Create self-contained cxx_zig_toolchain (#22)

Summary:
Closes #20

Adds `prelude//toolchains/cxx/zig` with a C/C++ toolchain based on `zig cc`.
The doc-string of the added module contains a description and examples
configurations.

Also adds an example to the repository that builds a C++ library and a binary
depending on that library using this toolchain.

Note, for that example to work in the open source buck2 tree the following additional patch needs to be applied:
```diff
 diff --git a/.buckconfig b/.buckconfig
deleted file mode 100644
index c32af92..0000000
 --- a/.buckconfig
+++ /dev/null
@@ -1,19 +0,0 @@
-[repositories]
-root = .
-prelude = prelude
-ovr_config = prelude
-shim = shim
-toolchains = shim
-fbcode = shim
-fbcode_macros = shim
-fbsource = shim
-buck = shim
-
-[buildfile]
-name = TARGETS
-
-[build]
-execution_platforms = ovr_config//platforms:default
-
-[parser]
-target_platform_detector_spec = target://...->ovr_config//platforms:default
 diff --git a/prelude/cxx/tools/TARGETS.v2 b/prelude/cxx/tools/TARGETS.v2
index 2030d2f..5db1689 100644
 --- a/prelude/cxx/tools/TARGETS.v2
+++ b/prelude/cxx/tools/TARGETS.v2
@@ -1,4 +1,3 @@
-load("fbcode_macros//build_defs/lib:python_common.bzl", "get_ldflags", "get_strip_mode")
 load(":defs.bzl", "cxx_hacks", "omnibus_environment")

 prelude = native
@@ -538,7 +537,7 @@ omnibus_environment(
     # We override the binary-level ldflags with library level ldfalgs for
     # shared roots. We include 2 important things: stripping binaries, and not
     # discarding GPU code.
-    shared_root_ld_flags = get_ldflags("", "", "omnibus_root", get_strip_mode("", ""), "") + [
+    shared_root_ld_flags = [
         "-Wl,--no-discard-section=.nv_fatbin",
         "-Wl,--no-discard-section=.nvFatBinSegment",
         # Reorder nv_fatbin after the bss section to avoid overflow
```

Pull Request resolved: #22

Reviewed By: arlyon

Differential Revision: D40179209

Pulled By: arlyon

fbshipit-source-id: d334f2ad4563281f78ba3f479b8defd09616cabf

examples/README.md

@@ -18,3 +18,7 @@ Preludeless examples for those wanting to use buck2 with their own
 rules and toolchains. In here you can learn about how BUILD
 files interact with rules, and how the provider abstraction can be
 used to encapsulate build logic.
+
+## toolchains
+
+Examples testing the various toolchains included in the prelude.

examples/toolchains/cxx_zig_toolchain/.buckconfig

@@ -0,0 +1,18 @@
+[repositories]
+self = .
+ovr_config = prelude
+prelude = prelude
+toolchains = toolchains
+buck = no
+fbcode = no
+fbcode_macros = no
+fbsource = no
+
+[buildfile]
+name = BUILD
+
+[build]
+execution_platforms = ovr_config//platforms:default
+
+[parser]
+target_platform_detector_spec = target://...->ovr_config//platforms:default

examples/toolchains/cxx_zig_toolchain/BUILD

@@ -0,0 +1,13 @@
+load("@prelude//platforms:defs.bzl", "execution_platform")
+
+cxx_library(
+    name = "lib",
+    srcs = ["lib.cc"],
+    headers = ["lib.h"],
+)
+
+cxx_binary(
+    name = "main",
+    srcs = ["main.cc"],
+    deps = [":lib"],
+)

examples/toolchains/cxx_zig_toolchain/lib.cc

@@ -0,0 +1,6 @@
+#include "lib.h"
+#include <iostream>
+
+void hello() {
+  std::cout << "Hello World!\n";
+}

examples/toolchains/cxx_zig_toolchain/lib.h

@@ -0,0 +1 @@
+void hello();

examples/toolchains/cxx_zig_toolchain/main.cc

@@ -0,0 +1,5 @@
+#include "lib.h"
+
+int main() {
+  hello();
+}

examples/toolchains/cxx_zig_toolchain/toolchains/.buckconfig

@@ -0,0 +1,6 @@
+[repositories]
+toolchains = .
+prelude = ../prelude
+
+[buildfile]
+name = BUILD

examples/toolchains/cxx_zig_toolchain/toolchains/BUILD

@@ -0,0 +1,19 @@
+load("@prelude//toolchains/cxx/zig:defs.bzl", "download_zig_distribution", "cxx_zig_toolchain")
+# TODO Replace by prelude toolchain once available
+load("@toolchains//python:defs.bzl", "system_python_bootstrap_toolchain")
+
+download_zig_distribution(
+    name = "zig",
+    version = "0.9.1",
+)
+
+cxx_zig_toolchain(
+    name = "cxx",
+    distribution = ":zig",
+    visibility = ["PUBLIC"],
+)
+
+system_python_bootstrap_toolchain(
+    name = "python_bootstrap",
+    visibility = ["PUBLIC"],
+)

examples/toolchains/cxx_zig_toolchain/toolchains/README.md

@@ -0,0 +1,32 @@
+This example tests the `zig cc` based self-contained C/C++ toolchain.
+
+To build it within the open source tree of buck2 to you need to
+* Create a symlink for the prelude
+  ```
+  ln -s ../../../prelude prelude
+  ```
+* Remove the top-level `.buckconfig`
+  ```
+  rm ../../../.buckconfig
+  ```
+* Apply the following patch to the prelude
+  ```
+  diff --git a/prelude/cxx/tools/TARGETS.v2 b/prelude/cxx/tools/TARGETS.v2
+  index 2030d2f..5db1689 100644
+  --- a/prelude/cxx/tools/TARGETS.v2
+  +++ b/prelude/cxx/tools/TARGETS.v2
+  @@ -1,4 +1,3 @@
+  -load("@fbcode_macros//build_defs/lib:python_common.bzl", "get_ldflags", "get_strip_mode")
+   load(":defs.bzl", "cxx_hacks", "omnibus_environment")
+
+   prelude = native
+  @@ -538,7 +537,7 @@ omnibus_environment(
+       # We override the binary-level ldflags with library level ldfalgs for
+       # shared roots. We include 2 important things: stripping binaries, and not
+       # discarding GPU code.
+  -    shared_root_ld_flags = get_ldflags("", "", "omnibus_root", get_strip_mode("", ""), "") + [
+  +    shared_root_ld_flags = [
+           "-Wl,--no-discard-section=.nv_fatbin",
+           "-Wl,--no-discard-section=.nvFatBinSegment",
+           # Reorder nv_fatbin after the bss section to avoid overflow
+  ```

examples/toolchains/cxx_zig_toolchain/toolchains/python/defs.bzl

@@ -0,0 +1,19 @@
+load(
+    "@prelude//python_bootstrap:python_bootstrap.bzl",
+    "PythonBootstrapToolchainInfo",
+)
+
+def _system_python_bootstrap_toolchain(_ctx):
+    return [
+        DefaultInfo(),
+        PythonBootstrapToolchainInfo(
+            interpreter = RunInfo(args = ["python3"]),
+        ),
+    ]
+
+system_python_bootstrap_toolchain = rule(
+    impl = _system_python_bootstrap_toolchain,
+    attrs = {
+    },
+    is_toolchain_rule = True,
+)