Added rust_unpretty_aspect and rust_unpretty rules (#2356)

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:
# rust-lang/rust#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.

```starlark
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
```

.bazelci/presubmit.yml

@@ -15,6 +15,7 @@ single_rust_channel_targets: &single_rust_channel_targets
   - "//..."
   # These tests are expected to fail as they require both a nightly and stable toolchain.
   - "-//test/unit/channel_transitions/..."
+  - "-//test/unpretty/..."
 default_linux_targets: &default_linux_targets
   - "--"
   - "//..."
@@ -169,6 +170,13 @@ tasks:
     test_targets: *default_windows_targets
     soft_fail: yes
     bazel: "rolling"
+  ubuntu2004_unpretty:
+    name: Unpretty
+    platform: ubuntu2004
+    build_targets: *default_linux_targets
+    test_targets: *default_linux_targets
+    build_flags:
+      - "--config=unpretty"
   ubuntu2004_bzlmod_only:
     name: With bzlmod
     platform: ubuntu2004

.bazelrc

@@ -32,6 +32,14 @@ build:rustfmt --output_groups=+rustfmt_checks
 build:clippy --aspects=//rust:defs.bzl%rust_clippy_aspect
 build:clippy --output_groups=+clippy_checks
 
+# Enable unpretty for all targets in the workspace
+build:unpretty --aspects=//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 --//rust/toolchain/channel=nightly
+
 ###############################################################################
 ## Incompatibility flags
 ###############################################################################

rust/defs.bzl

@@ -65,6 +65,11 @@ load(
     _rustfmt_aspect = "rustfmt_aspect",
     _rustfmt_test = "rustfmt_test",
 )
+load(
+    "//rust/private:unpretty.bzl",
+    _rust_unpretty = "rust_unpretty",
+    _rust_unpretty_aspect = "rust_unpretty_aspect",
+)
 
 rust_library = _rust_library
 # See @rules_rust//rust/private:rust.bzl for a complete description.
@@ -111,6 +116,12 @@ capture_clippy_output = _capture_clippy_output
 rustc_output_diagnostics = _rustc_output_diagnostics
 # See @rules_rust//rust/private:rustc.bzl for a complete description.
 
+rust_unpretty_aspect = _rust_unpretty_aspect
+# See @rules_rust//rust/private:unpretty.bzl for a complete description.
+
+rust_unpretty = _rust_unpretty
+# See @rules_rust//rust/private:unpretty.bzl for a complete description.
+
 error_format = _error_format
 # See @rules_rust//rust/private:rustc.bzl for a complete description.
 

rust/private/rust.bzl

@@ -266,6 +266,22 @@ def _rust_binary_impl(ctx):
 
     return providers
 
+def get_rust_test_flags(attr):
+    """Determine the desired rustc flags for test targets.
+
+    Args:
+        attr (dict): Attributes of a rule
+
+    Returns:
+        List: A list of test flags
+    """
+    if getattr(attr, "use_libtest_harness", True):
+        rust_flags = ["--test"]
+    else:
+        rust_flags = ["--cfg", "test"]
+
+    return rust_flags
+
 def _rust_test_impl(ctx):
     """The implementation of the `rust_test` rule.
 
@@ -394,7 +410,7 @@ def _rust_test_impl(ctx):
         attr = ctx.attr,
         toolchain = toolchain,
         crate_info_dict = crate_info_dict,
-        rust_flags = ["--test"] if ctx.attr.use_libtest_harness else ["--cfg", "test"],
+        rust_flags = get_rust_test_flags(ctx.attr),
         skip_expanding_rustc_env = True,
     )
     data = getattr(ctx.attr, "data", [])
@@ -487,6 +503,56 @@ def _stamp_attribute(default_value):
         values = [1, 0, -1],
     )
 
+# Internal attributes core to Rustc actions.
+RUSTC_ATTRS = {
+    "_cc_toolchain": attr.label(
+        doc = (
+            "In order to use find_cc_toolchain, your rule has to depend " +
+            "on C++ toolchain. See `@rules_cc//cc:find_cc_toolchain.bzl` " +
+            "docs for details."
+        ),
+        default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
+    ),
+    "_error_format": attr.label(
+        default = Label("//:error_format"),
+    ),
+    "_extra_exec_rustc_flag": attr.label(
+        default = Label("//:extra_exec_rustc_flag"),
+    ),
+    "_extra_exec_rustc_flags": attr.label(
+        default = Label("//:extra_exec_rustc_flags"),
+    ),
+    "_extra_rustc_flag": attr.label(
+        default = Label("//:extra_rustc_flag"),
+    ),
+    "_extra_rustc_flags": attr.label(
+        default = Label("//:extra_rustc_flags"),
+    ),
+    "_import_macro_dep": attr.label(
+        default = Label("//util/import"),
+        cfg = "exec",
+    ),
+    "_is_proc_macro_dep": attr.label(
+        default = Label("//rust/private:is_proc_macro_dep"),
+    ),
+    "_is_proc_macro_dep_enabled": attr.label(
+        default = Label("//rust/private:is_proc_macro_dep_enabled"),
+    ),
+    "_per_crate_rustc_flag": attr.label(
+        default = Label("//:experimental_per_crate_rustc_flag"),
+    ),
+    "_process_wrapper": attr.label(
+        doc = "A process wrapper for running rustc on all platforms.",
+        default = Label("//util/process_wrapper"),
+        executable = True,
+        allow_single_file = True,
+        cfg = "exec",
+    ),
+    "_rustc_output_diagnostics": attr.label(
+        default = Label("//:rustc_output_diagnostics"),
+    ),
+}
+
 _common_attrs = {
     "aliases": attr.label_keyed_string_dict(
         doc = dedent("""\
@@ -618,62 +684,18 @@ _common_attrs = {
         """),
         allow_files = [".rs"],
     ),
-    "stamp": _stamp_attribute(default_value = 0),
+    "stamp": _stamp_attribute(
+        default_value = 0,
+    ),
     "version": attr.string(
         doc = "A version to inject in the cargo environment variable.",
         default = "0.0.0",
     ),
-    "_cc_toolchain": attr.label(
-        doc = (
-            "In order to use find_cc_toolchain, your rule has to depend " +
-            "on C++ toolchain. See `@rules_cc//cc:find_cc_toolchain.bzl` " +
-            "docs for details."
-        ),
-        default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
-    ),
-    "_error_format": attr.label(
-        default = Label("//:error_format"),
-    ),
-    "_extra_exec_rustc_flag": attr.label(
-        default = Label("//:extra_exec_rustc_flag"),
-    ),
-    "_extra_exec_rustc_flags": attr.label(
-        default = Label("//:extra_exec_rustc_flags"),
-    ),
-    "_extra_rustc_flag": attr.label(
-        default = Label("//:extra_rustc_flag"),
-    ),
-    "_extra_rustc_flags": attr.label(
-        default = Label("//:extra_rustc_flags"),
-    ),
-    "_import_macro_dep": attr.label(
-        default = Label("//util/import"),
-        cfg = "exec",
-    ),
-    "_is_proc_macro_dep": attr.label(
-        default = Label("//rust/private:is_proc_macro_dep"),
-    ),
-    "_is_proc_macro_dep_enabled": attr.label(
-        default = Label("//rust/private:is_proc_macro_dep_enabled"),
-    ),
-    "_per_crate_rustc_flag": attr.label(
-        default = Label("//:experimental_per_crate_rustc_flag"),
-    ),
-    "_process_wrapper": attr.label(
-        doc = "A process wrapper for running rustc on all platforms.",
-        default = Label("//util/process_wrapper"),
-        executable = True,
-        allow_single_file = True,
-        cfg = "exec",
-    ),
-    "_rustc_output_diagnostics": attr.label(
-        default = Label("//:rustc_output_diagnostics"),
-    ),
     "_stamp_flag": attr.label(
         doc = "A setting used to determine whether or not the `--stamp` flag is enabled",
         default = Label("//rust/private:stamp"),
     ),
-}
+} | RUSTC_ATTRS
 
 _coverage_attrs = {
     "_collect_cc_coverage": attr.label(