Added rust_unpretty_aspect and rust_unpretty rules

[UebelAndre]

Proposal for #1642
Duplicates #1643 (special thanks to @freeformstu)

Summary

Rustc can be used to expand all macros so that you can inspect the generated source files easier.

This feature is enabled via -Zunpretty={mode}. The -Z flag is only available in the nightly
version of rustc (rust-lang/rust#43364).

Unprettying

Build and test your targets normally.

bazel build //:ok_binary   
INFO: Analyzed target //:ok_binary (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //:ok_binary up-to-date:
  bazel-bin/ok_binary
INFO: Elapsed time: 0.081s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

Use the aspect to generate the expanded files in as a one-off build.

(.bazelrc)

# Enable unpretty for all targets in the workspace
build:unpretty --aspects=@rules_rust//rust:defs.bzl%rust_unpretty_aspect
build:unpretty --output_groups=+rust_unpretty

# `unpretty` requires the nightly toolchain. See tracking issue:
# https://github.com/rust-lang/rust/issues/43364
build:unpretty --@rules_rust//rust/toolchain/channel=nightly

bazel build --config=unpretty //:ok_binary
INFO: Analyzed target //:ok_binary (1 packages loaded, 2 targets configured).
INFO: Found 1 target...
Aspect @rules_rust//rust/private:unpretty.bzl%rust_unpretty_aspect of //:ok_binary up-to-date:
  bazel-bin/ok_binary.expand.rs
INFO: Elapsed time: 0.149s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

Targeting tests is valid as well.

bazel build --config=unpretty //:ok_test  
INFO: Analyzed target //:ok_test (0 packages loaded, 2 targets configured).
INFO: Found 1 target...
Aspect @rules_rust//rust/private:unpretty.bzl%rust_expand_aspect of //:ok_test up-to-date:
  bazel-bin/test-397521499/ok_test.expand.rs
INFO: Elapsed time: 0.113s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

Finally, manually wire up a rust_unpretty target explicitly if you want a target to build. This rule is unique compared to the aspect in that it forces a transition to a nightly toolchain so that -Zunpretty can be used.

load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_unpretty")

rust_binary(
    name = "ok_binary",
    srcs = ["src/main.rs"],
    edition = "2021",
)

rust_unpretty(
    name = "ok_binary_expand",
    deps = [":ok_binary"],
)

bazel build //:ok_binary_expand
INFO: Analyzed target //:ok_binary_expand (0 packages loaded, 1 target configured).
INFO: Found 1 target...
Target //:ok_binary_expand up-to-date:
  bazel-bin/ok_binary.expand.rs
INFO: Elapsed time: 0.090s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action

[UebelAndre]

I still think these rules probably belong on @rules_rust//tools/unpretty. Hopefully my reviewers have opinions

[illicitonion]

Generally LGTM, I don't have strong opinions on where this lives.