Refactor DownlevelLibraryImportGenerator to inject types via RegisterPostInitializationOutput

[Copilot]

Refactors the DownlevelLibraryImportGenerator to inject internal definitions of LibraryImportAttribute and StringMarshalling directly into the user's compilation using RegisterPostInitializationOutput, instead of shipping them as separate .cs source files via eng/generators.targets.

Changes Made

• Switched to RegisterPostInitializationOutput — the downlevel LibraryImportAttribute and StringMarshalling types are now injected directly into the compilation rather than distributed as source files, removing the need for the corresponding entries in eng/generators.targets.
• Trimmed injected types — the injected LibraryImportAttribute definition only exposes properties supported on downlevel frameworks; StringMarshalling only exposes the Utf16 value.
• Added [Microsoft.CodeAnalysis.Embedded] — both injected types are annotated with this attribute so Roslyn treats them as compiler-internal, automatically suppressing CS0436 ("type conflicts with imported type") without needing explicit #pragma warning directives.
• Removed visibility #if ifdefs — the PUBLIC_CONTRACT / internal visibility ifdefs in the BCL source files for LibraryImportAttribute and StringMarshalling are no longer needed and have been removed.
• Removed #pragma warning disable/restore CS0436 — redundant now that [Embedded] suppresses CS0436 automatically.
• Removed #nullable restore — a nullable context restore at the very end of a generated file has no effect and has been removed.
• Updated unit tests — removed tests that relied on the old source-file injection path; added a test that verifies the downlevel attribute is injected and compiles correctly when no using for LibraryImportAttribute exists in the user's code.
• Fixed brittle tree-count assertion — OnlyTypeDefinitionsOutputTest now asserts that no stub output trees (with "LibraryImports" in their path) are added, rather than asserting an exact count of +2 injected trees, making the test resilient to future changes in Roslyn's embedded attribute handling.
• Restored CannotForwardToDllImport downlevel test — re-added StringMarshallingForwardingNotSupported_ReportsDiagnostic as a downlevel test that uses (StringMarshalling)1 (numeric cast for Utf8) instead of StringMarshalling.Utf8, since the injected downlevel enum only exposes Utf16 = 2, keeping the forwarder-stub diagnostic path covered.

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[jkoritzinsky]

@copilot please revise the PR title and description to cover all of the work that you've done in this PR, not just the last few changes.

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[Copilot]

@copilot please revise the PR title and description to cover all of the work that you've done in this PR, not just the last few changes.

Updated the PR description to cover the full set of changes across all commits: switching to RegisterPostInitializationOutput, trimming the injected types to downlevel-only members, adding [Embedded] to suppress CS0436, removing the now-redundant #pragma warning disable CS0436, removing the visibility #if ifdefs from BCL source, removing #nullable restore from the generated string, and updating the unit tests.

[Copilot]

Pull request overview

Refactors the downlevel LibraryImport source generator flow so required interop types are injected directly into the user compilation (via RegisterPostInitializationOutput) instead of being compiled in from shared .cs files via eng/generators.targets.

Changes:

• Inject downlevel System.Runtime.InteropServices.LibraryImportAttribute + StringMarshalling via RegisterPostInitializationOutput (and mark them [Microsoft.CodeAnalysis.Embedded]) to avoid CS0436 without pragmas.
• Remove the now-unneeded compile-time inclusion of LibraryImportAttribute.cs and StringMarshalling.cs from eng/generators.targets, and simplify the BCL sources accordingly.
• Update unit tests to stop relying on the old “include a source snippet for the attribute/enum” approach and adjust expectations for generator outputs.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Diagnostics.cs	Removes an OuterLoop downlevel diagnostic test tied to the old source-injection mechanism.
src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Compiles.cs	Updates downlevel compile snippets/tests to rely on post-init injected types; adds a new test class to assert only type-definition output when no [LibraryImport] is present.
src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/AdditionalAttributesOnStub.cs	Drops manual injection of LibraryImportAttribute snippet for downlevel tests.
src/libraries/System.Runtime.InteropServices/gen/DownlevelLibraryImportGenerator/DownlevelLibraryImportGenerator.cs	Adds post-initialization injection of embedded interop types into the compilation.
src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/StringMarshalling.cs	Removes visibility #if and makes the enum unconditionally public in CoreLib source.
src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/LibraryImportAttribute.cs	Removes downlevel-only internal/CS0436 pragmas and makes the attribute unconditionally public in CoreLib source.
eng/generators.targets	Removes compile inclusion of CoreLib shared LibraryImportAttribute.cs and StringMarshalling.cs for downlevel generator enablement.

[jkoritzinsky]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Applied both review suggestions in commit d446c83:

1. Compiles.cs — Replaced the brittle Assert.Equal(count + 2, ...) assertion in OnlyTypeDefinitionsOutputTest with Assert.DoesNotContain(addedTrees, tree => tree.FilePath.Contains("LibraryImports", StringComparison.Ordinal)), which checks the meaningful invariant (no stub output) without coupling to an exact tree count.

2. Diagnostics.cs — Added back StringMarshallingForwardingNotSupported_ReportsDiagnostic as a downlevel test. Since the injected StringMarshalling enum only exposes Utf16 = 2, the test uses (StringMarshalling)1 (numeric cast for Utf8) to trigger the CannotForwardToDllImport diagnostic, keeping the forwarder-stub path covered.

[jtschuster]

LGTM, love a copilot cleanup

[github-actions]

🤖 Copilot Code Review — PR #126056

Holistic Assessment

Motivation: This PR cleans up the DownlevelLibraryImportGenerator by replacing the old approach (conditional #if SYSTEM_PRIVATE_CORELIB visibility guards + CS0436 warning suppression) with Roslyn's [Embedded] attribute pattern for injecting internal type definitions into downlevel compilations. The motivation is sound — the old approach was fragile, required #pragma suppressions, and spread conditional compilation across shared source files.

Approach: Using RegisterPostInitializationOutput with AddEmbeddedAttributeDefinition() and [global::Microsoft.CodeAnalysis.Embedded]-marked types is a clean, idiomatic Roslyn pattern. The downlevel types are appropriately trimmed to only expose supported features (Utf16 marshalling, no StringMarshallingCustomType). The approach correctly narrows the downlevel API surface so that unsupported features produce compile errors rather than runtime diagnostics — a fail-fast improvement.

Summary: ✅ LGTM. The changes are well-structured, the tests are appropriately updated, and no public API surface is affected. The iterative commit history shows this PR has already undergone review feedback cycles (e.g., "fix brittle tree count assertion", "add CannotForwardToDllImport downlevel test").

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Detailed Findings

✅ Correctness — Conditional compilation removal is safe

Verified that:

• LibraryImportAttribute.cs is only compiled by System.Private.CoreLib (where SYSTEM_PRIVATE_CORELIB is always defined), so the type was always public in practice. Removing the #if guard just eliminates dead code.
• StringMarshalling.cs is compiled by CoreLib and Microsoft.Interop.SourceGeneration (where MICROSOFT_INTEROP_SOURCEGENERATION is defined). Both paths made it public. No other projects include this file via <Compile Include>.
• No ref/ assembly changes — no public API surface modification.

✅ Correctness — Downlevel type trimming is intentional and correct

The removal of StringMarshallingCustomType and Custom/Utf8 enum values from the injected downlevel types means users targeting downlevel frameworks will now get a compile error if they try to use unsupported features (e.g., StringMarshallingCustomType), rather than a CannotForwardToDllImport diagnostic at generator time. This is a behavioral improvement — unsupported features are caught earlier and more clearly.

✅ Test quality — Adequate coverage for the behavioral change

• OnlyTypeDefinitionsOutputTest: Correctly verifies that when no [LibraryImport] methods exist, the generator injects type definitions (via RegisterPostInitializationOutput) but does not emit stub output (no tree with "LibraryImports" in its path). This replaces the old Assert.Same(originalCompilation, newCompilation) check, which would now fail since the generator always injects types.
• StringMarshallingForwardingNotSupported_ReportsDiagnostic: The use of (StringMarshalling)1 to exercise a non-Utf16 value is correct. Verified in the analyzer code that pinvokeData.StringMarshalling uses the full Microsoft.Interop.StringMarshalling enum (which has Utf8 = 1), so ToString() on the value produces "Utf8", matching the test's expected diagnostic argument.
• The removal of the Method2 test case (Custom + StringMarshallingCustomType) is appropriate since that property no longer exists on the downlevel attribute — users now get a compile error CS0117 instead.

✅ Code style — Consistent with codebase patterns

• The OnlyTypeDefinitionsOutputTest naming follows existing test class naming conventions in the same file (e.g., NoChangeTest, PreprocessorTest, FallbackForwarderTest).
• The partial keyword on the injected LibraryImportAttribute is defensive but appropriate for source-generated types.
• Clean removal of now-unnecessary #pragma warning disable/restore CS0436 in the generated string.

💡 Observation — LibraryImportAttributeDeclaration cleanup

The removed LibraryImportAttributeDeclaration constant in CodeSnippets.cs has no remaining references in the codebase. Clean removal confirmed.

Generated by Code Review for issue #126056 · ◷

[jkoritzinsky]

/ba-g failures unrelated (code from this PR only validated on NetFX test leg)