Logging source generator: support generic methods (lift SYSLIB1011)

[Copilot]

Generic [LoggerMessage] methods were unconditionally rejected with SYSLIB1011. Methods with type parameters are fully supportable via the struct-based code generation path. The only remaining restriction is allows ref struct (C# 13), since the generated struct stores params in fields and cannot hold ref-struct type arguments.

Description

Parser (LoggerMessageGenerator.Parser.cs)

• Removed the method.Arity > 0 → SYSLIB1011 check
• Added LoggerMethodTypeParameter / LoggerMethodTypeParameterSpec record types to carry type parameter names + constraints through the pipeline
• GetTypeParameterConstraints serializes all constraint kinds: class/class?, struct, unmanaged, notnull, base/interface types (incl. nullable), new()
• Preserves all FullyQualifiedFormat.MiscellaneousOptions (EscapeKeywordIdentifiers | UseSpecialTypes) when adding IncludeNullableReferenceTypeModifier — fixes a pre-existing bug on the same call for parameter types
• Skips IErrorTypeSymbol constraint types (unresolvable in generated code)
• allows ref struct detected via Func<ITypeParameterSymbol, bool> delegate compiled from ITypeParameterSymbol.AllowsRefLikeType via Delegate.CreateDelegate (compiles against all supported Roslyn versions; no per-call boxing) — emits SYSLIB1011
• Renamed DiagnosticDescriptors.LoggingMethodIsGeneric → LoggingMethodHasAllowsRefStructConstraint; updated message + all XLF files (only changed entry marked needs-translation)

Emitter (LoggerMessageGenerator.Emitter.cs)

• UseLoggerMessageDefine excludes generic methods — the Define path caches a static delegate and cannot capture method type parameters
• GenStruct emits the state struct as generic (__M1Struct<T>) with matching where constraints
• GenLogMethod emits <T, U> and where clauses on the partial method signature
• GenHolder/Format references parameterized with concrete type arguments
• Replaced GetTypeParameterList/GetTypeConstraints (string-returning, allocating) with GenTypeParameterList/GenTypeConstraints writing directly to _builder
• Removed using System.Linq; IndexOf('{') replaces Contains('{') (no allocating LINQ path on netstandard2.0)

Incremental cache (LoggerMessageGenerator.Roslyn4.0.cs)

• FromSpec round-trips TypeParameters through the incremental cache

Docs

• docs/project/list-of-diagnostics.md updated: SYSLIB1011 now describes the allows ref struct restriction
• External learn.microsoft.com docs tracked in dotnet/docs#51840

Example

// Previously: SYSLIB1011. Now: works, no boxing of TCode.
[LoggerMessage(0, LogLevel.Trace, "Received {Code} ({Length} bytes)")]
public static partial void PacketReceived<TCode>(ILogger logger, TCode code, int length)
    where TCode : struct, Enum;

All constraint forms supported: class, class?, struct, unmanaged, notnull, base type, interface, nullable interface, new(). allows ref struct emits SYSLIB1011.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Logging source generator should support generic methods (SYSLIB1011 should be lifted)</issue_title>
<issue_description>From this section:

Logging methods cannot be generic.

This seems to be an unnecessary restriction that also works against the whole notion of high-performance logging.

I would like to be able to write a logging method like so:

[LoggerMessage(0, LogLevel.Trace, "C -> S {EndPoint}: {Channel}:{Code} ({Length} bytes)")]
public static partial void PacketReceived<TCode>(ILogger logger, IPEndPoint endPoint, ConnectionChannel channel, TCode code, int length)
    where TCode : struct, Enum;

Here, the type argument for TCode depends on the value of Channel. Of course, I could make the logging method non-generic and accept an Enum-typed value, but then I get unconditional boxing... certainly not what I want when logging in a fairly hot path. Alternatively, I could duplicate the method for as many TCodes as I have (which is what I'll do for now), but this is not great either, for all the expected maintenance reasons.

I think this restriction should just be lifted. Maybe there's a good reason for it, but if there is, it's not obvious to me.</issue_description>

<agent_instructions>Implement and test this. Make sure that all of these are supported: multiple type parameters, multiple constraints, various kinds of constraints.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

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Issue Details

---

From this section:

Logging methods cannot be generic.

This seems to be an unnecessary restriction that also works against the whole notion of high-performance logging.

I would like to be able to write a logging method like so:

[LoggerMessage(0, LogLevel.Trace, "C -> S {EndPoint}: {Channel}:{Code} ({Length} bytes)")]
public static partial void PacketReceived<TCode>(ILogger logger, IPEndPoint endPoint, ConnectionChannel channel, TCode code, int length)
    where TCode : struct, Enum;

Here, the type argument for TCode depends on the value of Channel. Of course, I could make the logging method non-generic and accept an Enum-typed value, but then I get unconditional boxing... certainly not what I want when logging in a fairly hot path. Alternatively, I could duplicate the method for as many TCodes as I have (which is what I'll do for now), but this is not great either, for all the expected maintenance reasons.

I think this restriction should just be lifted. Maybe there's a good reason for it, but if there is, it's not obvious to me.

Author:	alexrp
Assignees:	-
Labels:	untriaged, area-Extensions-Logging
Milestone:	-

@cincuranet Triage: Niche scenario, but would be nice to have it one day.

• Fixes Logging source generator should support generic methods (SYSLIB1011 should be lifted) #90589

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Pull request overview

Enables the LoggerMessage source generator to support generic logging methods by carrying method type parameters/constraints through parsing, caching, and emission, while narrowing SYSLIB1011 to only forbid the C# 13 allows ref struct anti-constraint.

Changes:

• Lifted the blanket rejection of generic logging methods; added parsing/serialization of method type parameters + constraint rendering (including nullable constraints).
• Updated emitter to generate generic state structs and generic partial method signatures (and to exclude generic methods from the LoggerMessage.Define fast-path).
• Updated SYSLIB1011 descriptor/message/resources/docs and added/updated parser + emitter baseline tests.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/LoggerMessageGenerator.Parser.cs	Parses method type parameters/constraints; diagnoses allows ref struct; preserves display format options.
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/LoggerMessageGenerator.Emitter.cs	Emits generic structs/methods/constraints; avoids Define path for generic methods; small perf cleanup.
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/LoggerMessageGenerator.Roslyn4.0.cs	Round-trips method type parameter data through the incremental cache.
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/DiagnosticDescriptors.cs	Renames/narrows SYSLIB1011 descriptor to the allows ref struct restriction.
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/Strings.resx	Updates SYSLIB1011 message text and resource key.
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.cs.xlf	Updates localized resource key/message for SYSLIB1011 (marked needs-translation).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.de.xlf	Same as above (German).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.es.xlf	Same as above (Spanish).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.fr.xlf	Same as above (French).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.it.xlf	Same as above (Italian).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.ja.xlf	Same as above (Japanese).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.ko.xlf	Same as above (Korean).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.pl.xlf	Same as above (Polish).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.pt-BR.xlf	Same as above (Portuguese - Brazil).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.ru.xlf	Same as above (Russian).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.tr.xlf	Same as above (Turkish).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.zh-Hans.xlf	Same as above (Chinese - Simplified).
src/libraries/Microsoft.Extensions.Logging.Abstractions/gen/Resources/xlf/Strings.zh-Hant.xlf	Same as above (Chinese - Traditional).
src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/LoggerMessageGeneratorParserTests.cs	Adds/updates tests for generic methods, constraints, nullable constraints, and allows ref struct diagnostic.
src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/LoggerMessageGeneratorEmitterTests.cs	Adds emitter baseline test covering generic methods.
src/libraries/Microsoft.Extensions.Logging.Abstractions/tests/Microsoft.Extensions.Logging.Generators.Tests/Baselines/TestWithGenericMethods.generated.txt	New baseline output verifying correct generic codegen.
docs/project/list-of-diagnostics.md	Updates SYSLIB1011 documentation to reflect the narrowed restriction.

[svick]

The equivalent PR in the telemetry logging source generator is at dotnet/extensions#7331.