Add build scripts HOST and OPT_LEVEL env vars (#336)

diff --git a/cargo/cargo_build_script.bzl b/cargo/cargo_build_script.bzl
index 3d80fda..022970d 100644
--- a/cargo/cargo_build_script.bzl
+++ b/cargo/cargo_build_script.bzl
@@ -1,4 +1,4 @@
-load("@io_bazel_rules_rust//rust:private/rustc.bzl", "BuildInfo", "rustc_compile_action")
+load("@io_bazel_rules_rust//rust:private/rustc.bzl", "BuildInfo", "get_compilation_mode_opts")
 load("@io_bazel_rules_rust//rust:private/utils.bzl", "find_toolchain")
 load("@io_bazel_rules_rust//rust:rust.bzl", "rust_binary")
 
@@ -8,13 +8,16 @@
     env_out = ctx.actions.declare_file(ctx.label.name + ".env")
     flags_out = ctx.actions.declare_file(ctx.label.name + ".flags")
     manifest_dir = "%s.runfiles/%s" % (script.path, ctx.label.workspace_name or ctx.workspace_name)
+    compilation_mode_opt_level = get_compilation_mode_opts(ctx, toolchain).opt_level
     env = {
-        "CARGO_MANIFEST_DIR": manifest_dir,
-        "RUSTC": toolchain.rustc.path,
-        "TARGET": toolchain.target_triple,
         "CARGO_CFG_TARGET_ARCH": toolchain.target_arch,
+        "CARGO_MANIFEST_DIR": manifest_dir,
+        "HOST": toolchain.exec_triple,
+        "OPT_LEVEL": compilation_mode_opt_level,
         "OUT_DIR": out_dir.path,
+        "RUSTC": toolchain.rustc.path,
         "RUST_BACKTRACE": "full",
+        "TARGET": toolchain.target_triple,
     }
 
     for f in ctx.attr.crate_features:
diff --git a/rust/private/rustc.bzl b/rust/private/rustc.bzl
index d54eb1a..50e9b4c 100644
--- a/rust/private/rustc.bzl
+++ b/rust/private/rustc.bzl
@@ -86,7 +86,7 @@
         "CARGO_CFG_TARGET_ARCH": toolchain.target_arch,
     }
 
-def _get_compilation_mode_opts(ctx, toolchain):
+def get_compilation_mode_opts(ctx, toolchain):
     comp_mode = ctx.var["COMPILATION_MODE"]
     if not comp_mode in toolchain.compilation_mode_opts:
         fail("Unrecognized compilation mode {} for toolchain.".format(comp_mode))
@@ -296,7 +296,7 @@
     args.add("--out-dir=" + output_dir)
     args.add("--codegen=extra-filename=" + extra_filename)
 
-    compilation_mode = _get_compilation_mode_opts(ctx, toolchain)
+    compilation_mode = get_compilation_mode_opts(ctx, toolchain)
     args.add("--codegen=opt-level=" + compilation_mode.opt_level)
     args.add("--codegen=debuginfo=" + compilation_mode.debug_info)