Propagate ConditionalModifierInfo from config_setting with single constraint

Summary:
RFC: https://fb.workplace.com/groups/buck2dev/permalink/4179858482302192/
---------

Extend `config_setting` to return a `ConditionalModifierInfo` provider when the `config_setting `has exactly one constraint value. This allows single-constraint `config_settings` to be used as modifiers, inheriting the conditional modifier behavior.

Differential Revision: D89706595

fbshipit-source-id: c4e039a18b0036b38a5a4aee5bce686b576d4394

Author: Nero5023

prelude/cfg/modifier/common.bzl

@@ -129,8 +129,7 @@ def get_modifier_info(
             return conditional_modifier_info.key, conditional_modifier_info.inner
         cfg_info = modifier_info[ConfigurationInfo]
         asserts.true(len(cfg_info.constraints) == 1, "Modifier should only be a single constraint value. Found multiple or none in `{}`".format(modifier))
-        constraint_value_info = list(cfg_info.constraints.values())[0]
-        return constraint_value_info.setting.label, constraint_value_info
+        fail("Internal error: Modifer (`{}` type `{}`) with single constraint value should have ConditionalModifierInfo provider.".format(modifier_info, type(modifier_info)))
     fail("Internal error: Found unexpected modifier `{}` type `{}`".format(modifier, type(modifier)))
 
 def _is_subset(a: ConfigurationInfo, b: ConfigurationInfo) -> bool:

prelude/configurations/rules.bzl

@@ -7,6 +7,7 @@
 # above-listed licenses.
 
 load("@prelude//cfg/modifier:constraint_modifier_info.bzl", "make_constraint_modifier_info")
+load("@prelude//cfg/modifier:types.bzl", "ConditionalModifierInfo")
 load(":util.bzl", "util")
 
 _ExecutionModifierInfo = provider(fields = {
@@ -16,7 +17,14 @@ _ExecutionModifierInfo = provider(fields = {
 def config_setting_impl(ctx):
     subinfos = [util.constraint_values_to_configuration(ctx.attrs.constraint_values)]
     subinfos.append(ConfigurationInfo(constraints = {}, values = ctx.attrs.values))
-    return [DefaultInfo(), util.configuration_info_union(subinfos)]
+    cfg_info = util.configuration_info_union(subinfos)
+    providers = [DefaultInfo(), cfg_info]
+    if len(ctx.attrs.constraint_values) == 1:
+        # When only have one constraint, we return an additional ConditionalModifierInfo provider for
+        # conditional modifier
+        conditional_modifier_info = ctx.attrs.constraint_values[0][ConditionalModifierInfo]
+        providers.append(conditional_modifier_info)
+    return providers
 
 def constraint_setting_impl(ctx):
     return [