Experimental -allow-deferral flag for enabling deferred actions on CLI

Now that deferred actions is a top-level per-plan option, here's a lil
command-line option for turning it on!

- It's called `-allow-deferral`.
- It is ONLY legal to use if this is an experimental build, i.e. compiled with
  `go build -ldflags "-w -s -X 'main.experimentsAllowed=yes'"` or the like.

Implementation notes: The design constraint here was, "please avoid having to
change the build infrastructure or alter the function of every args parsing
function". So, since `extendedFlagSet()` doesn't have access to the var that
says we're in an experimental build, I had to move the validation for that
particular flag to later in the process, when we're building the operation
request.

internal/backend/backendrun/operation.go

@@ -124,6 +124,13 @@ type Operation struct {
 	// the variables set in the plan are used instead, and they must be valid.
 	AllowUnsetVariables bool
 
+	// DeferralAllowed enables experimental support for automatically performing
+	// a partial plan if some objects are not yet plannable.
+	//
+	// IMPORTANT: When configuring an Operation, you should only set a value for
+	// this field if Terraform was built with experimental features enabled.
+	DeferralAllowed bool
+
 	// View implements the logic for all UI interactions.
 	View views.Operation
 

internal/backend/local/backend_local.go

@@ -204,6 +204,7 @@ func (b *Local) localRunDirect(op *backendrun.Operation, run *backendrun.LocalRu
 		SetVariables:       variables,
 		SkipRefresh:        op.Type != backendrun.OperationTypeRefresh && !op.PlanRefresh,
 		GenerateConfigPath: op.GenerateConfigOut,
+		DeferralAllowed:    op.DeferralAllowed,
 	}
 	run.PlanOpts = planOpts
 

internal/command/apply.go

@@ -280,6 +280,20 @@ func (c *ApplyCommand) OperationRequest(
 	opReq.Type = backendrun.OperationTypeApply
 	opReq.View = view.Operation()
 
+	// EXPERIMENTAL: maybe enable deferred actions
+	if c.AllowExperimentalFeatures {
+		opReq.DeferralAllowed = args.DeferralAllowed
+	} else if args.DeferralAllowed {
+		// Belated flag parse error, since we don't know about experiments
+		// support at actual parse time.
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Error,
+			"Failed to parse command-line flags",
+			"The -allow-deferral flag is only valid in experimental builds of Terraform.",
+		))
+		return nil, diags
+	}
+
 	var err error
 	opReq.ConfigLoader, err = c.initConfigLoader()
 	if err != nil {

internal/command/arguments/extended.go

@@ -78,6 +78,18 @@ type Operation struct {
 	// learn a use-case for broader matching.
 	ForceReplace []addrs.AbsResourceInstance
 
+	// DeferralAllowed enables experimental support for automatically performing
+	// a partial plan if some objects are not yet plannable (due to unknown
+	// values in count/for_each, or due to other missing dependencies that can't
+	// be resolved in a single plan/apply cycle).
+	//
+	// IMPORTANT: This feature should only be available when Terraform is built
+	// with experimental features enabled. Since extendedFlagSet can't currently
+	// test whether experimental features are enabled, the check needs to happen
+	// when _reading_ these Operation arguments and transferring values to the
+	// backendrun.Operation struct.
+	DeferralAllowed bool
+
 	// These private fields are used only temporarily during decoding. Use
 	// method Parse to populate the exported fields from these, validating
 	// the raw values in the process.
@@ -223,6 +235,7 @@ func extendedFlagSet(name string, state *State, operation *Operation, vars *Vars
 
 	if operation != nil {
 		f.IntVar(&operation.Parallelism, "parallelism", DefaultParallelism, "parallelism")
+		f.BoolVar(&operation.DeferralAllowed, "allow-deferral", false, "allow-deferral")
 		f.BoolVar(&operation.Refresh, "refresh", true, "refresh")
 		f.BoolVar(&operation.destroyRaw, "destroy", false, "destroy")
 		f.BoolVar(&operation.refreshOnlyRaw, "refresh-only", false, "refresh-only")

internal/command/plan.go

@@ -165,6 +165,20 @@ func (c *PlanCommand) OperationRequest(
 	opReq.Type = backendrun.OperationTypePlan
 	opReq.View = view.Operation()
 
+	// EXPERIMENTAL: maybe enable deferred actions
+	if c.AllowExperimentalFeatures {
+		opReq.DeferralAllowed = args.DeferralAllowed
+	} else if args.DeferralAllowed {
+		// Belated flag parse error, since we don't know about experiments
+		// support at actual parse time.
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Error,
+			"Failed to parse command-line flags",
+			"The -allow-deferral flag is only valid in experimental builds of Terraform.",
+		))
+		return nil, diags
+	}
+
 	var err error
 	opReq.ConfigLoader, err = c.initConfigLoader()
 	if err != nil {

internal/command/refresh.go

@@ -147,6 +147,20 @@ func (c *RefreshCommand) OperationRequest(be backendrun.OperationsBackend, view
 	opReq.Type = backendrun.OperationTypeRefresh
 	opReq.View = view.Operation()
 
+	// EXPERIMENTAL: maybe enable deferred actions
+	if c.AllowExperimentalFeatures {
+		opReq.DeferralAllowed = args.DeferralAllowed
+	} else if args.DeferralAllowed {
+		// Belated flag parse error, since we don't know about experiments
+		// support at actual parse time.
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Error,
+			"Failed to parse command-line flags",
+			"The -allow-deferral flag is only valid in experimental builds of Terraform.",
+		))
+		return nil, diags
+	}
+
 	var err error
 	opReq.ConfigLoader, err = c.initConfigLoader()
 	if err != nil {