Add variables to all commands that load configuration

.changes/v1.15/ENHANCEMENTS-20260313-162537.yaml

@@ -0,0 +1,5 @@
+kind: ENHANCEMENTS
+body: As part of supporting variables in module sources, most commands now accept variable values
+time: 2026-03-13T16:25:37.792809+01:00
+custom:
+    Issue: "38276"

internal/backend/backendrun/const_variables.go

@@ -0,0 +1,23 @@
+// Copyright IBM Corp. 2014, 2026
+// SPDX-License-Identifier: BUSL-1.1
+
+package backendrun
+
+import (
+	"context"
+
+	"github.com/hashicorp/terraform/internal/command/arguments"
+	"github.com/hashicorp/terraform/internal/tfdiags"
+)
+
+// ConstVariableSupplier is an optional interface that backends can implement
+// to supply variable values from their remote storage. This is used to fetch
+// const variable values that are needed during early configuration loading
+// (e.g., for module source resolution), before a full operation is started.
+type ConstVariableSupplier interface {
+	// FetchVariables retrieves Terraform variable values stored in the
+	// backend for the given workspace. Only variables that are relevant to
+	// Terraform (as opposed to environment variables or other categories)
+	// should be returned.
+	FetchVariables(ctx context.Context, workspace string) (map[string]arguments.UnparsedVariableValue, tfdiags.Diagnostics)
+}

internal/backend/backendrun/unparsed_value.go

@@ -146,13 +146,20 @@ func isDefinedAny(name string, maps ...terraform.InputValues) bool {
 // InputValues may be incomplete but will include the subset of variables
 // that were successfully processed, allowing for careful analysis of the
 // partial result.
-//
-// constOnly will only raise a diagnostic error if a required variable is
-// missing and is marked as const. Since configuration loading will always
-// require values for constant variables, this allows us to use this
-// function in both configuration loading and plan/apply contexts where all
-// variables are required.
-func ParseVariableValues(vv map[string]arguments.UnparsedVariableValue, decls map[string]*configs.Variable, constOnly bool) (terraform.InputValues, tfdiags.Diagnostics) {
+func ParseVariableValues(vv map[string]arguments.UnparsedVariableValue, decls map[string]*configs.Variable) (terraform.InputValues, tfdiags.Diagnostics) {
+	return parseVariableValues(vv, decls, false)
+}
+
+// ParseConstVariableValues is like ParseVariableValues but only produces
+// errors for missing const variables. Non-const required variables that are
+// missing will still receive placeholder values but won't produce errors.
+// This is used during early configuration loading (e.g. module installation)
+// where only const variables are needed for module source resolution.
+func ParseConstVariableValues(vv map[string]arguments.UnparsedVariableValue, decls map[string]*configs.Variable) (terraform.InputValues, tfdiags.Diagnostics) {
+	return parseVariableValues(vv, decls, true)
+}
+
+func parseVariableValues(vv map[string]arguments.UnparsedVariableValue, decls map[string]*configs.Variable, constOnly bool) (terraform.InputValues, tfdiags.Diagnostics) {
 	ret, diags := ParseDeclaredVariableValues(vv, decls)
 	undeclared, diagsUndeclared := ParseUndeclaredVariableValues(vv, decls)
 
@@ -212,3 +219,18 @@ func ParseVariableValues(vv map[string]arguments.UnparsedVariableValue, decls ma
 
 	return ret, diags
 }
+
+// HasUnsatisfiedConstVariables checks whether any const variables declared in
+// the given module are required but not yet present in the provided variable
+// values map. This is used to determine whether we need to fetch additional
+// variable values from a backend before loading the full configuration.
+func HasUnsatisfiedConstVariables(vv map[string]arguments.UnparsedVariableValue, decls map[string]*configs.Variable) bool {
+	for name, vc := range decls {
+		if vc.Const && vc.Required() {
+			if _, defined := vv[name]; !defined {
+				return true
+			}
+		}
+	}
+	return false
+}

internal/backend/backendrun/unparsed_value_test.go

@@ -162,7 +162,7 @@ func TestUnparsedValue(t *testing.T) {
 	})
 
 	t.Run("ParseVariableValues", func(t *testing.T) {
-		gotVals, diags := ParseVariableValues(vv, decls, false)
+		gotVals, diags := ParseVariableValues(vv, decls)
 		for _, diag := range diags {
 			t.Logf("%s: %s", diag.Description().Summary, diag.Description().Detail)
 		}
@@ -278,7 +278,7 @@ func TestUnparsedValue(t *testing.T) {
 			},
 		}
 
-		gotVals, diags := ParseVariableValues(vv, decls, true)
+		gotVals, diags := ParseConstVariableValues(vv, decls)
 
 		if got, want := len(diags), 1; got != want {
 			t.Fatalf("wrong number of diagnostics %d; want %d", got, want)
@@ -339,6 +339,80 @@ func TestUnparsedValue(t *testing.T) {
 	})
 }
 
+func TestHasUnsatisfiedConstVariables(t *testing.T) {
+	testCases := map[string]struct {
+		vv    map[string]arguments.UnparsedVariableValue
+		decls map[string]*configs.Variable
+		want  bool
+	}{
+		"no variables": {
+			vv:    nil,
+			decls: map[string]*configs.Variable{},
+			want:  false,
+		},
+		"no const variables": {
+			vv: nil,
+			decls: map[string]*configs.Variable{
+				"regular": {
+					Name: "regular",
+				},
+			},
+			want: false,
+		},
+		"const with default": {
+			vv: nil,
+			decls: map[string]*configs.Variable{
+				"has_default": {
+					Name:    "has_default",
+					Const:   true,
+					Default: cty.StringVal("default"),
+				},
+			},
+			want: false,
+		},
+		"const required and missing": {
+			vv: nil,
+			decls: map[string]*configs.Variable{
+				"required_const": {
+					Name:  "required_const",
+					Const: true,
+				},
+			},
+			want: true,
+		},
+		"const required but provided": {
+			vv: map[string]arguments.UnparsedVariableValue{
+				"required_const": testUnparsedVariableValue("value"),
+			},
+			decls: map[string]*configs.Variable{
+				"required_const": {
+					Name:  "required_const",
+					Const: true,
+				},
+			},
+			want: false,
+		},
+		"non-const required and missing": {
+			vv: nil,
+			decls: map[string]*configs.Variable{
+				"regular_required": {
+					Name: "regular_required",
+				},
+			},
+			want: false,
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			got := HasUnsatisfiedConstVariables(tc.vv, tc.decls)
+			if got != tc.want {
+				t.Errorf("got %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
 type testUnparsedVariableValue string
 
 func (v testUnparsedVariableValue) ParseVariableValue(mode configs.VariableParsingMode) (*terraform.InputValue, tfdiags.Diagnostics) {

internal/backend/local/backend_apply.go

@@ -266,7 +266,7 @@ func (b *Local) opApply(
 		// same parsing logic from the plan to generate the diagnostics.
 		undeclaredVariables := map[string]arguments.UnparsedVariableValue{}
 
-		parsedVars, _ := backendrun.ParseVariableValues(op.Variables, lr.Config.Module.Variables, false)
+		parsedVars, _ := backendrun.ParseVariableValues(op.Variables, lr.Config.Module.Variables)
 
 		for varName := range op.Variables {
 			parsedVar, parsed := parsedVars[varName]

internal/backend/local/backend_local.go

@@ -167,7 +167,7 @@ func (b *Local) localRunDirect(op *backendrun.Operation, run *backendrun.LocalRu
 		rawVariables = b.interactiveCollectVariables(context.TODO(), op.Variables, rootMod.Variables, op.UIIn)
 	}
 
-	variables, varDiags := backendrun.ParseVariableValues(rawVariables, rootMod.Variables, false)
+	variables, varDiags := backendrun.ParseVariableValues(rawVariables, rootMod.Variables)
 	diags = diags.Append(varDiags)
 	if diags.HasErrors() {
 		return nil, nil, diags
@@ -271,7 +271,7 @@ func (b *Local) localRunForPlanFile(op *backendrun.Operation, pf *planfile.Reade
 		return nil, nil, diags
 	}
 
-	variables, varDiags := backendrun.ParseVariableValues(op.Variables, rootMod.Variables, false)
+	variables, varDiags := backendrun.ParseVariableValues(op.Variables, rootMod.Variables)
 	diags = diags.Append(varDiags)
 	if diags.HasErrors() {
 		return nil, nil, diags
@@ -496,8 +496,8 @@ func (b *Local) interactiveCollectVariables(ctx context.Context, existing map[st
 func (b *Local) stubUnsetRequiredVariables(existing map[string]arguments.UnparsedVariableValue, vcs map[string]*configs.Variable) map[string]arguments.UnparsedVariableValue {
 	var missing bool // Do we need to add anything?
 	for name, vc := range vcs {
-		if !vc.Required() {
-			continue // We only stub required variables
+		if !vc.Required() || vc.Const {
+			continue // We only stub non-const required variables
 		}
 		if _, exists := existing[name]; !exists {
 			missing = true
@@ -512,7 +512,7 @@ func (b *Local) stubUnsetRequiredVariables(existing map[string]arguments.Unparse
 	maps.Copy(ret, existing) // don't use clone here, so we can return a non-nil map
 
 	for name, vc := range vcs {
-		if !vc.Required() {
+		if !vc.Required() || vc.Const {
 			continue
 		}
 		if _, exists := existing[name]; !exists {

internal/backend/remote/backend_common.go

@@ -259,7 +259,7 @@ func (b *Remote) hasExplicitVariableValues(op *backendrun.Operation) bool {
 	// goal here is just to make a best effort count of how many variable
 	// values are coming from -var or -var-file CLI arguments so that we can
 	// hint the user that those are not supported for remote operations.
-	variables, _ := backendrun.ParseVariableValues(op.Variables, config.Variables, false)
+	variables, _ := backendrun.ParseVariableValues(op.Variables, config.Variables)
 
 	// Check for explicitly-defined (-var and -var-file) variables, which the
 	// remote backend does not support. All other source types are okay,

internal/backend/remote/backend_context.go

@@ -135,7 +135,7 @@ func (b *Remote) LocalRun(op *backendrun.Operation) (*backendrun.LocalRun, state
 		}
 
 		if op.Variables != nil {
-			variables, varDiags := backendrun.ParseVariableValues(op.Variables, rootMod.Variables, false)
+			variables, varDiags := backendrun.ParseVariableValues(op.Variables, rootMod.Variables)
 			diags = diags.Append(varDiags)
 			if diags.HasErrors() {
 				return nil, nil, diags

internal/cloud/backend.go

@@ -123,6 +123,7 @@ type Cloud struct {
 var _ backend.Backend = (*Cloud)(nil)
 var _ backendrun.OperationsBackend = (*Cloud)(nil)
 var _ backendrun.Local = (*Cloud)(nil)
+var _ backendrun.ConstVariableSupplier = (*Cloud)(nil)
 
 // New creates a new initialized cloud backend.
 func New(services *disco.Disco) *Cloud {