Ergonomic Inspect

This guide covers the usage of the fuchsia_inspect_derive library, and assumes that you are familiar with Inspect and have basic experience with the fuchsia_inspect library.


The fuchsia_inspect_derive library provides ergonomic macros, traits and smart pointers around the fuchsia_inspect library, that makes it easier to integrate inspect with your Rust code base, by:

  • Owning source data and inspect data under the same RAII type
  • Being idiomatic. First class support for primitives, common interior mutability patterns and async.
  • Generating repetitive boilerplate code
  • Providing a unified way to attach a type to inspect
  • Supporting gradual integration with existing code bases, both those that don't yet support inspect, and the ones that are integrated with fuchsia_inspect directly.
  • Supporting foreign types that lack inspect integration. See IDebug<T> for usage and constraints.

At the same time, it preserves the performance and semantics of a manual inspect integration, by:

  • Committing granular inspect tree modifications, where logical leaf nodes are updated independently.
  • Applying static dispatch only, to avoid additional runtime overhead.
  • Not using any additional synchronization primitives.


When you integrate your Rust code base with this library, be aware that:

  • The library mirrors the internal type hierarchy of the Rust program. Limited structural modifications such as renaming, flattening and omitting fields are supported (similar to Serde). If the desired inspect tree structure is vastly different from the type hierarchy, you should consider using fuchsia_inspect directly.
  • Some features are not yet supported, requiring you to implement Inspect manually:
    • Lazy nodes, histograms and inspect arrays.
    • Option<T> and other enums.
    • Collection types, such as vectors and maps.
    • StringReferences
  • The library promotes custom smart pointers, which creates another layer of data wrapping.

Quick start

This section shows an example where you take an existing data structure and apply inspect to that structure. Let's start with a simple example, a Yak:

struct Yak {
    // TODO: Overflow risk at high altitudes?
    hair_length: u16,       // Current hair length in mm
    credit_card_no: String, // Super secret

impl Yak {
    pub fn new() -> Self {
        Self { hair_length: 5, credit_card_no: "<secret>".to_string() }

    pub fn shave(&mut self) {
        self.hair_length = 0;

Then, consider this construction site:

let mut yak = Yak::new();

Let's make the yak inspectable. In particular:

  • Expose the current hair length
  • Expose the number of times the Yak has been shaved
  • The credit card number should NOT be exposed

Now use fuchsia_inspect_derive to make this Yak inspectable:

use fuchsia_inspect_derive::{
    IValue,      // A RAII smart pointer that can be attached to inspect
    Inspect,     // The core trait and derive-macro
    WithInspect, // Provides `.with_inspect(..)`

struct Yak {
    #[inspect(rename = "hair_length_mm")] // Clarify that it's millimeters
    hair_length: IValue<u16>, // Encapsulate primitive in IValue

    #[inspect(skip)] // Credit card number should NOT be exposed
    credit_card_no: String,
    shaved_counter: fuchsia_inspect::UintProperty, // Write-only counter
    inspect_node: fuchsia_inspect::Node,           // Inspect node of this Yak, optional

impl Yak {
    pub fn new() -> Self {
        Self {
            hair_length: IValue::new(5), // Or if you prefer, `5.into()`
            credit_card_no: "<secret>".to_string(),

            // Inspect nodes and properties should be default-initialized
            shaved_counter: fuchsia_inspect::UintProperty::default(),
            inspect_node: fuchsia_inspect::Node::default(),

    pub fn shave(&mut self) {
        self.hair_length.iset(0); // Set the source value AND update the inspect property
        self.shaved_counter.add(1u64); // Increment counter

Now, in your main program (or in a unit test), construct the yak and attach it to the inspect tree:

// Initialization
let mut yak = Yak::new()
    .with_inspect(/* parent node */ inspector.root(), /* name */ "my_yak")?;

assert_data_tree!(inspector, root: {
    my_yak: { hair_length_mm: 5u64, shaved_counter: 0u64 }

// Mutation
assert_data_tree!(inspector, root: {
    my_yak: { hair_length_mm: 0u64, shaved_counter: 1u64 }

// Destruction
assert_data_tree!(inspector, root: {});

Now you have integrated a simple program with Inspect. The rest of this guide describes the types, traits and macros of this library, and how to apply them to real world programs.

Derive Inspect

derive(Inspect) can be added to any named struct, but each of its fields must also implement Inspect (except for inspect_node and skipped fields). The library provides implementations of Inspect for several types:

If you add a type that isn't Inspect, you get a compiler error:

struct Yakling {
    name: String, // Forgot to wrap, should be `name: IValue<String>`

// error[E0599]: no method named `iattach` found for struct
// `std::string::String` in the current scope

Nested Inspect Types

Inspect types can be freely nested, like so:

// Stable is represented as a node with two child nodes `yak` and `horse`
struct Stable {
    yak: Yak,     // Yak derives Inspect
    horse: Horse, // Horse implements Inspect manually
    inspect_node: fuchsia_inspect::Node,

Fields and Attributes

All fields, except for skipped fields and inspect_node, must implement Inspect, either for &mut T or &T.

If an inspect_node field is present, instances will have its own node in the inspect tree. It must be a fuchsia_inspect::Node:

struct Yak {
    name: IValue<String>,
    age: IValue<u16>,
    inspect_node: fuchsia_inspect::Node, // NOTE: Node is present

// Yak is represented as a node with `name` and `age` properties.

If inspect_node is absent, fields will be attached directly to the parent node (meaning that the name provided to with_inspect will be ignored):

struct YakName {
    title: IValue<String>, // E.g. "Lil"
    full_name: IValue<String>, // E.g. "Sebastian"
                           // NOTE: Node is absent

// YakName has no separate node. Instead, the `title` and `full_name`
// properties are attached directly to the parent node.

If your type needs to add or remove nodes or properties dynamically, it should own an inspect node. The inspect node is needed when nodes or properties are added or removed after the initial attachment.

derive(Inspect) supports the following field attributes:

  • inspect(skip): The field is ignored by inspect.
  • inspect(rename = "foo"): Use a different name. By default, the field name is used.
  • inspect(forward): Forwards the attachment to an inner Inspect type, omitting one layer of nesting from the inspect hierarchy. All other fields should not have any inspect attributes. The type must NOT have an inspect_node field. Useful for wrapper types. For example:
struct Wrapper {
    // key is not included, because inspect has been forwarded.
    _key: String,
    inner: RefCell<Inner>,

struct Inner {
    name: IValue<String>,
    age: IValue<u16>,
    inspect_node: fuchsia_inspect::Node,

// Wrapper is represented as a node with `name` and `age` properties.

Manually Managed Inspect Types

If you are integrating with a code base that uses fuchsia_inspect directly, its types are not be aware of fuchsia_inspect_derive. Do not add such manually managed types as fields to an Inspect type directly. Instead, implement Inspect manually for the type. Avoid attaching manually outside of the Inspect trait, since attachment in fuchsia_inspect_derive occurs after construction. Attaching in a constructor can silently cause its inspect state to be absent.

Attaching to the Inspect Tree

An inspect type should be attached once, and immediately after instantiation, using the with_inspect extension trait method:

let yak = Yak::new().with_inspect(inspector.root(), "my_yak")?;
assert_data_tree!(inspector, root: { my_yak: { name: "Lil Sebastian", age: 3u64 }});

If you have a nested Inspect structure, you should only attach the top-level type. The nested types are attached implicitly:

// Stable owns a Yak, which also implements Inspect.
let stable = Stable::new().with_inspect(inspector.root(), "stable")?;
    root: { stable: { yak: { name: "Lil Sebastian", age: 3u64 }}});

Note that when a Yak is constructed from within a Stable, there is no with_inspect call present. Instead, the Yak is automatically attached as a child of the Stable. However, you can still attach a Yak when it is the top-level type, such as in the unit tests for Yak. This allows you to test any Inspect type in isolation.

You can optionally choose to supply inspect nodes in constructors instead of explicitly calling with_inspect at the construction sites. First, ensure that the type is NOT nested under another Inspect type (as this would cause duplicate attachments). Sedondly, make sure to document this fact clearly, so the calling user is aware of your attachment convention.

Interior mutability

In Rust (and particularly async Rust), it is common to use interior mutability. This library provides Inspect implementations for several smart pointers and locks:

  • std: Box, Arc, Rc, RefCell, Mutex and RwLock
    • Note that Cell does NOT work. Instead, upgrade to a RefCell.
  • parking_lot: Mutex and RwLock
  • futures: Mutex

Generally, interior mutability within a derive(Inspect) type just works:

struct Stable {
-   yak: Yak,
+   yak: Arc<Mutex<Yak>>,
-   horse: Horse,
+   horse: RefCell<Horse>,
    inspect_node: fuchsia_inspect::Node,

Make sure to put your smart pointers inside your mutability wrapper:

struct Yak {
-   coins: IValue<Rc<RwLock<u32>>>,  // Won't compile
+   coins: Rc<RwLock<IValue<u32>>>,  // Correct

If an inner type is behind a lock, attachment will fail if the lock is acquired by someone else. Hence, always attach immediately after instantiation.

Implement Inspect Manually

The derive(Inspect) derive-macro generates an impl Inspect for &mut T { .. }. Oftentimes, this works fine, but in some cases you may need to implement Inspect manually. Fortunately, the Inspect trait is quite simple:

trait Inspect {
    /// Attach self to the inspect tree
    fn iattach(self, parent: &Node, name: AsRef<str>) -> Result<(), AttachError>;

Do not return an AttachError for structural errors in the data. Instead, report the error using logs or an inspect node. AttachError is reserved for irrecoverable invariant errors that fail the entire attachment.

IOwned Smart Pointers

Smart pointers may sound scary, but you probably use them everyday already. For instance, Arc and Box are smart pointers. They are statically dispatched, and have first-class support in Rust (through deref coercion). This makes them minimally invasive.

fuchsia_inspect_derive comes with a few useful smart pointers that implement Inspect and can be used to wrap primitives, debuggable types, and more. They all share the same behavior: An IOwned<T> smart pointer owns a generic source type T and some associated inspect data.

Here is a demonstration of the IOwned API:

let mut number = IValue::new(1337u16) // IValue is an IOwned smart pointer
    .with_inspect(inspector.root(), "my_number")?; // Attach to inspect tree

// Dereference the value behind the IValue, without mutating
assert_eq!(*number, 1337u16);
    // Mutate value behind an IOwned smart pointer, using a scope guard
    let mut number_guard = number.as_mut();
    *number_guard = 1338;
    *number_guard += 1;

    // Inspect state not yet updated
    assert_data_tree!(inspector, root: { my_number: 1337u64 });
// When the guard goes out of scope, the inspect state is updated
assert_data_tree!(inspector, root: { my_number: 1339u64 });

number.iset(1340); // Sets the source value AND updates inspect
assert_data_tree!(inspector, root: { my_number: 1340u64 });

let inner = number.into_inner(); // Detaches from inspect tree...
assert_eq!(inner, 1340u16); // ...and returns the inner value.

An IOwned<T> smart pointer should not be instantiated directly, but rather one of its variants:


The IValue<T> smart pointer wraps a primitive (or any type T: Unit). For example, an IValue<f32> is represented as a DoubleProperty, and an IValue<i16> is represented as an IntProperty.

An IValue of a primitive results in the same structure as using a plain inspect property directly. So, why would you use an IValue? If you only need to write or increment a value, you can use a plain inspect property. If you also need to read the value, you should use an IValue.


The IDebug<T> smart pointer wraps a debuggable type, and maintains the debug representation of T as a StringProperty. This is useful for:

  • Foreign types, where adding an inspect implementation is infeasible
  • Debugging, to quickly verify some state about your program

Avoid using debug representations in production code, since they come with the following issues:

  • Debug representations are written on every inspect update, which can result in unnecessary performance overhead.
  • Debug representations can exhaust the space of the inspect VMO, causing truncation of the entire inspect state.
  • Debug representations cannot be integrated with the privacy pipeline: if any PII is exposed as part of the debug string, the entire field must be considered PII. Managing your own structured data allows to granularly redact fields containing PII.

The Unit Trait

The Unit trait describes the inspect representation of a type, how to initialize it, and how to update it. It should be implemented for types that act as a logical leaf node, and does NOT support per-field updates. This library provides implementations of Unit for most primitives. For example, u8, u16, u32 and u64 are represented as a UintProperty.

Usage in IValue

A Unit type should be wrapped in an IValue<T: Unit> (see above), for a RAII managed inspectable type. It is NOT recommended to call methods on Unit directly.

Derive Unit

Sometimes a logical Unit is a composite type. Unit can be derived for a named struct, as long as its fields also implement Unit. For example:

// Represented as a Node with two properties, `x` and `y`, of type UintProperty
struct Point {
    x: f32,
    y: f32,

Unit can be nested, but keep in mind that all fields are still written at the same time:

// Represented as a Node with two child nodes `top_left` and `bottom_right`
struct Rect {
    #[inspect(rename = "top_left")]
    tl: Point,

    #[inspect(rename = "bottom_right")]
    br: Point,


derive(Unit) supports the following field attributes:

  • inspect(skip): The field is ignored by inspect.
  • inspect(rename = "foo"): Use a different name. By default, the field name is used.