command: Do CLI init of backend loaded from plan

If we don't do this, we can't produce any output when applying a saved
plan file.

Here we also introduce a check to the local backend's ReportResult
function so that it won't panic if CLI init is skipped, although that
will no longer happen in the apply-from-file case due to the change
described in the previous paragraph.

backend/local/backend.go

@@ -408,7 +408,15 @@ func (b *Local) ReportResult(op *backend.RunningOperation, diags tfdiags.Diagnos
 	} else {
 		op.Result = backend.OperationSuccess
 	}
-	b.ShowDiagnostics(diags)
+	if b.ShowDiagnostics != nil {
+		b.ShowDiagnostics(diags)
+	} else {
+		// Shouldn't generally happen, but if it does then we'll at least
+		// make some noise in the logs to help us spot it.
+		if len(diags) != 0 {
+			log.Printf("[ERROR] Local backend needs to report diagnostics but ShowDiagnostics callback is not set: %s", diags.ErrWithWarnings())
+		}
+	}
 }
 
 // Colorize returns the Colorize structure that can be used for colorizing

command/meta_backend.go

@@ -159,6 +159,19 @@ func (m *Meta) BackendForPlan(settings plans.Backend) (backend.Enhanced, tfdiags
 	configureDiags := b.Configure(configVal)
 	diags = diags.Append(configureDiags)
 
+	// If the backend supports CLI initialization, do it.
+	if cli, ok := b.(backend.CLI); ok {
+		cliOpts := m.backendCLIOpts()
+		if err := cli.CLIInit(cliOpts); err != nil {
+			diags = diags.Append(fmt.Errorf(
+				"Error initializing backend %T: %s\n\n"+
+					"This is a bug; please report it to the backend developer",
+				b, err,
+			))
+			return nil, diags
+		}
+	}
+
 	// If the result of loading the backend is an enhanced backend,
 	// then return that as-is. This works even if b == nil (it will be !ok).
 	if enhanced, ok := b.(backend.Enhanced); ok {