1 [build constraints]: https://golang.org/pkg/go/build/#hdr-Build_Constraints
2 [select]: https://docs.bazel.build/versions/master/be/functions.html#select
3 [config_setting]: https://docs.bazel.build/versions/master/be/general.html#config_setting
4 [Gazelle]: https://github.com/bazelbuild/bazel-gazelle
5
6
7## Platform-specific dependencies
8
9When cross-compiling, you may have some platform-specific sources and
10dependencies. Source files from all platforms can be mixed freely in a single
11`srcs` list. Source files are filtered using [build constraints] (filename
12suffixes and `+build` tags) before being passed to the compiler.
13
14Platform-specific dependencies are another story. For example, if you are
15building a binary for Linux, and it has dependency that should only be built
16when targeting Windows, you will need to filter it out using Bazel [select]
17expressions:
18
19``` bzl
20go_binary(
21 name = "cmd",
22 srcs = [
23 "foo_linux.go",
24 "foo_windows.go",
25 ],
26 deps = [
27 # platform agnostic dependencies
28 "//bar",
29 ] + select({
30 # OS-specific dependencies
31 "@io_bazel_rules_go//go/platform:linux": [
32 "//baz_linux",
33 ],
34 "@io_bazel_rules_go//go/platform:windows": [
35 "//quux_windows",
36 ],
37 "//conditions:default": [],
38 }),
39)
40```
41
42`select` accepts a dictionary argument. The keys are labels that reference [config_setting] rules.
43The values are lists of labels. Exactly one of these
44lists will be selected, depending on the target configuration. rules_go has
45pre-declared `config_setting` rules for each OS, architecture, and
46OS-architecture pair. For a full list, run this command:
47
48``` bash
49$ bazel query 'kind(config_setting, @io_bazel_rules_go//go/platform:all)'
50```
51
52[Gazelle] will generate dependencies in this format automatically.
53
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