Added TemplateVariableInfo to rust_toolchain

[UebelAndre]

See bazelbuild/bazel#14009 (comment)

[illicitonion]

LGTM - thanks!

[uhthomas]

I need to use TemplateVariableInfo provider for #1287, but this PR does not return the provider from the implementation function. This means the make variables are not substituted for rules which use the toolchain.

(10:58:10) ERROR: /private/var/tmp/_bazel_thomas/796dd52d4d981101108548a9446ddca6/external/crate_index/BUILD.bazel:11:21: in args attribute of cargo_bazel rule @crate_index//:repin: $(CARGO) not defined. Since this rule was created by the macro 'cargo_bazel_generate', the error might have been caused by the macro implementation

The provider needs to be returned to allow this to work properly. For example:

diff --git a/rust/toolchain.bzl b/rust/toolchain.bzl
index 90dea6e..caa3627 100644
--- a/rust/toolchain.bzl
+++ b/rust/toolchain.bzl
@@ -518,7 +518,7 @@ def _rust_toolchain_impl(ctx):
         _rename_first_party_crates = rename_first_party_crates,
         _third_party_dir = third_party_dir,
     )
-    return [toolchain]
+    return [toolchain, toolchain.make_variables]
 
 rust_toolchain = rule(
     implementation = _rust_toolchain_impl,

I'll open a PR :)

[uhthomas]

See #1412