Blazor WASM app leaves untrimmed System.IO.Pipelines assembly by default

[eerhardt]

Describe the bug

When publishing a default blazorwasm app, System.IO.Pipelines.dll is getting published to the output folder and is not trimmed at all.

The reason this isn't trimmed by default is because System.IO.Pipelines isn't part of the shared fx / runtimepack in .NET Core. And the SDK only trims System assemblies that are in the runtimepack, and not assemblies that come as part of a NuGet package.

We should be marking System.IO.Pipelines.dll as trimmable, so it gets trimmed when it isn't used.

To Reproduce

1. dotnet new blazorwasm
2. dotnet publish -c Release
3. Inspect the published app folder bin\Release\net5.0\publish\wwwroot\_framework\

See that System.IO.Pipelines.dll exists, and it doesn't have any IL trimmed from it.

Further technical details

• ASP.NET Core version: 5.0
• Include the output of dotnet --info

.NET SDK (reflecting any global.json):
 Version:   5.0.100
 Commit:    5044b93829

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19042
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.100\

Host (useful for support):
  Version: 5.0.0
  Commit:  cf258a14b7

.NET SDKs installed:
  2.1.811 [C:\Program Files\dotnet\sdk]
  5.0.100-rc.1.20420.7 [C:\Program Files\dotnet\sdk]
  5.0.100-rc.2.20479.15 [C:\Program Files\dotnet\sdk]
  5.0.100-rtm.20509.5 [C:\Program Files\dotnet\sdk]
  5.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-rc.2.20475.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-rtm.20508.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-rc.2.20475.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-rtm.20508.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0-rc.2.20475.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0-rtm.20508.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

cc @pranavkm @SteveSandersonMS

[pranavkm]

@mkArtakMSFT we might want to consider patching this in 5.0.1. Addressing this issue removes an entire assembly so it's one fewer download, and 20kb (compressed) in download size.

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[eerhardt]

This has been addressed with the combination of the following:

• Support attribute trimming opt-in linker#1839
• Enable additional ways to opt in to trimming sdk#16094
• Annotate framework assemblies as trimmable runtime#48428