }

internal/terraform/node_resource_abstract_instance.go

@@ -1555,7 +1555,7 @@ func (n *NodeAbstractResourceInstance) providerMetas(ctx EvalContext) (cty.Value
 // value, but it still matches the previous state, then we can record a NoNop
 // change. If the states don't match then we record a Read change so that the
 // new value is applied to the state.
-func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRuleSeverity tfdiags.Severity) (*plans.ResourceInstanceChange, *states.ResourceInstanceObject, instances.RepetitionData, tfdiags.Diagnostics) {
+func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRuleSeverity tfdiags.Severity, skipPlanChanges bool) (*plans.ResourceInstanceChange, *states.ResourceInstanceObject, instances.RepetitionData, tfdiags.Diagnostics) {
 	var diags tfdiags.Diagnostics
 	var keyData instances.RepetitionData
 	var configVal cty.Value
@@ -1609,6 +1609,17 @@ func (n *NodeAbstractResourceInstance) planDataSource(ctx EvalContext, checkRule
 	// producing a "Read" change for this resource, and a placeholder value for
 	// it in the state.
 	if depsPending || !configKnown {
+		// We can't plan any changes if we're only refreshing, so the only
+		// value we can set here is whatever was in state previously.
+		if skipPlanChanges {
+			plannedNewState := &states.ResourceInstanceObject{
+				Value:  priorVal,
+				Status: states.ObjectReady,
+			}
+
+			return nil, plannedNewState, keyData, diags
+		}
+
 		var reason plans.ResourceInstanceChangeActionReason
 		switch {
 		case !configKnown:

internal/terraform/node_resource_plan_instance.go

@@ -87,7 +87,7 @@ func (n *NodePlannableResourceInstance) dataResourceExecute(ctx EvalContext) (di
 		checkRuleSeverity = tfdiags.Warning
 	}
 
-	change, state, repeatData, planDiags := n.planDataSource(ctx, checkRuleSeverity)
+	change, state, repeatData, planDiags := n.planDataSource(ctx, checkRuleSeverity, n.skipPlanChanges)
 	diags = diags.Append(planDiags)
 	if diags.HasErrors() {
 		return diags