Add Cargo expand functionality

[freeformstu]

Cargo expand is an extremely useful command for debugging Rust source files with complicated proc macros. cargo expand is just a wrapper around rustc which passes in the flag -Zunpretty=expanded.

Proposal

Add a rust_expand and rust_expand_aspect to the core rules which functions similarly to the clippy rules. The rust_expand rule can be used to generated an expanded .rs file and the rust_expand_aspect can be used to produce such files on demand.

Example:

# .bazelrc
build:expanded --aspects=@rules_rust//rust:defs.bzl%rust_expand_aspect
build:expanded --output_groups=+expanded

bazel build --config=expanded //:ok_binary
INFO: Analyzed target //:ok_binary (1 packages loaded, 2 targets configured).
INFO: Found 1 target...
Aspect @rules_rust//rust/private:expand.bzl%rust_expand_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

ok_binary's src/main.rs

fn main() {
    println!("Hello world");
}

And the expanded output:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
fn main() {
    {
        ::std::io::_print(::core::fmt::Arguments::new_v1(&["Hello world\n"],
                &[]));
    };
}