将Nlohmann/JSON构建为bazel库会出现“nothing to build”错误

Question

我有一个非常简单的bazel项目，试图将https://github.com/nlohmann/json添加为依赖项。为此，我在本地克隆了JSONrepo，并在repo的根目录中添加了build文件，以生成一个cc_library，其中包含一个includejson.hpp文件。但当我建造那个的时候，它总是抱怨没有什么可建造的。

├── json
│   ├── BUILD
│   ├── // all files under nlohmann/json repo.
├── myproject
│   ├── BUILD
│   └── Main.cpp
└── WORKSPACE

JSON/Build：

cc_library(
    name = "json",
    hdrs = ["single_include/nlohmann/json.hpp"],
    includes = ["json"],
    visibility = ["//visibility:public"],
)

上面的构建成功了，但是没有生成库。注意输出中的'(nothing to build)'消息。

bazel build :json
INFO: Analyzed target //json:json (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //json:json up-to-date (nothing to build)
INFO: Elapsed time: 0.065s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

myProject/main.cpp：

#include <single_include/nlohmann/json.hpp>

int main() {
    ...
}

cc_binary(
  name = "main",
  srcs = ["Main.cpp"],
  deps = [ "//json:json"]
)

myproject/Main.cpp:1:44: fatal error: single_include/nlohmann/json.hpp: No such file or directory
compilation terminated.

你知道我错过了什么吗？我的目标是将JSONrepo用作我的bazel项目MyProject中的依赖项。

Answer 1

最好使用http_repository/git_repository机制获取外部依赖项，因为它们不会扰乱repo的历史记录，更容易更新，而且如果不构建依赖于它的任何东西，甚至不会获取它们。

# /WORKSPACE file
http_archive(
    name = "com_github_nlohmann_json",
    build_file = "//third_party:json.BUILD", # see below
    sha256 = "4cf0df69731494668bdd6460ed8cb269b68de9c19ad8c27abc24cd72605b2d5b",
    strip_prefix = "json-3.9.1",
    urls = ["https://github.com/nlohmann/json/archive/v3.9.1.tar.gz"],
)

# /third_party/json.BUILD file
package(default_visibility = ["//visibility:public"])

load("@rules_cc//cc:defs.bzl", "cc_library")

licenses(["notice"]) # MIT license

cc_library(
    name = "json",
    hdrs = ["single_include/nlohmann/json.hpp"],
    strip_include_prefix = "single_include/",
)

后来呢：

cc_binary(
  name = "main",
  srcs = ["Main.cpp"],
  deps = [ "@com_github_nlohmann_json//:json"]
)

// Main.cpp file
#include "nlohmann/json.hpp"