hack in Windows support

This is an ugly hack, and hopefully can be done in a nicer way.

worker/bootstrap.bzl

@@ -3,23 +3,49 @@ A simplified rust_binary implementation based on cargo so we can compile
 the worker binary.
 """
 
+load("@io_bazel_rules_rust//rust:private/rustc.bzl", "get_cc_toolchain", "get_linker_and_args")
+
+def _rewrite_to_unix_path(path):
+    drive = path[0]
+    return "/" + drive + path[2:].replace("\\", "/")
+
 def _rust_binary_impl(ctx):
     toolchain = ctx.toolchains["@io_bazel_rules_rust//rust:toolchain"]
     output = ctx.actions.declare_file(ctx.label.name + toolchain.binary_ext)
     cache_dir = ctx.actions.declare_directory(ctx.label.name + "_cache")
     folder = ctx.files.srcs[0].dirname
     rust_bin = toolchain.cargo.dirname
 
-    # + [toolchain.rust_lib, toolchain.rustc_lib]
+    # get linker env for building on Windows
+    cc_toolchain, feature_configuration = get_cc_toolchain(ctx)
+    _, _, link_env = get_linker_and_args(ctx, cc_toolchain, feature_configuration, depset())
+
+    # this is an awful hack to extract the path to MSVC on Windows, so we can inject it
+    # into the path after use_default_shell_env has set it up. Without default_shell_env,
+    # compilation fails on other platforms because `cc` is not in the path. There is no doubt
+    # a better way to handle this.
+    path = link_env.get("PATH")
+    if path:
+        # cc toolchain has placed it at head of path
+        msvc_path = path.split(";")[0]
+        msvc_path = _rewrite_to_unix_path(msvc_path) + ":"
+    else:
+        msvc_path = ""
+
+    lib = link_env.get("LIB", "")
+    if lib:
+        lib = _rewrite_to_unix_path(lib)
+
     ctx.actions.run_shell(
         inputs = ctx.files.srcs,
         outputs = [output, cache_dir],
-        #        executable = toolchain.cargo,
         command = """\
 export RUSTC="$(pwd)/{rustc}"; \
 export CARGO="$(pwd)/{cargo}"; \
 export OUTPUT="$(pwd)/{output}"; \
 export CARGO_HOME="$(pwd)/{cache_dir}"; \
+export PATH="{msvc_path}$PATH"; \
+export LIB="{lib}"; \
 cd {folder} && \
 "$CARGO" build -q --release && \
 mv target/release/rustc-worker "$OUTPUT" """.format(
@@ -29,6 +55,8 @@ mv target/release/rustc-worker "$OUTPUT" """.format(
             rustc = toolchain.rustc.path,
             output = output.path,
             cache_dir = cache_dir.path,
+            msvc_path = msvc_path,
+            lib = lib,
         ),
         tools = [toolchain.cargo, toolchain.rustc] + toolchain.rust_lib.files.to_list() + toolchain.rustc_lib.files.to_list(),
         use_default_shell_env = True,
@@ -45,6 +73,8 @@ rust_cargo_binary = rule(
             default = "@bazel_tools//tools/cpp:current_cc_toolchain",
         ),
     },
+    fragments = ["cpp"],
+    host_fragments = ["cpp"],
     executable = True,
     toolchains = [
         "@bazel_tools//tools/cpp:toolchain_type",