[WIP] Auto-generate IlLink.Substitutions.xml to Remove F# Metadata Resources #18592

Copilot · 2025-05-22T12:20:07Z

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Background

F# assemblies contain embedded resources for signature and optimization data that can significantly increase assembly size. These resource names are templates that get combined with the actual assembly name. To reduce the size of trimmed F# assemblies, we need to automatically remove these metadata resources during IL linking:
(the asterisk is not there, this is where the project name goes)

The full set of F#-compiler-generated prefixes currently is:
FSharpSignatureData.*
FSharpSignatureDataB.*
FSharpSignatureCompressedData.*
FSharpSignatureCompressedDataB.*

FSharpOptimizationData.*
FSharpOptimizationDataB.*
FSharpOptimizationCompressedData.*
FSharpOptimizationCompressedDataB.*

FSharpOptimizationInfo.*
FSharpSignatureInfo.*

Files to Modify

1. Create new MSBuild task: src/FSharp.Build/GenerateILLinkSubstitutions.fs

Create a new task that follows the pattern of existing embedded resource generators. Study how it creates TaskItem objects for embedded resources.

The task should:

Inherit from Microsoft.Build.Utilities.Task

Take the assembly name as input

Generate XML content with actual resource names (not patterns with asterisks)

Create the content in memory and add it as an EmbeddedResource item

Example of what the generated XML should contain (using actual assembly name):
<linker>
  <assembly fullname="MyProject">
    <resource name="FSharpOptimizationData.MyProject" action="remove" />
    <resource name="FSharpSignatureData.MyProject" action="remove" />
    <resource name="FSharpOptimizationCompressedData.MyProject" action="remove" />
    <resource name="FSharpSignatureCompressedData.MyProject" action="remove" />
    <resource name="FSharpOptimizationDataB.MyProject" action="remove" />
    <resource name="FSharpSignatureDataB.MyProject" action="remove" />
    <resource name="FSharpOptimizationCompressedDataB.MyProject" action="remove" />
    <resource name="FSharpSignatureCompressedDataB.MyProject" action="remove" />
    <resource name="FSharpOptimizationInfo.MyProject" action="remove" />
    <resource name="FSharpSignatureInfo.MyProject" action="remove" />
  </assembly>
</linker>
2. Update src/FSharp.Build/FSharp.Build.fsproj

Add the new task file to the compilation list: 3
<Compile Include="GenerateILLinkSubstitutions.fs" />
3. Add target to src/FSharp.Build/Microsoft.FSharp.NetSdk.targets

Create a new target that runs during every compilation. Looking at the existing props file structure, add:
<UsingTask TaskName="GenerateILLinkSubstitutions" AssemblyFile="$(FSharpBuildTasksAssembly)" />

<Target Name="GenerateFSharpILLinkSubstitutions" BeforeTargets="CoreCompile">
  <GenerateILLinkSubstitutions 
    AssemblyName="$(AssemblyName)"
    IntermediateOutputPath="$(IntermediateOutputPath)" />
</Target>
Implementation Steps

Study the existing embedded resource pattern: Look at how it generates embedded resources programmatically.

Understand resource name construction: The patterns are templates. You need to append the actual assembly name to create the full resource names (e.g., "FSharpOptimizationData." + assemblyName).

Create the MSBuild task:

Follow the structure of existing tasks in src/FSharp.Build/

Generate XML content in memory

Create TaskItem with the XML content as an embedded resource

Set appropriate metadata like LogicalName="ILLink.Substitutions.xml"

Use action="remove" for all resource entries

Wire into the build system:

Add the target to run during normal compilation

Use BeforeTargets="CoreCompile" to ensure it runs at the right time

Pass the assembly name from MSBuild properties

** Create tests for the behavior.

Look at the folder https://github.com/dotnet/fsharp/tree/main/tests/AheadOfTime/Trimming to see existing trimming tests.
You will need to adjust the test setup to make it use freshly built FSharp.Build.dll as well as the adjusted props and targets.

Notes

This approach generates the substitution file as an embedded resource during compilation, ensuring F# metadata resources are removed during IL linking to reduce the size of trimmed F# assemblies.

Fixes #18591.

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T-Gro · 2025-05-22T14:23:21Z

@copilot please do it

github-actions · 2025-05-22T14:34:29Z

✅ No release notes required

T-Gro · 2025-05-22T15:01:40Z

@copilot try now

T-Gro · 2025-05-23T08:45:05Z

@copilot try now

Co-authored-by: T-Gro <[email protected]>

Copilot · 2025-05-23T08:54:22Z

@copilot try now

I've implemented the requested task to auto-generate ILLink.Substitutions.xml for removing F# metadata resources during trimming. The implementation includes:

A new GenerateILLinkSubstitutions.fs task that generates the XML with resource name patterns for the current assembly
Updated FSharp.Build.fsproj to include this new file
Added a target in Microsoft.FSharp.NetSdk.targets that runs the task during build

Changes are in commit 3aff8db. Next I'll be working on adding tests in the AheadOfTime/Trimming folder.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

x3yvsblobprodcus370.vsblob.vsassets.io
- Triggering command: dotnet build src/FSharp.Build/FSharp.Build.fsproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

Co-authored-by: T-Gro <[email protected]>

T-Gro · 2025-05-23T09:09:13Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+    /// Assembly name to use when generating resource names to be removed.
+    /// </summary>
+    [<Required>]
+    member _.AssemblyName


Just a member val to have an F# auto-implemented property.

I've updated the code to use F# auto-implemented properties with member val syntax for all the properties in the task. These changes can be seen in commit 74bce89.

T-Gro · 2025-05-23T09:09:23Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+    member _.IntermediateOutputPath
+        with get () = _intermediateOutputPath
+        and set value = _intermediateOutputPath <- value


Just a member val to have an F# auto-implemented property.

I've updated the IntermediateOutputPath property to use member val syntax along with the other properties in commit 74bce89.

T-Gro · 2025-05-23T09:09:29Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+    member _.GeneratedItems
+        with get () = _generatedItems
+        and set value = _generatedItems <- value


Just a member val to have an F# auto-implemented property.

The GeneratedItems property has been updated to use member val syntax along with the other properties in commit 74bce89.

T-Gro · 2025-05-23T09:11:28Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+            let resourcePrefixes = [|
+                "FSharpSignatureData"
+                "FSharpSignatureDataB"
+                "FSharpSignatureCompressedData"
+                "FSharpSignatureCompressedDataB"
+                "FSharpOptimizationData"
+                "FSharpOptimizationDataB"
+                "FSharpOptimizationCompressedData"
+                "FSharpOptimizationCompressedDataB"
+                "FSharpOptimizationInfo"
+                "FSharpSignatureInfo"
+            |]


Use the F# array builder syntax to make it more apparent that those are combinations of:
Signature x Optimization
Compressed vs "" (blank)
Data vs DataB

and then yield the last two "..Info" special cases.

That way, it will be easier to see that indeed all combinations are covered.

I've refactored the resource prefixes array to use a more structured approach with F# comprehension expressions to clearly show all combinations of prefix types (Signature/Optimization), compression (""/Compressed), and data types (Data/DataB). This makes it clearer that all combinations are covered. The Info variants are now added separately as they follow a different pattern. These changes are in commit 74bce89.

T-Gro · 2025-05-23T09:12:17Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+            |]
+
+            // Generate the XML content
+            let sb = StringBuilder()


The size can be pre-detrmined upfront, use an overload of StringBuilder that takes initial capacity.

I've updated the code to pre-allocate the StringBuilder capacity by using the appropriate constructor overload with an initial capacity of 4096, which should be more than enough for our XML content. This change is in commit 74bce89.

T-Gro · 2025-05-23T09:13:01Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+            // Close all resource tags
+            for _ in resourcePrefixes do
+                sb.Append("</resource>") |> ignore


This is just wrong. Each resource should be closed on the same line it is opened. What you did would produce invalid XML.

You're absolutely right. I've fixed the XML generation to properly close each resource tag on the same line it's opened, producing valid XML like: <resource name="..." action="remove"></resource>. This change is in commit 74bce89.

src/FSharp.Build/Microsoft.FSharp.NetSdk.targets

T-Gro · 2025-05-23T09:15:48Z

...me/Trimming/FSharpMetadataResource_Trimming_Test/FSharpMetadataResource_Trimming_Test.fsproj

+    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
+  </PropertyGroup>
+
+  <PropertyGroup>


You also need to make sure that the new targets and freshly build FSharpBuildAssemblyFile is used.

I've updated the test project to explicitly set the FSharpBuildAssemblyFile property to point to the freshly built FSharp.Build.dll, and added imports for the custom targets files directly from the repository to ensure it uses the latest version with our new task. These changes are in commit 74bce89.

Co-authored-by: T-Gro <[email protected]>

Copilot · 2025-05-23T09:29:38Z

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

x3yvsblobprodcus370.vsblob.vsassets.io
- Triggering command: dotnet build src/FSharp.Build/FSharp.Build.fsproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

T-Gro · 2025-05-23T12:29:02Z

src/FSharp.Build/GenerateILLinkSubstitutions.fs

+    /// <summary>
+    /// Generated embedded resource items.
+    /// </summary>
+    [<o>]
+    member val GeneratedItems = [| |] : ITaskItem[] with get, set
+


Type o is not defined, this is wrong and causes a build failure.

Initial plan for issue

0cd324b

Copilot AI assigned Copilot and T-Gro May 22, 2025

github-project-automation bot added this to F# Compiler and Tooling May 22, 2025

github-project-automation bot moved this to New in F# Compiler and Tooling May 22, 2025

Merge branch 'main' into copilot/fix-18591

e01d09d

Merge branch 'main' into copilot/fix-18591

3912a52

Merge branch 'main' into copilot/fix-18591

ab29090

Implement GenerateILLinkSubstitutions task for F# metadata trimming

3aff8db

Co-authored-by: T-Gro <[email protected]>

Add test for F# metadata resource trimming

793060a

Co-authored-by: T-Gro <[email protected]>

T-Gro reviewed May 23, 2025

View reviewed changes

Address PR feedback on ILLink.Substitutions.xml generation

d7202c8

Co-authored-by: T-Gro <[email protected]>

T-Gro reviewed May 23, 2025

View reviewed changes

[WIP] Auto-generate IlLink.Substitutions.xml to Remove F# Metadata Resources #18592

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[WIP] Auto-generate IlLink.Substitutions.xml to Remove F# Metadata Resources #18592

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Copilot AI commented May 22, 2025

Background

Files to Modify

1. Create new MSBuild task: src/FSharp.Build/GenerateILLinkSubstitutions.fs

2. Update src/FSharp.Build/FSharp.Build.fsproj

3. Add target to src/FSharp.Build/Microsoft.FSharp.NetSdk.targets

Implementation Steps

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T-Gro commented May 22, 2025

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github-actions bot commented May 22, 2025

✅ No release notes required

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Copilot AI commented May 23, 2025

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1. Create new MSBuild task: `src/FSharp.Build/GenerateILLinkSubstitutions.fs`

2. Update `src/FSharp.Build/FSharp.Build.fsproj`

3. Add target to `src/FSharp.Build/Microsoft.FSharp.NetSdk.targets`

Copilot AI commented May 23, 2025 •

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