From eee26758c2e111c53c43eaffc6603039b97c102e Mon Sep 17 00:00:00 2001
From: am11 <3840695+am11@users.noreply.github.com>
Date: Mon, 26 Oct 2026 18:04:16 +0200
Subject: [PATCH 1/2] Trim OperatingSystem.IsXxx checks in library build

---
 eng/RuntimeIdentifier.props | 18 +++++++++++++++
 eng/illink.targets          | 44 +++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/eng/RuntimeIdentifier.props b/eng/RuntimeIdentifier.props
index ea53b9e2d4e923..e2f854402c3364 100644
--- a/eng/RuntimeIdentifier.props
+++ b/eng/RuntimeIdentifier.props
@@ -69,6 +69,24 @@
     <TargetsWindows Condition="'$(TargetOS)' == 'windows'">true</TargetsWindows>
     <TargetsUnix Condition="'$(TargetsOpenBSD)' == 'true' or '$(TargetsFreeBSD)' == 'true' or '$(Targetsillumos)' == 'true' or '$(TargetsSolaris)' == 'true' or '$(TargetsHaiku)' == 'true' or '$(TargetsLinux)' == 'true' or '$(TargetsNetBSD)' == 'true' or '$(TargetsOSX)' == 'true' or '$(TargetsMacCatalyst)' == 'true' or '$(TargetstvOS)' == 'true' or '$(TargetsiOS)' == 'true' or '$(TargetsAndroid)' == 'true'">true</TargetsUnix>
   </PropertyGroup>
+  <!-- Values of the public OperatingSystem.IsXxx() checks for the TargetOS. These must match the
+       TARGET_* conditions in System.Private.CoreLib's OperatingSystem.cs. -->
+  <ItemGroup Label="TargetOperatingSystemChecks">
+    <TargetOperatingSystemCheck Include="IsAndroid" Value="$([MSBuild]::ValueOrDefault('$(TargetsAndroid)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsBrowser" Value="$([MSBuild]::ValueOrDefault('$(TargetsBrowser)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsFreeBSD" Value="$([MSBuild]::ValueOrDefault('$(TargetsFreeBSD)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsIOS" Value="true" Condition="'$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true'" />
+    <TargetOperatingSystemCheck Include="IsIOS" Value="false" Condition="'$(TargetsiOS)' != 'true' and '$(TargetsMacCatalyst)' != 'true'" />
+    <TargetOperatingSystemCheck Include="IsLinux" Value="true" Condition="'$(TargetsLinux)' == 'true' and '$(TargetsAndroid)' != 'true'" />
+    <TargetOperatingSystemCheck Include="IsLinux" Value="false" Condition="'$(TargetsLinux)' != 'true' or '$(TargetsAndroid)' == 'true'" />
+    <TargetOperatingSystemCheck Include="IsMacCatalyst" Value="$([MSBuild]::ValueOrDefault('$(TargetsMacCatalyst)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsMacOS" Value="$([MSBuild]::ValueOrDefault('$(TargetsOSX)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsOpenBSD" Value="$([MSBuild]::ValueOrDefault('$(TargetsOpenBSD)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsTvOS" Value="$([MSBuild]::ValueOrDefault('$(TargetstvOS)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsWasi" Value="$([MSBuild]::ValueOrDefault('$(TargetsWasi)', 'false'))" />
+    <TargetOperatingSystemCheck Include="IsWatchOS" Value="false" />
+    <TargetOperatingSystemCheck Include="IsWindows" Value="$([MSBuild]::ValueOrDefault('$(TargetsWindows)', 'false'))" />
+  </ItemGroup>
   <PropertyGroup>
     <_ImportedRuntimeIdentifierProps>true</_ImportedRuntimeIdentifierProps>
   </PropertyGroup>
diff --git a/eng/illink.targets b/eng/illink.targets
index 714ac859810554..695be57bfbf175 100644
--- a/eng/illink.targets
+++ b/eng/illink.targets
@@ -192,6 +192,50 @@
     </PropertyGroup>
   </Target>
 
+  <!-- OperatingSystem.IsXxx() are constants in the OS-specific System.Private.CoreLib, but the library build
+       resolves them from the System.Runtime reference assembly. Substitute their values for the OS being
+       built so that the library build trims code guarded by checks for other operating systems.
+       Only applied to shared framework libraries built for the current TargetOS: binaries shipped in
+       packages (e.g. a runtimes/unix asset) must remain valid on every OS that their flavor covers. -->
+  <PropertyGroup>
+    <!-- The linux, ios and tvos flavors are also used by the TargetOS values that fall back to them:
+         android and openharmony (linux), maccatalyst and iossimulator (ios), tvossimulator (tvos). -->
+    <_OSSubstitutionsApply Condition="'$(TargetPlatformIdentifier)' != '' and
+                                      '$(TargetOS)' != '' and
+                                      '$(IsPackable)' != 'true' and
+                                      '$(BuildAllConfigurations)' != 'true' and
+                                      '$(ILLinkSubstitutionsLibraryBuildXml)' == '' and
+                                      ('$(TargetPlatformIdentifier)' == '$(TargetOS)' or
+                                       ('$(TargetPlatformIdentifier)' == 'unix' and '$(TargetsUnix)' == 'true') or
+                                       ('$(TargetPlatformIdentifier)' == 'linux' and '$(TargetsLinux)' == 'true') or
+                                       ('$(TargetPlatformIdentifier)' == 'ios' and ('$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true')) or
+                                       ('$(TargetPlatformIdentifier)' == 'tvos' and '$(TargetstvOS)' == 'true'))">true</_OSSubstitutionsApply>
+  </PropertyGroup>
+
+  <Target Name="_GenerateOperatingSystemSubstitutions"
+          Condition="'$(_OSSubstitutionsApply)' == 'true' and '$(ILLinkTrimAssembly)' == 'true'"
+          BeforeTargets="ILLinkTrimAssembly">
+    <PropertyGroup>
+      <ILLinkSubstitutionsLibraryBuildXml>$(IntermediateOutputPath)ILLink.Substitutions.OperatingSystem.LibraryBuild.xml</ILLinkSubstitutionsLibraryBuildXml>
+      <_OSSubstitutions><![CDATA[<linker>
+  <assembly fullname="System.Runtime">
+    <type fullname="System.OperatingSystem">
+@(TargetOperatingSystemCheck->'      <method signature="System.Boolean %(Identity)()" body="stub" value="%(Value)" />', '%0A')
+    </type>
+  </assembly>
+</linker>]]></_OSSubstitutions>
+    </PropertyGroup>
+
+    <WriteLinesToFile File="$(ILLinkSubstitutionsLibraryBuildXml)"
+                      Lines="$(_OSSubstitutions)"
+                      Overwrite="true"
+                      WriteOnlyWhenDifferent="true" />
+
+    <ItemGroup>
+      <FileWrites Include="$(ILLinkSubstitutionsLibraryBuildXml)" />
+    </ItemGroup>
+  </Target>
+
   <!-- Examines the "input assembly" for IL that is unreachable from public API and trims that,
        rewriting the assembly to an "output assembly" -->
   <UsingTask TaskName="ILLink"

From 24d3805aea8c902b8c7e3bba95bd233fd014f532 Mon Sep 17 00:00:00 2001
From: am11 <3840695+am11@users.noreply.github.com>
Date: Tue, 27 Oct 2026 14:38:29 +0200
Subject: [PATCH 2/2] Substitute OS checks for each flavor's own OS

---
 eng/illink.targets | 42 +++++++++++++++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/eng/illink.targets b/eng/illink.targets
index 695be57bfbf175..7cdbe7ffb079e2 100644
--- a/eng/illink.targets
+++ b/eng/illink.targets
@@ -195,32 +195,52 @@
   <!-- OperatingSystem.IsXxx() are constants in the OS-specific System.Private.CoreLib, but the library build
        resolves them from the System.Runtime reference assembly. Substitute their values for the OS being
        built so that the library build trims code guarded by checks for other operating systems.
-       Only applied to shared framework libraries built for the current TargetOS: binaries shipped in
-       packages (e.g. a runtimes/unix asset) must remain valid on every OS that their flavor covers. -->
+       Not applied to packages: binaries shipped in them (e.g. a runtimes/unix asset) must remain valid on every
+       OS that their flavor covers. -->
   <PropertyGroup>
     <!-- The linux, ios and tvos flavors are also used by the TargetOS values that fall back to them:
          android and openharmony (linux), maccatalyst and iossimulator (ios), tvossimulator (tvos). -->
-    <_OSSubstitutionsApply Condition="'$(TargetPlatformIdentifier)' != '' and
-                                      '$(TargetOS)' != '' and
+    <_OSSubstitutionsFlavorMatchesTargetOS Condition="'$(TargetOS)' != '' and
+                                                      ('$(TargetPlatformIdentifier)' == '$(TargetOS)' or
+                                                       ('$(TargetPlatformIdentifier)' == 'unix' and '$(TargetsUnix)' == 'true') or
+                                                       ('$(TargetPlatformIdentifier)' == 'linux' and '$(TargetsLinux)' == 'true') or
+                                                       ('$(TargetPlatformIdentifier)' == 'ios' and ('$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true')) or
+                                                       ('$(TargetPlatformIdentifier)' == 'tvos' and '$(TargetstvOS)' == 'true'))">true</_OSSubstitutionsFlavorMatchesTargetOS>
+    <!-- The OS whose values are substituted: the TargetOS for the flavors it uses, otherwise the OS of an OS-specific
+         flavor, so that a flavor produces the same binary regardless of the TargetOS the build was invoked for.
+         The unix flavor covers multiple operating systems, so it is only substituted when built for one of them. -->
+    <_OSSubstitutionsTargetOS Condition="'$(_OSSubstitutionsFlavorMatchesTargetOS)' == 'true'">$(TargetOS)</_OSSubstitutionsTargetOS>
+    <_OSSubstitutionsTargetOS Condition="'$(_OSSubstitutionsFlavorMatchesTargetOS)' != 'true' and '$(TargetPlatformIdentifier)' != 'unix'">$(TargetPlatformIdentifier.ToLowerInvariant())</_OSSubstitutionsTargetOS>
+    <_OSSubstitutionsApply Condition="'$(_OSSubstitutionsTargetOS)' != '' and
                                       '$(IsPackable)' != 'true' and
                                       '$(BuildAllConfigurations)' != 'true' and
-                                      '$(ILLinkSubstitutionsLibraryBuildXml)' == '' and
-                                      ('$(TargetPlatformIdentifier)' == '$(TargetOS)' or
-                                       ('$(TargetPlatformIdentifier)' == 'unix' and '$(TargetsUnix)' == 'true') or
-                                       ('$(TargetPlatformIdentifier)' == 'linux' and '$(TargetsLinux)' == 'true') or
-                                       ('$(TargetPlatformIdentifier)' == 'ios' and ('$(TargetsiOS)' == 'true' or '$(TargetsMacCatalyst)' == 'true')) or
-                                       ('$(TargetPlatformIdentifier)' == 'tvos' and '$(TargetstvOS)' == 'true'))">true</_OSSubstitutionsApply>
+                                      '$(ILLinkSubstitutionsLibraryBuildXml)' == ''">true</_OSSubstitutionsApply>
   </PropertyGroup>
 
+  <Target Name="_GetTargetOperatingSystemChecks"
+          Returns="@(TargetOperatingSystemCheck)" />
+
   <Target Name="_GenerateOperatingSystemSubstitutions"
           Condition="'$(_OSSubstitutionsApply)' == 'true' and '$(ILLinkTrimAssembly)' == 'true'"
           BeforeTargets="ILLinkTrimAssembly">
+    <ItemGroup Condition="'$(_OSSubstitutionsTargetOS)' == '$(TargetOS)'">
+      <_OSSubstitutionsCheck Include="@(TargetOperatingSystemCheck)" />
+    </ItemGroup>
+
+    <!-- Evaluate the checks for the flavor's OS from the same definitions in RuntimeIdentifier.props. -->
+    <MSBuild Projects="$(MSBuildProjectFullPath)"
+             Targets="_GetTargetOperatingSystemChecks"
+             Properties="TargetOS=$(_OSSubstitutionsTargetOS);TargetFramework=$(TargetFramework)"
+             Condition="'$(_OSSubstitutionsTargetOS)' != '$(TargetOS)'">
+      <Output TaskParameter="TargetOutputs" ItemName="_OSSubstitutionsCheck" />
+    </MSBuild>
+
     <PropertyGroup>
       <ILLinkSubstitutionsLibraryBuildXml>$(IntermediateOutputPath)ILLink.Substitutions.OperatingSystem.LibraryBuild.xml</ILLinkSubstitutionsLibraryBuildXml>
       <_OSSubstitutions><![CDATA[<linker>
   <assembly fullname="System.Runtime">
     <type fullname="System.OperatingSystem">
-@(TargetOperatingSystemCheck->'      <method signature="System.Boolean %(Identity)()" body="stub" value="%(Value)" />', '%0A')
+@(_OSSubstitutionsCheck->'      <method signature="System.Boolean %(Identity)()" body="stub" value="%(Value)" />', '%0A')
     </type>
   </assembly>
 </linker>]]></_OSSubstitutions>