## Keyboard shortcuts

Press `←` or `→` to navigate between chapters

Press `S` or `/` to search in the book

Press `?` to show this help

Press `Esc` to hide this help

# `aarch64-unknown-fuchsia` and `x86_64-unknown-fuchsia`

Tier: 2

Fuchsia is a modern open source operating system that's simple, secure, updatable, and performant.

## Target maintainers

@erickt @Nashenas88

The up-to-date list can be also found via the fuchsia marker team.

## Table of contents

1. Requirements
2. Walkthrough structure
3. Compiling a Rust binary targeting Fuchsia
   1. Targeting Fuchsia with rustup and cargo
   2. Targeting Fuchsia with a compiler built from source
4. Creating a Fuchsia package
   1. Creating a Fuchsia component
   2. Building a Fuchsia package
5. Publishing a Fuchsia package
   1. Creating a Fuchsia package repository
   2. Publishing Fuchsia package to repository
6. Running a Fuchsia component on an emulator
   1. Starting the Fuchsia emulator
   2. Watching emulator logs
   3. Serving a Fuchsia package
   4. Running a Fuchsia component
7. `.gitignore` extensions
8. Testing
   1. Running unit tests
   2. Running the compiler test suite
9. Debugging
   1. `zxdb`
   2. Attaching `zxdb`
   3. Using `zxdb`
   4. Displaying source code in `zxdb`

## Requirements

This target is cross-compiled from a host environment. You will need a recent copy of the Fuchsia SDK, which provides the tools, libraries, and binaries required to build and link programs for Fuchsia.

Development may also be done from the source tree.

Fuchsia targets support `std` and follow the `sysv64` calling convention on x86_64. Fuchsia binaries use the ELF file format.

## Walkthrough structure

This walkthrough will cover:

1. Compiling a Rust binary targeting Fuchsia.
2. Building a Fuchsia package.
3. Publishing and running a Fuchsia package to a Fuchsia emulator.

For the purposes of this walkthrough, we will only target `x86_64-unknown-fuchsia`.

## Compiling a Rust binary targeting Fuchsia

Today, there are two main ways to build a Rust binary targeting Fuchsia using the Fuchsia SDK:

1. Allow rustup to handle the installation of Fuchsia targets for you.
2. Build a toolchain locally that can target Fuchsia.

### Targeting Fuchsia with rustup and cargo

The easiest way to build a Rust binary targeting Fuchsia is by allowing rustup to handle the installation of Fuchsia targets for you. This can be done by issuing the following commands:

    rustup target add x86_64-unknown-fuchsia
    rustup target add aarch64-unknown-fuchsia

After installing our Fuchsia targets, we can now compile a Rust binary that targets Fuchsia.

To create our Rust project, we can use `cargo` as follows:

From base working directory

    cargo new hello_fuchsia

The rest of this walkthrough will take place from `hello_fuchsia`, so we can change into that directory now:

    cd hello_fuchsia

Note: From this point onwards, all commands will be issued from the `hello_fuchsia/` directory, and all `hello_fuchsia/` prefixes will be removed from references for sake of brevity.

We can edit our `src/main.rs` to include a test as follows:

`src/main.rs`

    fn main() {
        println!("Hello Fuchsia!");
    }

    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }

In addition to the standard workspace created, we will want to create a `.cargo/config.toml` file to link necessary libraries during compilation:

`.cargo/config.toml`

    [target.x86_64-unknown-fuchsia]

    rustflags = [
        "-Lnative=<SDK_PATH>/arch/x64/lib",
        "-Lnative=<SDK_PATH>/arch/x64/sysroot/lib"
    ]

Note: Make sure to fill out `<SDK_PATH>` with the path to the downloaded Fuchsia SDK.

These options configure the following:

- `-Lnative=${SDK_PATH}/arch/${ARCH}/lib`: Link against Fuchsia libraries from the SDK
- `-Lnative=${SDK_PATH}/arch/${ARCH}/sysroot/lib`: Link against Fuchsia sysroot libraries from the SDK

In total, our new project will look like:

Current directory structure

    hello_fuchsia/
    ┣━ src/
    ┃  ┗━ main.rs
    ┣━ Cargo.toml
    ┗━ .cargo/
       ┗━ config.toml

Finally, we can build our rust binary as:

    cargo build --target x86_64-unknown-fuchsia

Now we have a Rust binary at `target/x86_64-unknown-fuchsia/debug/hello_fuchsia`, targeting our desired Fuchsia target.

Current directory structure

    hello_fuchsia/
    ┣━ src/
    ┃  ┗━ main.rs
    ┣━ target/
    ┃  ┗━ x86_64-unknown-fuchsia/
    ┃     ┗━ debug/
    ┃        ┗━ hello_fuchsia
    ┣━ Cargo.toml
    ┗━ .cargo/
       ┗━ config.toml

### Targeting Fuchsia with a compiler built from source

An alternative to the first workflow is to target Fuchsia by using `rustc` built from source.

Before building Rust for Fuchsia, you'll need a clang toolchain that supports Fuchsia as well. A recent version (14+) of clang should be sufficient to compile Rust for Fuchsia.

x86-64 and AArch64 Fuchsia targets can be enabled using the following configuration in `bootstrap.toml`:

    [build]
    target = ["<host_platform>", "aarch64-unknown-fuchsia", "x86_64-unknown-fuchsia"]

    [rust]
    lld = true

    [llvm]
    download-ci-llvm = false

    [target.x86_64-unknown-fuchsia]
    cc = "clang"
    cxx = "clang++"

    [target.aarch64-unknown-fuchsia]
    cc = "clang"
    cxx = "clang++"

Though not strictly required, you may also want to use `clang` for your host target as well:

    [target.<host_platform>]
    cc = "clang"
    cxx = "clang++"

By default, the Rust compiler installs itself to `/usr/local` on most UNIX systems. You may want to install it to another location (e.g. a local `install` directory) by setting a custom prefix in `bootstrap.toml`:

    [install]
    # Make sure to use the absolute path to your install directory
    prefix = "<RUST_SRC_PATH>/install"

Next, the following environment variables must be configured. For example, using a script we name `config-env.sh`:

    # Configure this environment variable to be the path to the downloaded SDK
    export SDK_PATH="<SDK path goes here>"

    export CFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -I${SDK_PATH}/pkg/fdio/include"
    export CXXFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -I${SDK_PATH}/pkg/fdio/include"
    export LDFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -L${SDK_PATH}/arch/arm64/lib"
    export CARGO_TARGET_AARCH64_UNKNOWN_FUCHSIA_RUSTFLAGS="-C link-arg=--sysroot=${SDK_PATH}/arch/arm64/sysroot -Lnative=${SDK_PATH}/arch/arm64/sysroot/lib -Lnative=${SDK_PATH}/arch/arm64/lib"
    export CFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -I${SDK_PATH}/pkg/fdio/include"
    export CXXFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -I${SDK_PATH}/pkg/fdio/include"
    export LDFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -L${SDK_PATH}/arch/x64/lib"
    export CARGO_TARGET_X86_64_UNKNOWN_FUCHSIA_RUSTFLAGS="-C link-arg=--sysroot=${SDK_PATH}/arch/x64/sysroot -Lnative=${SDK_PATH}/arch/x64/sysroot/lib -Lnative=${SDK_PATH}/arch/x64/lib"

Finally, the Rust compiler can be built and installed:

    (source config-env.sh && ./x.py install)

Once `rustc` is installed, we can create a new working directory to work from, `hello_fuchsia` along with `hello_fuchsia/src`:

    mkdir hello_fuchsia
    cd hello_fuchsia
    mkdir src

Note: From this point onwards, all commands will be issued from the `hello_fuchsia/` directory, and all `hello_fuchsia/` prefixes will be removed from references for sake of brevity.

There, we can create a new file named `src/hello_fuchsia.rs`:

`src/hello_fuchsia.rs`

    fn main() {
        println!("Hello Fuchsia!");
    }

    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }

Current directory structure

    hello_fuchsia/
    ┗━ src/
        ┗━ hello_fuchsia.rs

Using your freshly installed `rustc`, you can compile a binary for Fuchsia using the following options:

- `--target x86_64-unknown-fuchsia`/`--target aarch64-unknown-fuchsia`: Targets the Fuchsia platform of your choice
- `-Lnative ${SDK_PATH}/arch/${ARCH}/lib`: Link against Fuchsia libraries from the SDK
- `-Lnative ${SDK_PATH}/arch/${ARCH}/sysroot/lib`: Link against Fuchsia sysroot libraries from the SDK

Putting it all together:

    # Configure these for the Fuchsia target of your choice
    TARGET_ARCH="<x86_64-unknown-fuchsia|aarch64-unknown-fuchsia>"
    ARCH="<x64|aarch64>"

    rustc \
        --target ${TARGET_ARCH} \
        -Lnative=${SDK_PATH}/arch/${ARCH}/lib \
        -Lnative=${SDK_PATH}/arch/${ARCH}/sysroot/lib \
        --out-dir bin src/hello_fuchsia.rs

Current directory structure

    hello_fuchsia/
    ┣━ src/
    ┃   ┗━ hello_fuchsia.rs
    ┗━ bin/
       ┗━ hello_fuchsia

## Creating a Fuchsia package

Before moving on, double check your directory structure:

Current directory structure

    hello_fuchsia/
    ┣━ src/                         (if using rustc)
    ┃   ┗━ hello_fuchsia.rs         ...
    ┣━ bin/                         ...
    ┃  ┗━ hello_fuchsia             ...
    ┣━ src/                         (if using cargo)
    ┃  ┗━ main.rs                   ...
    ┗━ target/                      ...
       ┗━ x86_64-unknown-fuchsia/   ...
          ┗━ debug/                 ...
             ┗━ hello_fuchsia       ...

With our Rust binary built, we can move to creating a Fuchsia package. On Fuchsia, a package is the unit of distribution for software. We'll need to create a new package directory where we will place files like our finished binary and any data it may need.

To start, make the `pkg`, and `pkg/meta` directories:

    mkdir pkg
    mkdir pkg/meta

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┗━ meta/

Now, create the following files inside:

`pkg/meta/package`

    {
      "name": "hello_fuchsia",
      "version": "0"
    }

The `package` file describes our package's name and version number. Every package must contain one.

`pkg/hello_fuchsia.manifest` if using cargo

    bin/hello_fuchsia=target/x86_64-unknown-fuchsia/debug/hello_fuchsia
    lib/ld.so.1=<SDK_PATH>/arch/x64/sysroot/dist/lib/ld.so.1
    lib/libfdio.so=<SDK_PATH>/arch/x64/dist/libfdio.so
    meta/package=pkg/meta/package
    meta/hello_fuchsia.cm=pkg/meta/hello_fuchsia.cm

`pkg/hello_fuchsia.manifest` if using rustc

    bin/hello_fuchsia=bin/hello_fuchsia
    lib/ld.so.1=<SDK_PATH>/arch/x64/sysroot/dist/lib/ld.so.1
    lib/libfdio.so=<SDK_PATH>/arch/x64/dist/libfdio.so
    meta/package=pkg/meta/package
    meta/hello_fuchsia.cm=pkg/meta/hello_fuchsia.cm

Note: Relative manifest paths are resolved starting from the working directory of `ffx`. Make sure to fill out `<SDK_PATH>` with the path to the downloaded SDK.

The `.manifest` file will be used to describe the contents of the package by relating their location when installed to their location on the file system. The `bin/hello_fuchsia=` entry will be different depending on how your Rust binary was built, so choose accordingly.

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┣━ meta/
       ┃  ┗━ package
       ┗━ hello_fuchsia.manifest

### Creating a Fuchsia component

On Fuchsia, components require a component manifest written in Fuchsia's markup language called CML. The Fuchsia devsite contains an overview of CML and a reference for the file format. Here's a basic one that can run our single binary:

`pkg/hello_fuchsia.cml`

    {
        include: [ "syslog/client.shard.cml" ],
        program: {
            runner: "elf",
            binary: "bin/hello_fuchsia",
        },
    }

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┣━ meta/
       ┃  ┗━ package
       ┣━ hello_fuchsia.manifest
       ┗━ hello_fuchsia.cml

Now we can compile that CML into a component manifest:

    ${SDK_PATH}/tools/${ARCH}/cmc compile \
        pkg/hello_fuchsia.cml \
        --includepath ${SDK_PATH}/pkg \
        -o pkg/meta/hello_fuchsia.cm

Note: `--includepath` tells the compiler where to look for `include`s from our CML. In our case, we're only using `syslog/client.shard.cml`.

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┣━ meta/
       ┃  ┣━ package
       ┃  ┗━ hello_fuchsia.cm
       ┣━ hello_fuchsia.manifest
       ┗━ hello_fuchsia.cml

### Building a Fuchsia package

Next, we'll build a package manifest as defined by our manifest:

    ${SDK_PATH}/tools/${ARCH}/ffx package build \
        --api-level $(${SDK_PATH}/tools/${ARCH}/ffx --machine json version | jq .tool_version.api_level) \
        --out pkg/hello_fuchsia_manifest \
        pkg/hello_fuchsia.manifest

This will produce `pkg/hello_fuchsia_manifest/` which is a package manifest we can publish directly to a repository.

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┣━ meta/
       ┃  ┣━ package
       ┃  ┗━ hello_fuchsia.cm
       ┣━ hello_fuchsia_manifest/
       ┃  ┗━ ...
       ┣━ hello_fuchsia.manifest
       ┣━ hello_fuchsia.cml
       ┗━ hello_fuchsia_package_manifest

We are now ready to publish the package.

## Publishing a Fuchsia package

With our package and component manifests setup, we can now publish our package. The first step will be to create a Fuchsia package repository to publish to.

### Creating a Fuchsia package repository

We can set up our repository with:

    ${SDK_PATH}/tools/${ARCH}/ffx repository create pkg/repo

Current directory structure

    hello_fuchsia/
    ┗━ pkg/
       ┣━ meta/
       ┃  ┣━ package
       ┃  ┗━ hello_fuchsia.cm
       ┣━ hello_fuchsia_manifest/
       ┃  ┗━ ...
       ┣━ repo/
       ┃  ┗━ ...
       ┣━ hello_fuchsia.manifest
       ┣━ hello_fuchsia.cml
       ┗━ hello_fuchsia_package_manifest

## Publishing Fuchsia package to repository

We can publish our new package to that repository with:

    ${SDK_PATH}/tools/${ARCH}/ffx repository publish \
        --package pkg/hello_fuchsia_package_manifest \
        pkg/repo

## Running a Fuchsia component on an emulator

At this point, we are ready to run our Fuchsia component. For reference, our final directory structure will look like:

Final directory structure

    hello_fuchsia/
    ┣━ src/                         (if using rustc)
    ┃   ┗━ hello_fuchsia.rs         ...
    ┣━ bin/                         ...
    ┃  ┗━ hello_fuchsia             ...
    ┣━ src/                         (if using cargo)
    ┃  ┗━ main.rs                   ...
    ┣━ target/                      ...
    ┃  ┗━ x86_64-unknown-fuchsia/   ...
    ┃     ┗━ debug/                 ...
    ┃        ┗━ hello_fuchsia       ...
    ┗━ pkg/
       ┣━ meta/
       ┃  ┣━ package
       ┃  ┗━ hello_fuchsia.cm
       ┣━ hello_fuchsia_manifest/
       ┃  ┗━ ...
       ┣━ repo/
       ┃  ┗━ ...
       ┣━ hello_fuchsia.manifest
       ┣━ hello_fuchsia.cml
       ┗━ hello_fuchsia_package_manifest

### Starting the Fuchsia emulator

Start a Fuchsia emulator in a new terminal using:

    ${SDK_PATH}/tools/${ARCH}/ffx product-bundle get workstation_eng.qemu-${ARCH}
    ${SDK_PATH}/tools/${ARCH}/ffx emu start workstation_eng.qemu-${ARCH} --headless

### Watching emulator logs

Once the emulator is running, open a separate terminal to watch the emulator logs:

In separate terminal

    ${SDK_PATH}/tools/${ARCH}/ffx log \
        --since now

### Serving a Fuchsia package

Now, start a package repository server to serve our package to the emulator:

    ${SDK_PATH}/tools/${ARCH}/ffx repository server start \
        --background --repository hello-fuchsia --repo-path pkg-repo

Once the repository server is up and running, register it with the target Fuchsia system running in the emulator:

    ${SDK_PATH}/tools/${ARCH}/ffx target repository register \
        --repository hello-fuchsia

### Running a Fuchsia component

Finally, run the component:

    ${SDK_PATH}/tools/${ARCH}/ffx component run \
        /core/ffx-laboratory:hello_fuchsia \
        fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm

On reruns of the component, the `--recreate` argument may also need to be passed.

    ${SDK_PATH}/tools/${ARCH}/ffx component run \
        --recreate \
        /core/ffx-laboratory:hello_fuchsia \
        fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm

## `.gitignore` extensions

Optionally, we can create/extend our `.gitignore` file to ignore files and directories that are not helpful to track:

    pkg/repo
    pkg/meta/hello_fuchsia.cm
    pkg/hello_fuchsia_manifest
    pkg/hello_fuchsia_package_manifest

## Testing

### Running unit tests

Tests can be run in the same way as a regular binary.

- If using `cargo`, you can simply pass `test --no-run` to the `cargo` invocation and then repackage and rerun the Fuchsia package. From our previous example, this would look like `cargo test --target x86_64-unknown-fuchsia --no-run`, and moving the executable binary path found from the line `Executable unittests src/main.rs (target/x86_64-unknown-fuchsia/debug/deps/hello_fuchsia-<HASH>)` into `pkg/hello_fuchsia.manifest`.

- If using the compiled `rustc`, you can simply pass `--test` to the `rustc` invocation and then repackage and rerun the Fuchsia package.

The test harness will run the applicable unit tests.

Often when testing, you may want to pass additional command line arguments to your binary. Additional arguments can be set in the component manifest:

`pkg/hello_fuchsia.cml`

    {
        include: [ "syslog/client.shard.cml" ],
        program: {
            runner: "elf",
            binary: "bin/hello_fuchsia",
            args: ["it_works"],
        },
    }

This will pass the argument `it_works` to the binary, filtering the tests to only those tests that match the pattern. There are many more configuration options available in CML including environment variables. More documentation is available on the Fuchsia devsite.

### Running the compiler test suite

The commands in this section assume that they are being run from inside your local Rust source checkout:

    cd ${RUST_SRC_PATH}

To run the Rust test suite on an emulated Fuchsia device, you'll also need to download a copy of the Fuchsia SDK. The current minimum supported SDK version is 20.20240412.3.1.

Fuchsia's test runner interacts with the Fuchsia emulator and is located at `src/ci/docker/scripts/fuchsia-test-runner.py`. First, add the following variables to your existing `config-env.sh`:

    # TEST_TOOLCHAIN_TMP_DIR can point anywhere, but it:
    #  - must be less than 108 characters, otherwise qemu can't handle the path
    #  - must be consistent across calls to this file (don't use `mktemp -d` here)
    export TEST_TOOLCHAIN_TMP_DIR="/tmp/rust-tmp"

    # Keep existing contents of `config-env.sh` from earlier, including SDK_PATH

We can then use the script to start our test environment with:

    ( \
        source config-env.sh &&                                                   \
        src/ci/docker/scripts/fuchsia-test-runner.py start                        \
        --rust-build ${RUST_SRC_PATH}/build                                       \
        --sdk ${SDK_PATH}                                                         \
        --target {x86_64-unknown-fuchsia|aarch64-unknown-fuchsia}                 \
        --verbose                                                                 \
    )

Where `${RUST_SRC_PATH}/build` is the `build-dir` set in `bootstrap.toml`.

Once our environment is started, we can run our tests using `x.py` as usual. The test runner script will run the compiled tests on an emulated Fuchsia device. To run the full `tests/ui` test suite:

    ( \
        source config-env.sh &&                                                   \
        ./x.py                                                                    \
        --config bootstrap.toml                                                      \
        --stage=2                                                                 \
        test tests/ui                                                             \
        --target x86_64-unknown-fuchsia                                           \
        --run=always                                                              \
        --test-args --target-rustcflags                                           \
        --test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/sysroot/lib             \
        --test-args --target-rustcflags                                           \
        --test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/lib                     \
        --test-args --target-rustcflags                                           \
        --test-args -Clink-arg=--undefined-version                                \
        --test-args --remote-test-client                                          \
        --test-args src/ci/docker/scripts/fuchsia-test-runner.py                  \
    )

By default, `x.py` compiles test binaries with `panic=unwind`. If you built your Rust toolchain with `-Cpanic=abort`, you need to tell `x.py` to compile test binaries with `panic=abort` as well:

        --test-args --target-rustcflags                                           \
        --test-args -Cpanic=abort                                                 \
        --test-args --target-rustcflags                                           \
        --test-args -Zpanic_abort_tests                                           \

When finished testing, the test runner can be used to stop the test environment:

    src/ci/docker/scripts/fuchsia-test-runner.py stop

## Debugging

### `zxdb`

Debugging components running on a Fuchsia emulator can be done using the console-mode debugger: zxdb. We will demonstrate attaching necessary symbol paths to debug our `hello-fuchsia` component.

### Attaching `zxdb`

In a separate terminal, issue the following command from our `hello_fuchsia` directory to launch `zxdb`:

In separate terminal

    ${SDK_PATH}/tools/${ARCH}/ffx debug connect -- \
        --symbol-path target/x86_64-unknown-fuchsia/debug

- `--symbol-path` gets required symbol paths, which are necessary for stepping through your program.

The "displaying source code in `zxdb`" section describes how you can display Rust and/or Fuchsia source code in your debugging session.

### Using `zxdb`

Once launched, you will be presented with the window:

    Connecting (use "disconnect" to cancel)...
    Connected successfully.
    👉 To get started, try "status" or "help".
    [zxdb]

To attach to our program, we can run:

    [zxdb] attach hello_fuchsia

Expected output

    Waiting for process matching "hello_fuchsia".
    Type "filter" to see the current filters.

Next, we can create a breakpoint at main using "b main":

    [zxdb] b main

Expected output

    Created Breakpoint 1 @ main

Finally, we can re-run the "hello_fuchsia" component from our original terminal:

    ${SDK_PATH}/tools/${ARCH}/ffx component run \
        --recreate \
        fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm

Once our component is running, our `zxdb` window will stop execution in our main as desired:

Expected output

    Breakpoint 1 now matching 1 addrs for main
    🛑 on bp 1 hello_fuchsia::main() • main.rs:2
       1 fn main() {
     ▶ 2     println!("Hello Fuchsia!");
       3 }
       4
    [zxdb]

`zxdb` has similar commands to other debuggers like gdb. To list the available commands, run "help" in the `zxdb` window or visit the zxdb documentation.

    [zxdb] help

Expected output

    Help!

      Type "help <command>" for command-specific help.

    Other help topics (see "help <topic>")
    ...

### Displaying source code in `zxdb`

By default, the debugger will not be able to display source code while debugging. For our user code, we displayed source code by pointing our debugger to our debug binary via the `--symbol-path` arg. To display library source code in the debugger, you must provide paths to the source using `--build-dir`. For example, to display the Rust and Fuchsia source code:

    ${SDK_PATH}/tools/${ARCH}/ffx debug connect -- \
        --symbol-path target/x86_64-unknown-fuchsia/debug \
        --build-dir ${RUST_SRC_PATH}/rust \
        --build-dir ${FUCHSIA_SRC_PATH}/fuchsia/out/default

- `--build-dir` links against source code paths, which are not strictly necessary for debugging, but is a nice-to-have for displaying source code in `zxdb`.

Linking to a Fuchsia checkout can help with debugging Fuchsia libraries, such as fdio.

### Debugging the compiler test suite

Debugging the compiler test suite requires some special configuration:

First, we have to properly configure zxdb so it will be able to find debug symbols and source information for our test. The test runner can do this for us with:

    src/ci/docker/scripts/fuchsia-test-runner.py debug                            \
        --rust-src ${RUST_SRC_PATH}                                               \
        --fuchsia-src ${FUCHSIA_SRC_PATH}                                         \
        --test ${TEST}

where `${TEST}` is relative to Rust's `tests` directory (e.g. `ui/abi/...`).

This will start a zxdb session that is properly configured for the specific test being run. All three arguments are optional, so you can omit `--fuchsia-src` if you don't have it downloaded. Now is a good time to set any desired breakpoints, like `b main`.

Next, we have to tell `x.py` not to optimize or strip debug symbols from our test suite binaries. We can do this by passing some new arguments to `rustc` through our `x.py` invocation. The full invocation is:

    ( \
        source config-env.sh &&                                                   \
        ./x.py                                                                    \
        --config bootstrap.toml                                                      \
        --stage=2                                                                 \
        test tests/${TEST}                                                        \
        --target x86_64-unknown-fuchsia                                           \
        --run=always                                                              \
        --test-args --target-rustcflags                                           \
        --test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/sysroot/lib             \
        --test-args --target-rustcflags                                           \
        --test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/lib                     \
        --test-args --target-rustcflags                                           \
        --test-args -Clink-arg=--undefined-version                                \
        --test-args --target-rustcflags                                           \
        --test-args -Cdebuginfo=2                                                 \
        --test-args --target-rustcflags                                           \
        --test-args -Copt-level=0                                                 \
        --test-args --target-rustcflags                                           \
        --test-args -Cstrip=none                                                  \
        --test-args --remote-test-client                                          \
        --test-args src/ci/docker/scripts/fuchsia-test-runner.py                  \
    )

If you built your Rust toolchain with `panic=abort`, make sure to include the previous flags so your test binaries are also compiled with `panic=abort`.

Upon running this command, the test suite binary will be run and zxdb will attach and load any relevant debug symbols.