Add fail_on_missing_constraint parameter to apple_support.platform_info_from_rule_ctx.

This allows callers to control whether a missing Apple platform constraint should result in a failure or a warning, or if a fallback should be silently applied, without the need to be on an allowlist.

Generally this should be assumed "True" for rules-level logic as it can identify correctness issues quickly. However, users who want top level builds to default to Apple Silicon macOS as previous Apple fragment APIs allowed (albeit for Intel macOS) can still use the same API with this argument set to "False".

PiperOrigin-RevId: 936570577

Author: nglevin

lib/apple_support.bzl

@@ -649,12 +649,34 @@ Internal Error: Found unrecognized target environment of {target_environment} fo
         ),
     )
 
-def _apple_platform_info_from_rule_ctx(ctx):
-    """Returns an ApplePlatformInfo provider from a rule context, needed to resolve constraints."""
-    target_os = _target_os_from_rule_ctx(ctx, fail_on_missing_constraint = False)
+def _platform_info_from_rule_ctx(ctx, fail_on_missing_constraint = True):
+    """Returns `ApplePlatformInfo` from a rule context, required for Apple platform information.
+
+    In order to use `apple_support.platform_info_from_rule_ctx()`, you'll need to modify your rule
+    definition to add the following:
+
+      * Add the `apple_support.platform_constraint_attrs()` attributes to the `attrs` dictionary.
+        This can be done using the `dicts.add()` method from Skylib or via the `|` operator.
+
+    Args:
+        ctx: The context of the rule that has Apple platform constraint attributes.
+        fail_on_missing_constraint: Whether to fail if no constraint is found. If `False`, a
+            fallback platform of Apple Silicon macOS will be used if no Apple platform was found
+            from the current target configuration. If `True`, the function will fail if no Apple
+            platform is found and the target is not in the
+            `ALLOWED_USERS_OF_MISSING_PLATFORM_FALLBACK` allowlist.
+
+    Returns:
+        An `ApplePlatformInfo` representing the resolved Apple platform.
+    """
+    target_os = _target_os_from_rule_ctx(
+        ctx,
+        fail_on_missing_constraint = False,
+    )
     if not target_os:
         full_label = "//{package}:{name}".format(package = ctx.label.package, name = ctx.label.name)
-        if full_label not in ALLOWED_USERS_OF_MISSING_PLATFORM_FALLBACK:
+        if (fail_on_missing_constraint and
+            full_label not in ALLOWED_USERS_OF_MISSING_PLATFORM_FALLBACK):
             fail("""
 ERROR: A valid Apple platform constraint could not be found for target {full_label}
 
@@ -668,9 +690,10 @@ through `--platforms` rather than relying on the defaults.
                 full_label = full_label,
             ))
 
-        # Starlark-only default fallback when Apple constraints are missing (e.g. Linux host analysis)
-        # buildifier: disable=print
-        print("Warning: Target {} is analyzed without Apple platform constraints. Applying temporary macos fallback.".format(full_label))
+        if fail_on_missing_constraint:
+            # Starlark-only default fallback when Apple constraints are missing (e.g. Linux host analysis)
+            # buildifier: disable=print
+            print("Warning: Target {} is analyzed without Apple platform constraints. Applying temporary macos fallback.".format(full_label))
         target_os = "macos"
         target_env = "device"
         target_arch = "arm64"
@@ -698,7 +721,7 @@ apple_support = struct(
         xcode = _xcode_path_placeholder,
     ),
     platform_constraint_attrs = _platform_constraint_attrs,
-    platform_info_from_rule_ctx = _apple_platform_info_from_rule_ctx,
+    platform_info_from_rule_ctx = _platform_info_from_rule_ctx,
     run = _run,
     run_shell = _run_shell,
     target_arch_from_rule_ctx = _target_arch_from_rule_ctx,