add simple error indicating backend removal

There are no good options for inserting diagnostics into the backend
lookup, or creating a backend which reports it's removal because none of
the init or GetSchema functions return any errors.

Keep a registry of the removed backend so that we can at least notify
users that a backend was removed vs an invalid name.

internal/backend/init/init.go

@@ -43,6 +43,10 @@ import (
 var backends map[string]backend.InitFn
 var backendsLock sync.Mutex
 
+// RemovedBackends is a record of previously supported backends which have
+// since been deprecated and removed.
+var RemovedBackends map[string]string
+
 // Init initializes the backends map with all our hardcoded backends.
 func Init(services *disco.Disco) {
 	backendsLock.Lock()
@@ -101,6 +105,10 @@ func Init(services *disco.Disco) {
 			)
 		},
 	}
+
+	RemovedBackends = map[string]string{
+		"etcd": `The "etcd" backend is not supported in Terraform v1.3 or later.`,
+	}
 }
 
 // Backend returns the initialization factory for the given backend, or

internal/builtin/providers/terraform/data_source_state.go

@@ -193,10 +193,15 @@ func getBackend(cfg cty.Value) (backend.Backend, cty.Value, tfdiags.Diagnostics)
 	log.Printf("[DEBUG] Initializing remote state backend: %s", backendType)
 	f := getBackendFactory(backendType)
 	if f == nil {
+		detail := fmt.Sprintf("There is no backend type named %q.", backendType)
+		if msg, removed := backendInit.RemovedBackends[backendType]; removed {
+			detail = msg
+		}
+
 		diags = diags.Append(tfdiags.AttributeValue(
 			tfdiags.Error,
 			"Invalid backend configuration",
-			fmt.Sprintf("There is no backend type named %q.", backendType),
+			detail,
 			cty.Path(nil).GetAttr("backend"),
 		))
 		return nil, cty.NilVal, diags

internal/command/init.go

@@ -424,10 +424,15 @@ func (c *InitCommand) initBackend(root *configs.Module, extraConfig rawFlags) (b
 
 		bf := backendInit.Backend(backendType)
 		if bf == nil {
+			detail := fmt.Sprintf("There is no backend type named %q.", backendType)
+			if msg, removed := backendInit.RemovedBackends[backendType]; removed {
+				detail = msg
+			}
+
 			diags = diags.Append(&hcl.Diagnostic{
 				Severity: hcl.DiagError,
 				Summary:  "Unsupported backend type",
-				Detail:   fmt.Sprintf("There is no backend type named %q.", backendType),
+				Detail:   detail,
 				Subject:  &root.Backend.TypeRange,
 			})
 			return nil, true, diags

internal/command/meta_backend.go

@@ -466,10 +466,15 @@ func (m *Meta) backendConfig(opts *BackendOpts) (*configs.Backend, int, tfdiags.
 
 	bf := backendInit.Backend(c.Type)
 	if bf == nil {
+		detail := fmt.Sprintf("There is no backend type named %q.", c.Type)
+		if msg, removed := backendInit.RemovedBackends[c.Type]; removed {
+			detail = msg
+		}
+
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagError,
 			Summary:  "Invalid backend type",
-			Detail:   fmt.Sprintf("There is no backend type named %q.", c.Type),
+			Detail:   detail,
 			Subject:  &c.TypeRange,
 		})
 		return nil, 0, diags