allow interpolation in import block id

The import block id field can now reference variables, attributes, and module outputs, as long as the result is a known non-empty string at plan time. A null or unknown value will result in an error.

This commit slightly modifies the legacy CLI terraform import code path to construct a synthetic hcl.Expression from the import id passed in from the command line, with no intended change of functionality.

internal/command/import.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/zclconf/go-cty/cty"
 
 	"github.com/hashicorp/terraform/internal/addrs"
 	"github.com/hashicorp/terraform/internal/backend"
@@ -236,7 +237,10 @@ func (c *ImportCommand) Run(args []string) int {
 		Targets: []*terraform.ImportTarget{
 			{
 				Addr: addr,
-				ID:   args[1],
+
+				// In the import block, the ID can be an arbitrary hcl.Expression,
+				// but here it's always interpreted as a literal string.
+				ID: hcl.StaticExpr(cty.StringVal(args[1]), configs.SynthBody("import", nil).MissingItemRange()),
 			},
 		},
 

internal/configs/import.go

@@ -5,12 +5,11 @@ package configs
 
 import (
 	"github.com/hashicorp/hcl/v2"
-	"github.com/hashicorp/hcl/v2/gohcl"
 	"github.com/hashicorp/terraform/internal/addrs"
 )
 
 type Import struct {
-	ID string
+	ID hcl.Expression
 	To addrs.AbsResourceInstance
 
 	ProviderConfigRef *ProviderConfigRef
@@ -30,9 +29,7 @@ func decodeImportBlock(block *hcl.Block) (*Import, hcl.Diagnostics) {
 	diags = append(diags, moreDiags...)
 
 	if attr, exists := content.Attributes["id"]; exists {
-		attrDiags := gohcl.DecodeExpression(attr.Expr, nil, &imp.ID)
-		diags = append(diags, attrDiags...)
-
+		imp.ID = attr.Expr
 	}
 
 	if attr, exists := content.Attributes["to"]; exists {

internal/configs/import_test.go

@@ -14,6 +14,11 @@ import (
 	"github.com/zclconf/go-cty/cty"
 )
 
+var (
+	typeComparer  = cmp.Comparer(cty.Type.Equals)
+	valueComparer = cmp.Comparer(cty.Value.RawEquals)
+)
+
 func TestImportBlock_decode(t *testing.T) {
 	blockRange := hcl.Range{
 		Filename: "mock.tf",
@@ -52,7 +57,7 @@ func TestImportBlock_decode(t *testing.T) {
 			},
 			&Import{
 				To:        mustAbsResourceInstanceAddr("test_instance.bar"),
-				ID:        "foo",
+				ID:        foo_str_expr,
 				DeclRange: blockRange,
 			},
 			``,
@@ -76,7 +81,7 @@ func TestImportBlock_decode(t *testing.T) {
 			},
 			&Import{
 				To:        mustAbsResourceInstanceAddr("test_instance.bar[\"one\"]"),
-				ID:        "foo",
+				ID:        foo_str_expr,
 				DeclRange: blockRange,
 			},
 			``,
@@ -100,7 +105,7 @@ func TestImportBlock_decode(t *testing.T) {
 			},
 			&Import{
 				To:        mustAbsResourceInstanceAddr("module.bar.test_instance.bar"),
-				ID:        "foo",
+				ID:        foo_str_expr,
 				DeclRange: blockRange,
 			},
 			``,
@@ -138,7 +143,7 @@ func TestImportBlock_decode(t *testing.T) {
 				DefRange: blockRange,
 			},
 			&Import{
-				ID:        "foo",
+				ID:        foo_str_expr,
 				DeclRange: blockRange,
 			},
 			"Missing required argument",
@@ -160,7 +165,7 @@ func TestImportBlock_decode(t *testing.T) {
 				t.Fatal("expected error")
 			}
 
-			if !cmp.Equal(got, test.want, cmp.AllowUnexported(addrs.MoveEndpoint{})) {
+			if !cmp.Equal(got, test.want, typeComparer, valueComparer) {
 				t.Fatalf("wrong result: %s", cmp.Diff(got, test.want))
 			}
 		})

internal/terraform/context_import.go

@@ -6,6 +6,7 @@ package terraform
 import (
 	"log"
 
+	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/terraform/internal/addrs"
 	"github.com/hashicorp/terraform/internal/configs"
 	"github.com/hashicorp/terraform/internal/states"
@@ -22,7 +23,8 @@ type ImportOpts struct {
 	SetVariables InputValues
 }
 
-// ImportTarget is a single resource to import.
+// ImportTarget is a single resource to import,
+// in legacy (CLI) import mode.
 type ImportTarget struct {
 	// Config is the original import block for this import. This might be null
 	// if the import did not originate in config.
@@ -33,7 +35,7 @@ type ImportTarget struct {
 	Addr addrs.AbsResourceInstance
 
 	// ID is the ID of the resource to import. This is resource-specific.
-	ID string
+	ID hcl.Expression
 }
 
 // Import takes already-created external resources and brings them