Add compile_only aspect

Adds an aspect that can be used to skip actions which are not compilation related. This is useful for warming projects in CI to maintain a Bazel remote cache for users to use. It is also useful when compilation actions are the only thing that need to be validated as it allows skipping potentially longer actions like bundling, signing, etc.

Example usage: `bazel run //:LocalDevCacheWarming_xcodeproj -- 'build --aspects=//:compile_only_aspect.bzl%compile_only_aspect --output_groups=compiles $_GENERATOR_LABEL_'`

Signed-off-by: Luis Padron <heyluispadron@gmail.com>

Author: luispadron

docs/usage.md

@@ -1,3 +1,5 @@
+- [Bazel aspects](#bazel-aspects)
+  - [`compile_only_aspect`](#compile_only_aspect)
 - [Bazel configs](#bazel-configs)
   - [`rules_xcodeproj`](#rules_xcodeproj)
   - [`rules_xcodeproj_generator`](#rules_xcodeproj_generator)
@@ -11,6 +13,41 @@
   - [Options](#options)
   - [Substitutions](#substitutions)
 
+# Bazel aspects
+
+## `compile_only_aspect`
+
+The `compile_only_aspect` aspect is useful for skipping non-compilation
+related actions. For example, in CI it can be used to help with disk
+space usage by skipping actions that are typically not cached during
+a cache warming job. It can also be used to validate that the targets
+compile while skipping costly actions like bundling, signing, etc.
+
+To use the aspect, you apply it at the command line:
+
+```
+bazel build //some:target \
+  --aspects=@rules_xcodeproj//xcodeproj:compile_only_aspect.bzl%compile_only_aspect \
+  --output_groups=compile_only
+```
+
+You can also create a Bazel configuration in a `.bazelrc` file to reuse the
+aspect easily:
+
+```
+common:compile_only --aspects=@rules_xcodeproj//xcodeproj:compile_only_aspect.bzl%compile_only_aspect
+common:compile_only --output_groups=compile_only
+```
+
+And use it, for example, with the command-line API:
+
+```
+bazel run //label/to:xcodeproj \
+  -- \
+  --generator_output_groups=all_targets \
+  'build --config=compile_only --remote_download_minimal'
+```
+
 # Bazel configs
 
 The way your project is generated, and the way Bazel builds it inside of Xcode,

xcodeproj/compile_only_aspect.bzl

@@ -0,0 +1,102 @@
+"""
+Aspect that collects the outputs of all compile actions of a given build.
+Useful to be able to build and cache compile actions on CI, without having to
+link, bundle, or codesign as well (which we don't cache anyway).
+"""
+
+_DOWNSTREAM_VALID_RULE_KINDS = {
+    "apple_dynamic_framework_import": None,
+    "apple_dynamic_xcframework_import": None,
+    "apple_static_framework_import": None,
+    "apple_static_xcframework_import": None,
+    "cc_binary": None,
+    "ios_app_clip": None,
+    "ios_application": None,
+    "ios_extension": None,
+    "ios_framework": None,
+    "ios_ui_test": None,
+    "ios_unit_test": None,
+    "swift_binary": None,
+    "swift_test": None,
+    "_ios_internal_ui_test_bundle": None,
+    "_ios_internal_unit_test_bundle": None,
+    "_precompiled_apple_resource_bundle": None,
+}
+
+_SWIFT_LIBRARY_KINDS = [
+    "swift_library",
+    "swift_test",
+]
+
+def _compile_only_aspect_impl(target, ctx):
+    outs = []
+    deps = []
+    if ctx.rule.kind in _DOWNSTREAM_VALID_RULE_KINDS or CcInfo in target:
+        if ctx.rule.kind in _SWIFT_LIBRARY_KINDS:
+            for action in target.actions:
+                if action.mnemonic == "SwiftCompile":
+                    outs = [action.outputs]
+                    break
+        elif ctx.rule.kind == "objc_library":
+            outs = [
+                action.outputs
+                for action in target.actions
+                if action.mnemonic == "ObjcCompile"
+            ]
+        deps = (
+            getattr(ctx.rule.attr, "deps", []) +
+            getattr(ctx.rule.attr, "implementation_deps", []) +
+            getattr(ctx.rule.attr, "private_deps", [])
+        )
+        swift_target = getattr(ctx.rule.attr, "swift_target", None)
+        if swift_target:
+            deps.append(swift_target)
+        clang_target = getattr(ctx.rule.attr, "clang_target", None)
+        if clang_target:
+            deps.append(clang_target)
+    elif ctx.rule.kind == "test_suite":
+        deps = ctx.rule.attr.tests
+    elif ctx.rule.kind == "ios_build_test":
+        deps = ctx.rule.attr.targets
+    elif ctx.rule.kind == "xcodeproj":
+        deps = (
+            getattr(ctx.rule.attr, "top_level_device_targets", []) +
+            getattr(ctx.rule.attr, "top_level_simulator_targets", [])
+        )
+    else:
+        return []
+
+    for dep in deps:
+        if OutputGroupInfo in dep and not hasattr(dep[OutputGroupInfo], "compiles"):
+            fail(target, dep)
+
+    return [
+        OutputGroupInfo(
+            compiles = depset(
+                transitive = outs + [
+                    dep[OutputGroupInfo].compiles
+                    for dep in deps
+                    if OutputGroupInfo in dep
+                ],
+            ),
+        ),
+    ]
+
+compile_only_aspect = aspect(
+    implementation = _compile_only_aspect_impl,
+    attr_aspects = [
+        "deps",
+        "implementation_deps",
+        "private_deps",
+        # from `mixed_language_library`
+        "clang_target",
+        "swift_target",
+        # from `test_suite`
+        "targets",
+        # from `*_build_test`
+        "tests",
+        # from `xcodeproj`
+        "top_level_device_targets",
+        "top_level_simulator_targets",
+    ],
+)