Rust Hello World

First we'll create a "hello world" rust binary. Then we'll create the rust library and switch our rust binary to call it. We'll also create a unit test for our rust library.

Hello world rust binary

Once we're done this chapter, this binary will print out a random number gint from 0 to a user provided arg. We'll start with just making sure we can build a rust binary and print hello world!.

We're going to put all the summation related code in the directory $HOME/repo/src/summation, let's go ahead and make and cd that:

mkdir -p "${HOME}/repo/src/summation"
cd "${HOME}/repo/src/summation"

BUILD files

Bazel uses BUILD files to describe all the "targets" (things that get built) in a directory, and the rules for how to build them. The rules_rust rust_binary rule provides options you can set to control how things are built.

WARNING: Bazel aims for hermetic builds; code is built in a sandbox and configuration is stored in BUILD files, not environment variables. To pass rustc environment variables you set them in the BUILD file since setting environment variables won't pass through to the sandbox

Open up $HOME/repo/src/summation/BUILD in your favorite text editor and lets setup the build rules for our binary.

# This tells bazel to load the rust_binary rule from the rules_rust package
load("@rules_rust//rust:defs.bzl", "rust_binary")

rust_binary(
    #We are going to call the target/binary summation
    name = "executable",
    #The list of src files it needs (just main.rs)
    srcs = ["main.rs"],
    #Any libraries/crates it depends on, for now we'll leave this blank
    deps = [],
    #The crate_root file, this would default to main.rs but we put it in for clarity
    crate_root = "main.rs",
)

Let's also create our main.rs file:

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

Now lets try to build it by running:

bazel build :executable

And it fails! If you get what I got you'll see something like:

$ bazel build :executable
INFO: Repository local_config_cc instantiated at:
  /DEFAULT.WORKSPACE.SUFFIX:509:13: in <toplevel>
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/cc_configure.bzl:184:16: in cc_configure
Repository rule cc_autoconf defined at:
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/cc_configure.bzl:143:30: in <toplevel>
ERROR: An error occurred during the fetch of repository 'local_config_cc':
   Traceback (most recent call last):
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/cc_configure.bzl", line 125, column 33, in cc_autoconf_impl
                configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 349, column 17, in configure_unix_toolchain
                cc = find_cc(repository_ctx, overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 314, column 23, in find_cc
                cc = _find_generic(repository_ctx, "gcc", "CC", overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 310, column 32, in _find_generic
                auto_configure_fail(msg)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/lib_cc_configure.bzl", line 112, column 9, in auto_configure_fail
                fail("\n%sAuto-Configuration Error:%s %s\n" % (red, no_color, msg))
Error in fail:
Auto-Configuration Error: Cannot find gcc or CC; either correct your path or set the CC environment variable
ERROR: /DEFAULT.WORKSPACE.SUFFIX:509:13: fetching cc_autoconf rule //external:local_config_cc: Traceback (most recent call last):
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/cc_configure.bzl", line 125, column 33, in cc_autoconf_impl
                configure_unix_toolchain(repository_ctx, cpu_value, overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 349, column 17, in configure_unix_toolchain
                cc = find_cc(repository_ctx, overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 314, column 23, in find_cc
                cc = _find_generic(repository_ctx, "gcc", "CC", overriden_tools)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/unix_cc_configure.bzl", line 310, column 32, in _find_generic
                auto_configure_fail(msg)
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/cpp/lib_cc_configure.bzl", line 112, column 9, in auto_configure_fail
                fail("\n%sAuto-Configuration Error:%s %s\n" % (red, no_color, msg))
Error in fail:
Auto-Configuration Error: Cannot find gcc or CC; either correct your path or set the CC environment variable
INFO: Repository rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools instantiated at:
  /home/parallels/repo/WORKSPACE:18:25: in <toplevel>
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/repositories.bzl:203:14: in rust_register_toolchains
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/bazel_tools/tools/build_defs/repo/utils.bzl:233:18: in maybe
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/repositories.bzl:874:65: in rust_repository_set
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/repositories.bzl:496:36: in rust_toolchain_repository
Repository rule rust_toolchain_tools_repository defined at:
  /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/repositories.bzl:333:50: in <toplevel>
ERROR: /home/parallels/repo/src/summation/BUILD:4:12: //src/summation:executable depends on @local_config_cc//:cc-compiler-k8 in repository @local_config_cc which failed to fetch. no such package '@local_config_cc//':
Auto-Configuration Error: Cannot find gcc or CC; either correct your path or set the CC environment variable
ERROR: Analysis of target '//src/summation:executable' failed; build aborted: Analysis failed
INFO: Elapsed time: 8.170s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (93 packages loaded, 187 targets configured)
    Fetching https://static.rust-lang.org/dist/rustc-1.68.1-x86_64-unknown-linux-gnu.tar.gz

For most dependencies, you'll tell Bazel where to find them and it'll pull them down for you. One exception is the C++ toolchain, which rules_rust depends on. To get around this we can install the build-essential package on debian which includes gcc, g++, and libc.

sudo apt-get install build-essential

Once you've installed that let's try bazel build :executable again and see what happens

$ bazel build :executable
ERROR: /home/parallels/repo/src/summation/BUILD:4:12: in rust_binary rule //src/summation:executable:
Traceback (most recent call last):
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/private/rust.bzl", line 351, column 34, in _rust_binary_impl
                edition = get_edition(ctx.attr, toolchain, ctx.label),
        File "/home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/external/rules_rust/rust/private/rust.bzl", line 125, column 13, in get_edition
                fail("Attribute `edition` is required for {}.".format(label))
Error in fail: Attribute `edition` is required for @//src/summation:executable.
ERROR: /home/parallels/repo/src/summation/BUILD:4:12: Analysis of target '//src/summation:executable' failed
ERROR: Analysis of target '//src/summation:executable' failed; build aborted:
INFO: Elapsed time: 17.253s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (9 packages loaded, 325 targets configured)

This time it fails again saying we didn't set the edition. We could manually set the edition on the rule, but that's kind of annoying if we want to use the same edition across the repo, so lets open up our $HOME/repo/WORKSPACE file and specify an edition on the rust_register_toolchains() call by changing it to:

rust_register_toolchains(edition = "2021")

Hopefully the third time is a charm? Let's see what bazel build :executable does this time:

$ bazel build :executable
INFO: Analyzed target //src/summation:executable (1 packages loaded, 60 targets configured).
INFO: Found 1 target...
Target //src/summation:executable up-to-date:
  bazel-bin/src/summation/executable
INFO: Elapsed time: 16.776s, Critical Path: 5.44s
INFO: 94 processes: 91 internal, 3 linux-sandbox.
INFO: Build completed successfully, 94 total actions

Success! Now before we get up for a coffee break lets just make sure it actually runs. You can use the bazel run subcommand to run the binary.

$ bazel run :executable
INFO: Analyzed target //src/summation:executable (24 packages loaded, 172 targets configured).
INFO: Found 1 target...
Target //src/summation:executable up-to-date:
  bazel-bin/src/summation/executable
INFO: Elapsed time: 0.455s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/src/summation/executable
Hello world

It worked! One thing to note is this ran it inside bazel, and it output a bunch of bazel log messages. The second to last line of the output says INFO: Running command line: bazel-bin/src/summation/executable.

What is that? Let's go to the repo directory and see:

$ ls -l $HOME/repo
total 28
-rw-r--r-- 1 parallels parallels  798 Apr 20 14:40 WORKSPACE
-rw-r--r-- 1 parallels parallels  782 Apr 20 14:36 WORKSPACE~
lrwxrwxrwx 1 parallels parallels  123 Apr 20 14:40 bazel-bin -> /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/execroot/__main__/bazel-out/k8-fastbuild/bin
lrwxrwxrwx 1 parallels parallels  106 Apr 20 14:40 bazel-out -> /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/execroot/__main__/bazel-out
lrwxrwxrwx 1 parallels parallels   96 Apr 20 14:40 bazel-repo -> /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/execroot/__main__
lrwxrwxrwx 1 parallels parallels  128 Apr 20 14:40 bazel-testlogs -> /home/parallels/.cache/bazel/_bazel_parallels/db6a46b6510c6ee4dba1a9500854830b/execroot/__main__/bazel-out/k8-fastbuild/testlogs
drwxr-xr-x 3 parallels parallels 4096 Apr 20 14:37 src

You can see bazel created a bunch of symlinks to a mysterious .cache/bazel directory. When you run bazel, it caches build artifacts to avoid rebuilding things that didn't change, and these symlinks give us a way to access the artifacts bazel produces. If we want, we can run the binary directly by running $HOME/repo/bazel-bin/src/summation/executable. Let's try that:

$ $HOME/repo/bazel-bin/src/summation/executable
Hello world

Now we see the output of the binary without any of the bazel messages because we are invoking it directly.

With this we've successfully configured bazel to compile rust and get a binary out. When we're back from our coffee break we'll add in a C++ library, make a rust crate that links to it, and make our Hello world program call the rust crate.

Using Bazel with Rust, gRPC, protobuf, and Docker

Rust Hello World

Hello world rust binary

BUILD files