Set --sysroot in for rust_doc_test

rust/private/rustdoc.bzl

@@ -120,6 +120,19 @@ def rustdoc_compile_action(
         force_link = True,
     )
 
+    # Because rustdoc tests compile tests outside of the sandbox, the sysroot
+    # must be updated to the `short_path` equivilant as it will now be
+    # a part of runfiles.
+    if is_test:
+        # Determine what the `short_path` would be for the sysroot
+        sysroot, _, _ = toolchain.sysroot_anchor.short_path.rpartition("/")
+
+        env.update({"SYSROOT": "${{pwd}}/{}".format(sysroot)})
+
+        # `rustdoc` does not support the SYSROOT environment variable. To account
+        # for this, the flag must be explicitly passed to the `rustdoc` binary.
+        args.rustc_flags.add("--sysroot=${{pwd}}/{}".format(sysroot))
+
     return struct(
         executable = ctx.executable._process_wrapper,
         inputs = depset([crate_info.output], transitive = [compile_inputs]),

rust/toolchain.bzl

@@ -293,6 +293,7 @@ def _rust_toolchain_impl(ctx):
         rust_std_paths = depset([file.dirname for file in sysroot.rust_std.to_list()]),
         rust_lib = sysroot.rust_std,  # `rust_lib` is deprecated and only exists for legacy support.
         sysroot = sysroot.sysroot_anchor.dirname,
+        sysroot_anchor = sysroot.sysroot_anchor,
         binary_ext = ctx.attr.binary_ext,
         staticlib_ext = ctx.attr.staticlib_ext,
         dylib_ext = ctx.attr.dylib_ext,