chore: setup a buildifier pre-commit hook (#661)

Matches the one just landed in rules_apple: bazelbuild/rules_apple@61bc7c0

Author: alexeagle

.bazelci/config.yaml

@@ -187,4 +187,5 @@ tasks:
 
 buildifier:
   version: latest
+  # keep this argument in sync with .pre-commit-config.yaml
   warnings: "all"

.pre-commit-config.yaml

@@ -0,0 +1,13 @@
+# See CONTRIBUTING.md for instructions.
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+  - repo: https://github.com/keith/pre-commit-buildifier
+    rev: 4.0.1.1
+    hooks:
+      - id: buildifier
+        args: &args
+          # Keep this argument in sync with .bazelci/config.yaml
+          - --warnings=all
+      - id: buildifier-lint
+        args: *args

CONTRIBUTING.md

@@ -3,6 +3,19 @@
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
+## Formatting
+
+Starlark files should be formatted by buildifier.
+We suggest using a pre-commit hook to automate this.
+First [install pre-commit](https://pre-commit.com/#installation),
+then run
+
+```shell
+pre-commit install
+```
+
+Otherwise the Buildkite CI will yell at you about formatting/linting violations.
+
 ## Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License

examples/cmake_crosstool/compilers/linaro_linux_gcc_5.3.1.BUILD

@@ -1,81 +1,81 @@
-package(default_visibility = ['//visibility:public'])
+package(default_visibility = ["//visibility:public"])
 
 filegroup(
-  name = 'gcc',
-  srcs = [
-    'bin/arm-linux-gnueabihf-gcc',
-  ],
+    name = "gcc",
+    srcs = [
+        "bin/arm-linux-gnueabihf-gcc",
+    ],
 )
 
 filegroup(
-  name = 'ar',
-  srcs = [
-    'bin/arm-linux-gnueabihf-ar',
-  ],
+    name = "ar",
+    srcs = [
+        "bin/arm-linux-gnueabihf-ar",
+    ],
 )
 
 filegroup(
-  name = 'ld',
-  srcs = [
-    'bin/arm-linux-gnueabihf-ld',
-  ],
+    name = "ld",
+    srcs = [
+        "bin/arm-linux-gnueabihf-ld",
+    ],
 )
 
 filegroup(
-  name = 'nm',
-  srcs = [
-    'bin/arm-linux-gnueabihf-nm',
-  ],
+    name = "nm",
+    srcs = [
+        "bin/arm-linux-gnueabihf-nm",
+    ],
 )
 
 filegroup(
-  name = 'objcopy',
-  srcs = [
-    'bin/arm-linux-gnueabihf-objcopy',
-  ],
+    name = "objcopy",
+    srcs = [
+        "bin/arm-linux-gnueabihf-objcopy",
+    ],
 )
 
 filegroup(
-  name = 'objdump',
-  srcs = [
-    'bin/arm-linux-gnueabihf-objdump',
-  ],
+    name = "objdump",
+    srcs = [
+        "bin/arm-linux-gnueabihf-objdump",
+    ],
 )
 
 filegroup(
-  name = 'strip',
-  srcs = [
-    'bin/arm-linux-gnueabihf-strip',
-  ],
+    name = "strip",
+    srcs = [
+        "bin/arm-linux-gnueabihf-strip",
+    ],
 )
 
 filegroup(
-  name = 'as',
-  srcs = [
-    'bin/arm-linux-gnueabihf-as',
-  ],
+    name = "as",
+    srcs = [
+        "bin/arm-linux-gnueabihf-as",
+    ],
 )
 
 filegroup(
-  name = 'compiler_pieces',
-  srcs = glob([
-    'arm-linux-gnueabihf/**',
-    'libexec/**',
-    'lib/gcc/arm-linux-gnueabihf/**',
-    'include/**',
-  ]),
+    name = "compiler_pieces",
+    srcs = glob([
+        "arm-linux-gnueabihf/**",
+        "libexec/**",
+        "lib/gcc/arm-linux-gnueabihf/**",
+        "include/**",
+    ]),
 )
 
 filegroup(
-  name = 'compiler_components',
-  srcs = [
-    ':gcc',
-    ':ar',
-    ':ld',
-    ':nm',
-    ':objcopy',
-    ':objdump',
-    ':strip',
-    ':as',
-  ],
+    name = "compiler_components",
+    srcs = [
+        ":ar",
+        ":as",
+        ":gcc",
+        ":ld",
+        ":nm",
+        ":objcopy",
+        ":objdump",
+        ":strip",
+    ],
 )