tracing: Add noop tracing

BUG=b:213154559
TEST=presubmit
Change-Id: Idfe23544c8381e664d913c4a80742f4e25a30006

Change-Id: I2bb0b59098ae183c45e92fcf15e1bcfe834df204
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3707622
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Vikram Auradkar <auradkar@google.com>

Cargo.toml

@@ -76,6 +76,7 @@ members = [
     "serde_keyvalue",
     "tpm2",
     "tpm2-sys",
+    "tracing",
     "usb_sys",
     "usb_util",
     "vfio_sys",

tracing/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "tracing"
+version = "0.1.0"
+authors = ["The Chromium OS Authors"]
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tracing/src/lib.rs

@@ -0,0 +1,8 @@
+// Copyright 2022 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// A crate that provides noop tracing.
+mod noop;
+
+pub use noop::*;

tracing/src/noop.rs

@@ -0,0 +1,23 @@
+// Copyright 2022 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Empty macros for when the tracing feature isn't used.
+#[macro_export]
+macro_rules! trace_event {
+    ($category:ident, $name:expr) => {
+        None as Option<bool>
+    };
+}
+
+#[macro_export]
+macro_rules! trace_event_begin {
+    ($category:ident, $name:expr) => {};
+}
+
+#[macro_export]
+macro_rules! trace_event_end {
+    ($category:ident) => {};
+}
+
+pub fn init() {}