bazel: don't use cargo in $PATH, make the build hermetic.

Background:
tools/bazel/vendor is just running 'cargo', hoping to find it somewhere
in $PATH.

Problem:
rules_rust try to be hermetic, and not use the cargo of the system, but
the cargo of the toolchain installed.
This means that if the cargo on the system is different from the cargo
used by cxx.rs, or the system does not have a cargo at all, bad things
will happen when using cxx.rs.

In this change:
use the recommended way to find/install the tool. Thanks to Andre in the
bazel rust slack channel for pointers and help.

Author: ccontavalli

WORKSPACE

@@ -1,7 +1,6 @@
 workspace(name = "cxx.rs")
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-load("//tools/bazel:vendor.bzl", "vendor")
 
 http_archive(
     name = "rules_rust",
@@ -15,12 +14,17 @@ http_archive(
 
 load("@rules_rust//rust:repositories.bzl", "rust_repositories")
 
+RUST_VERSION = "1.51.0"
+
 rust_repositories(
     edition = "2018",
-    version = "1.51.0",
+    version = RUST_VERSION,
 )
 
+load("//tools/bazel:vendor.bzl", "vendor")
+
 vendor(
     name = "third-party",
     lockfile = "//third-party:Cargo.lock",
+    cargo_version = RUST_VERSION,
 )

tools/bazel/vendor.bzl

@@ -2,6 +2,8 @@
 of a crate in the current workspace.
 """
 
+load("@rules_rust//rust:repositories.bzl", "load_arbitrary_tool", "DEFAULT_RUST_VERSION")
+
 def _impl(repository_ctx):
     # Link cxx repository into @third-party.
     lockfile = repository_ctx.path(repository_ctx.attr.lockfile)
@@ -14,8 +16,27 @@ def _impl(repository_ctx):
     root_lockfile = repository_ctx.path("workspace/Cargo.lock")
     _copy_file(repository_ctx, src = vendor_lockfile, dst = root_lockfile)
 
-    # Execute cargo vendor.
-    cmd = ["cargo", "vendor", "--versioned-dirs", "third-party/vendor"]
+    # Figure out which version of cargo to use.
+    if repository_ctx.attr.target_triple:
+        target_triple = repository_ctx.attr.target_triple
+    elif "mac" in repository_ctx.os.name:
+        target_triple = "x86_64-apple-darwin"
+    elif "windows" in repository_ctx.os.name:
+        target_triple = "x86_64-pc-windows-msvc"
+    else:
+        target_triple = "x86_64-unknown-linux-gnu"
+
+    # Download cargo.
+    load_arbitrary_tool(
+        ctx = repository_ctx,
+        tool_name = "cargo",
+        tool_subdirectories = ["cargo"],
+        version = repository_ctx.attr.cargo_version,
+        iso_date = repository_ctx.attr.cargo_iso_date,
+        target_triple = target_triple,
+    )
+    
+    cmd = ["{}/bin/cargo".format(repository_ctx.path(".")), "vendor", "--versioned-dirs", "third-party/vendor"]
     result = repository_ctx.execute(
         cmd,
         quiet = True,
@@ -54,6 +75,16 @@ def _log_cargo_vendor(repository_ctx, result):
 vendor = repository_rule(
     doc = "A rule used to vendor the dependencies of a crate in the current workspace",
     attrs = {
+        "cargo_version": attr.string(
+            doc = "The version of cargo to use",
+            default = DEFAULT_RUST_VERSION,
+        ),
+        "cargo_iso_date": attr.string(
+            doc = "The date of the tool (or None, if the version is a specific version)",
+        ),
+        "target_triple": attr.string(
+            doc = "The target triple of the cargo binary to download",
+        ),
         "lockfile": attr.label(
             doc = "A lockfile providing the set of crates to vendor",
         ),