CoreCLR trimming incorrectly makes IsDynamicCodeCompiled true on interpreter-only targets #133615

Description

CoreCLR embeds src/coreclr/System.Private.CoreLib/src/ILLink/ILLink.Substitutions.xml unconditionally. That XML substitutes RuntimeFeature.IsDynamicCodeCompiled with true when IsDynamicCodeSupported is enabled, even when CoreLib was built without FEATURE_DYNAMIC_CODE_COMPILED.

This contradicts RuntimeFeature.NonNativeAot.cs, whose CoreCLR getter explicitly returns false without that feature. Browser CoreCLR supports dynamic code through its interpreter but does not dynamically compile it.

This is a trimming configuration bug, not an R2R-only failure. Both aggressively trimmed R2R and clean aggressively trimmed non-R2R library runs fail the same two existing RuntimeFeatureTests.

Reproduction Steps

Build browser CoreCLR and its libraries/packs, then run the existing Runtime tests with trimming enabled:

XHARNESS_COMMAND=test-browser ./dotnet.sh build /t:Test \
  src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj \
  /p:TargetOS=browser /p:TargetArchitecture=wasm /p:RuntimeFlavor=CoreCLR /p:Configuration=Release \
  /p:PublishReadyToRun=false /p:EnableAggressiveTrimming=true \
  /p:Scenario=WasmTestOnChrome /p:InstallChromeForTests=true \
  '/p:WasmTestAppArgs=-method System.Runtime.CompilerServices.Tests.RuntimeFeatureTests.DynamicCode_Browser -method System.Runtime.CompilerServices.Tests.RuntimeFeatureTests.DynamicCode'

The library-test trimming configuration uses the existing mobile test descriptors, as part of #133193. The same failures occur with PublishReadyToRun=true.

Expected behavior

RuntimeFeature.IsDynamicCodeCompiled remains false for interpreter-only browser CoreCLR, and its direct getter agrees with RuntimeFeature.IsSupported("IsDynamicCodeCompiled").

Actual behavior

[FAIL] RuntimeFeatureTests.DynamicCode_Browser
Assert.False() Failure
Expected: False
Actual:   True

[FAIL] RuntimeFeatureTests.DynamicCode
Assert.Equal() Failure
Expected: True
Actual:   False

The embedded substitution contains:

<type fullname="System.Runtime.CompilerServices.RuntimeFeature" feature="System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeSupported" featurevalue="true">
  <method signature="System.Boolean get_IsDynamicCodeCompiled()" body="stub" value="true" />
</type>

Regression?

Unknown. The non-JIT getter was introduced separately from the unconditional substitution. The issue is exposed by enabling browser library-test trimming.

Known Workarounds

Do not apply this JIT-specific substitution when FeatureDynamicCodeCompiled is not true. The original getter already has the correct false implementation for those builds.

Configuration

Browser-wasm CoreCLR Release, macOS arm64 build host, Chromium 153.0.8010.0. Runtime base 798449390515fa02f4b20160bedc5e34f8fbdfb9, with publishing prerequisite #133378 (245cb70d593f2c8f3a4e23bcc422e7da5926e15b).

Other information

Found during #133193. The two existing RuntimeFeature tests should remain enabled; the trimming configuration should be corrected instead of quarantining them.