Making rust_std attr in rust_toolchain mandatory (#1984)

`rust_toolchain`'s `rust_std` attribute should be mandatory because when it's empty, [this line](https://github.com/bazelbuild/rules_rust/blob/56e7604879ac74cb8c3d65cbb5d41611f7424977/rust/toolchain.bzl#L147) will always fail with

```
if not rust_common.stdlib_info in rust_std:
Error: unsupported binary operation: Provider in NoneType
```
3 files changed
tree: 43a8972c0c26e5893dbb470751c409fd334152f4
  1. .bazelci/
  2. .github/
  3. bindgen/
  4. cargo/
  5. crate_universe/
  6. docs/
  7. examples/
  8. proto/
  9. rust/
  10. test/
  11. tools/
  12. util/
  13. wasm_bindgen/
  14. .bazelignore
  15. .bazelrc
  16. .clang-format
  17. .gitattributes
  18. .gitignore
  19. .rustfmt.toml
  20. ARCHITECTURE.md
  21. AUTHORS
  22. BUILD.bazel
  23. CODEOWNERS
  24. COMPATIBILITY.md
  25. CONTRIBUTING.md
  26. CONTRIBUTORS
  27. LICENSE.txt
  28. MODULE.bazel
  29. README.md
  30. version.bzl
  31. WORKSPACE.bazel
README.md

Rust Rules

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Overview

This repository provides rules for building Rust projects with Bazel.

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