Fix silent zero coverage on .NET Framework since 8.0.0

[tobiwae]

Fixes #1984.

Problem

Instrumenter.AddCustomModuleTrackerToModule copies the ModuleTrackerTemplate IL from the
running coverlet.core assembly (typeof(ModuleTrackerTemplate).Assembly.Location). Since 8.0.0
coverlet.core targets net8.0+, so the C# compiler lowers the template's string interpolations to
DefaultInterpolatedStringHandler — a type that does not exist on .NET Framework.

When that IL is injected into a .NET Framework module, the tracker's UnloadModule (the
AppDomain.ProcessExit / DomainUnload flush handler) fails to JIT at process exit. The CLR
swallows exceptions thrown from unload handlers, so the test run passes green while the coverage
report is silently 0%, with no error. The #1818 scope-remap doesn't save it — it points the
reference at mscorlib, where the type still doesn't exist.

Affected drivers: coverlet.console and coverlet.collector (both run the net8.0+ core).
coverlet.msbuild is unaffected because it runs the netstandard2.0 core, where the same
interpolations lower to string.Format.

Fix

Add a small coverlet.template project that compiles ModuleTrackerTemplate to netstandard2.0
(linking the existing source file — single source of truth), embed that assembly into coverlet.core,
and read the injected template IL from the embedded resource instead of from the running build. The
injected IL is then .NET Framework-safe regardless of which TFM coverlet.core runs as. No new file
ships and there is no type collision (the reference uses ReferenceOutputAssembly="false").

Verification

1. Regression test — new TestInstrument_NetFrameworkAssembly_TrackerDoesNotReferenceInterpolatedStringHandler
   ([Trait("Category", "NetFramework")]) instruments the net-framework sample and asserts the injected
   tracker references no DefaultInterpolatedStringHandler. It fails on master and passes with this
   change. (The existing [BUG] TypeInitializationException when targeting .NET Framework #1818 test can't catch this — the scope-remap makes its "not System.Runtime"
   assertion pass while the type is still missing.)

2. No regressions — full InstrumenterTests suite green (55/55).

3. End-to-end — built coverlet.console from this branch and ran it over a stock SDK-style net48
   library + NUnit tests via nunit3-console, on the same sample used in [BUG] Silent zero coverage on .NET Framework 4.8 (console & collector) since 8.0.0 #1984 (Windows x64, .NET SDK 10):

   coverlet.console	Tests	Coverage collected
   10.0.1 (shipped)	7/7 passed	0 / 13 sequence points (0%) — silent bug
   this branch	7/7 passed	13 / 13 (100%)

A minimal standalone reproduction is attached to #1984.

[tobiwae]

@dotnet-policy-service agree

[Bertk]

Thank you for the PR. 👍

[Copilot]

Pull request overview

This PR fixes a regression where .NET Framework targets could report silent 0% coverage (tests pass green, but no hits are flushed) when driven by coverlet.console or coverlet.collector, by ensuring the injected ModuleTrackerTemplate IL is sourced from a netstandard2.0-compiled template rather than the running net8.0+ coverlet.core build.

Changes:

• Add a new coverlet.template project that compiles ModuleTrackerTemplate for netstandard2.0 (linked source) so the resulting IL avoids net6+ only types like DefaultInterpolatedStringHandler.
• Embed the coverlet.template.dll output into coverlet.core and update Instrumenter to read the template IL from the embedded resource stream.
• Add a regression test asserting the injected tracker in a .NET Framework module does not reference DefaultInterpolatedStringHandler, and document the fix in the changelog.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/coverlet.core.tests/Instrumentation/InstrumenterTests.cs	Adds a regression test verifying the injected tracker IL does not reference DefaultInterpolatedStringHandler for .NET Framework modules.
src/coverlet.template/coverlet.template.csproj	Introduces a netstandard2.0-only project that links ModuleTrackerTemplate.cs to produce a framework-safe IL template.
src/coverlet.core/Instrumentation/Instrumenter.cs	Switches tracker template sourcing from on-disk assembly location to an embedded coverlet.template.dll resource stream.
src/coverlet.core/coverlet.core.csproj	Embeds the coverlet.template build output as a manifest resource in coverlet.core.
Documentation/Changelog.md	Adds an “Unreleased” entry describing the .NET Framework silent-zero-coverage fix.
coverlet.slnx	Adds src/coverlet.template/coverlet.template.csproj to the solution.