Add test that rustfmt binary works

test/rustfmt/BUILD

@@ -0,0 +1,17 @@
+load("@io_bazel_rules_rust//test/rustfmt:rustfmt_generator.bzl", "rustfmt_generator")
+
+rustfmt_generator(
+    name = "formatted",
+    src = ":unformatted.rs",
+)
+
+sh_test(
+    name = "rustfmt_test",
+    size = "small",
+    data = [
+        ":formatted.rs",
+        ":unformatted.rs",
+    ],
+    srcs = [":rustfmt_test.sh"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)

test/rustfmt/rustfmt_generator.bzl

@@ -0,0 +1,29 @@
+load("@io_bazel_rules_rust//rust:private/utils.bzl", "find_toolchain")
+
+def _rustfmt_generator_impl(ctx):
+    toolchain = find_toolchain(ctx)
+    rustfmt_bin = toolchain.rustfmt
+    output = ctx.outputs.out
+
+    ctx.actions.run_shell(
+        inputs = depset([rustfmt_bin, ctx.file.src]),
+        outputs = [output],
+        command = "{} --emit stdout --quiet {} > {}".format(rustfmt_bin.path, ctx.file.src.path, output.path),
+        tools = [rustfmt_bin],
+    )
+
+
+rustfmt_generator = rule(
+    _rustfmt_generator_impl,
+    doc = "Given an unformatted Rust source file, output the file after being run through rustfmt.",
+    attrs = {
+        "src": attr.label(
+            doc = "The file to be formatted.",
+            allow_single_file = True,
+        )
+    },
+    outputs = {"out": "%{name}.rs"},
+    toolchains = [
+        "@io_bazel_rules_rust//rust:toolchain",
+    ],
+)

test/rustfmt/rustfmt_test.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -euxo pipefail
+
+formatted="$(rlocation io_bazel_rules_rust/test/rustfmt/formatted.rs)"
+unformatted="$(rlocation io_bazel_rules_rust/test/rustfmt/unformatted.rs)"
+
+# Ensure that the file was formatted
+! diff "$unformatted" "$formatted"

test/rustfmt/unformatted.rs

@@ -0,0 +1 @@
+fn example(){println!("test");}