Add additional validation that the backend used in a run is a supported type

internal/command/test.go

@@ -130,21 +130,37 @@ func (c *TestCommand) Run(rawArgs []string) int {
 		return 1
 	}
 
-	// Per file, ensure backends aren't reused
-	var duplicateBackendDiags tfdiags.Diagnostics
+	// Per file, ensure backends:
+	// * aren't reused
+	// * are valid types
+	var backendDiags tfdiags.Diagnostics
 	for _, tf := range config.Module.Tests {
 		bucketHashes := make(map[int]string)
 		// Use an ordered list of backends, so that errors are raised by 2nd+ time
 		// that a backend config is used in a file.
 		for _, bc := range orderBackendsByDeclarationLine(tf.BackendConfigs) {
 			f := backendInit.Backend(bc.Backend.Type)
+			if f == nil {
+				detail := fmt.Sprintf("There is no backend type named %q.", bc.Backend.Type)
+				if msg, removed := backendInit.RemovedBackends[bc.Backend.Type]; removed {
+					detail = msg
+				}
+				backendDiags = backendDiags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Unsupported backend type",
+					Detail:   detail,
+					Subject:  &bc.Backend.TypeRange,
+				})
+				continue
+			}
+
 			b := f()
 			schema := b.ConfigSchema()
 			hash := bc.Backend.Hash(schema)
 
 			if runName, exists := bucketHashes[hash]; exists {
 				// This backend's been encountered before
-				duplicateBackendDiags = duplicateBackendDiags.Append(
+				backendDiags = backendDiags.Append(
 					&hcl.Diagnostic{
 						Severity: hcl.DiagError,
 						Summary:  "Repeat use of the same backend block",
@@ -157,8 +173,8 @@ func (c *TestCommand) Run(rawArgs []string) int {
 			bucketHashes[bc.Backend.Hash(schema)] = bc.Run.Name
 		}
 	}
-	diags = diags.Append(duplicateBackendDiags)
-	if duplicateBackendDiags.HasErrors() {
+	diags = diags.Append(backendDiags)
+	if backendDiags.HasErrors() {
 		view.Diagnostics(nil, nil, diags)
 		return 1
 	}

internal/command/test_test.go

@@ -2564,12 +2564,13 @@ Failure! 0 passed, 1 failed.
 	}
 }
 
-// TestTest_ReusedBackendConfiguration asserts that it's not valid to re-use the same backend config (i.e the same state file)
-// in parallel runs. This would result in multiple actions attempting to set state, potentially with different resource configurations.
+// TestTest_ValidateBackendConfiguration tests validation of how backends are declared in test files:
+// * it's not valid to re-use the same backend config (i.e the same state file)
+// * it's not valid to use a deprecated backend type
+// * it's not valid to use a non-existent backend type
 //
-// Note - this test is written to assert that diagnostics are returned about re-used backend blocks between run blocks, but it allows either
-// of the conflicting run blocks to cause the error to be raised. This is because run blocks without matching state keys aren't run in a
-// deterministic order.
+// Backend validaton performed in the command package is dependent on the internal/backend/init package,
+// which cannot be imported in configuration parsing packages without creating an import cycle.
 func TestTest_ReusedBackendConfiguration(t *testing.T) {
 
 	testCases := map[string]struct {
@@ -2600,6 +2601,28 @@ Error: Repeat use of the same backend block
 The run "test_2" contains a backend configuration that's already been used in
 run "test_1". Sharing the same backend configuration between separate runs
 will result in conflicting state updates.
+`,
+		},
+		"validation detects when a deprecated backend type is used": {
+			dirName: "removed-backend-type",
+			expectErr: `
+Error: Unsupported backend type
+
+  on main.tftest.hcl line 7, in run "test_removed_backend":
+   7:   backend "etcd" {
+
+The "etcd" backend is not supported in Terraform v1.3 or later.
+`,
+		},
+		"validation detects when a non-existent backend type": {
+			dirName: "non-existent-backend-type",
+			expectErr: `
+Error: Unsupported backend type
+
+  on main.tftest.hcl line 7, in run "test_invalid_backend":
+   7:   backend "foobar" {
+
+There is no backend type named "foobar".
 `,
 		},
 	}

internal/command/testdata/test/non-existent-backend-type/main.tf

@@ -0,0 +1,10 @@
+
+variable "input" {
+  type = string
+}
+
+resource "test_resource" "a" {
+  value = var.input
+}
+
+resource "test_resource" "c" {}

internal/command/testdata/test/non-existent-backend-type/main.tftest.hcl

@@ -0,0 +1,9 @@
+# The "foobar" backend does not exist and isn't a removed backend either
+run "test_invalid_backend" {
+  variables {
+    input = "foobar"
+  }
+
+  backend "foobar" {
+  }
+}

internal/command/testdata/test/removed-backend-type/main.tf

@@ -0,0 +1,10 @@
+
+variable "input" {
+  type = string
+}
+
+resource "test_resource" "a" {
+  value = var.input
+}
+
+resource "test_resource" "c" {}

internal/command/testdata/test/removed-backend-type/main.tftest.hcl

@@ -0,0 +1,9 @@
+# The "etcd" backend has been removed from Terraform versions 1.3+
+run "test_removed_backend" {
+  variables {
+    input = "foobar"
+  }
+
+  backend "etcd" {
+  }
+}