feat: Strip debug info from opt builds

Attempts to follow the cargo proposal linked below to remove debug info
for release builds. Furthermore this should uphold the expected behavior
of bazel for cc builds which automatically strips debug symbols for
optimized builds.

rust-lang/cargo#4122 (comment)

Author: matte1

rust/private/rustc.bzl

@@ -959,6 +959,7 @@ def construct_arguments(
     compilation_mode = get_compilation_mode_opts(ctx, toolchain)
     rustc_flags.add(compilation_mode.opt_level, format = "--codegen=opt-level=%s")
     rustc_flags.add(compilation_mode.debug_info, format = "--codegen=debuginfo=%s")
+    rustc_flags.add(compilation_mode.strip_level, format = "--codegen=strip=%s")
 
     # For determinism to help with build distribution and such
     if remap_path_prefix != None:

rust/toolchain.bzl

@@ -490,11 +490,13 @@ def _rust_toolchain_impl(ctx):
         list: A list containing the target's toolchain Provider info
     """
     compilation_mode_opts = {}
-    for k, v in ctx.attr.opt_level.items():
+    for k, opt_level in ctx.attr.opt_level.items():
         if not k in ctx.attr.debug_info:
             fail("Compilation mode {} is not defined in debug_info but is defined opt_level".format(k))
-        compilation_mode_opts[k] = struct(debug_info = ctx.attr.debug_info[k], opt_level = v)
-    for k, v in ctx.attr.debug_info.items():
+        if not k in ctx.attr.strip_level:
+            fail("Compilation mode {} is not defined in strip_level but is defined opt_level".format(k))
+        compilation_mode_opts[k] = struct(debug_info = ctx.attr.debug_info[k], opt_level = opt_level, strip_level = ctx.attr.strip_level[k])
+    for k in ctx.attr.debug_info.keys():
         if not k in ctx.attr.opt_level:
             fail("Compilation mode {} is not defined in opt_level but is defined debug_info".format(k))
 
@@ -813,6 +815,14 @@ rust_toolchain = rule(
             ),
             mandatory = True,
         ),
+        "strip_level": attr.string_dict(
+            doc = "Rustc strip levels.",
+            default = {
+                "dbg": "none",
+                "fastbuild": "none",
+                "opt": "debuginfo",
+            },
+        ),
         "target_json": attr.string(
             doc = ("Override the target_triple with a custom target specification. " +
                    "For more details see: https://doc.rust-lang.org/rustc/targets/custom.html"),