Use host Cargo label in host tools repo (#127)

Author: dzbarsky

rs/private/host_tools_repository.bzl

@@ -1,20 +1,12 @@
 def _host_tools_repository_impl(rctx):
-    defs_bzl_content = """\
-RS_HOST_CARGO_LABEL = Label("@{host_cargo_repo}//:bin/cargo{binary_suffix}")
-""".format(
-        host_cargo_repo = rctx.attr.host_cargo_repo,
-        binary_suffix = rctx.attr.binary_suffix,
-    )
-
-    rctx.file("defs.bzl", defs_bzl_content)
+    rctx.file("defs.bzl", 'RS_HOST_CARGO_LABEL = Label("%s")' % rctx.attr.host_cargo)
     rctx.file("BUILD.bazel", 'exports_files(["defs.bzl"])')
 
     return rctx.repo_metadata(reproducible = True)
 
 host_tools_repository = repository_rule(
     implementation = _host_tools_repository_impl,
     attrs = {
-        "host_cargo_repo": attr.string(mandatory = True),
-        "binary_suffix": attr.string(mandatory = True),
+        "host_cargo": attr.label(allow_single_file = True, mandatory = True),
     },
 )

rs/toolchains/module_extension.bzl

@@ -292,10 +292,16 @@ def _toolchains_impl(mctx):
                 sha256 = _sha_for("rust-analyzer", base_version, iso_date, exec_triple),
             )
 
+    if host_cargo_repo == None:
+        fail("Could not find host Cargo repository for {}-{}".format(host_os, host_arch))
+    host_cargo = "@{}//:bin/cargo{}".format(
+        host_cargo_repo,
+        ".exe" if host_os == "windows" else "",
+    )
+
     host_tools_repository(
         name = "rs_rust_host_tools",
-        host_cargo_repo = host_cargo_repo,
-        binary_suffix = ".exe" if host_os == "windows" else "",
+        host_cargo = host_cargo,
     )
 
     # `rs_rust_host_tools` is an implementation detail of rules_rs itself.