Diagnostics

Diagnostics

The union rules live in one shared library (Diagnostics/DiagnosticLibrary.cs, Diagnostics/ResultUnionDiagnostics.cs, Diagnostics/ResultDiagnostic.cs) that both hosts consume:

• ResultsDiagnosticAnalyzer reports the per-target rules (RSG1000–RSG1004, RSG1007) in the IDE and in build output.
• The generator reports the compilation-wide rules (RSG1005, RSG1006) that need every opted-in union in the compilation.
• The generator never reports a rule the analyzer reports. It still runs the same shared analysis, so DiagnosticLibrary.IsBlocking is the single blocking policy, but a finding is never surfaced twice.

Rules

ID	Severity	Reported by	Blocking	Description
RSG1000	Error	Analyzer	Yes	[GenerateResult] was applied to a type that is not a union.
RSG1001	Error	Analyzer	Yes	The union declares no union cases.
RSG1002	Error	Analyzer	Union: yes / case: no	A generic union, or a case type that contains type parameters.
RSG1003	Error	Analyzer	Union: yes / case: no	A type that generated code must reference is not accessible (for example a file type).
RSG1004	Error	Analyzer	No	The same union is configured more than once (for example on two partial declarations); the helpers are generated once.
RSG1005	Error	Generator	Yes (the colliding union is skipped)	Two unions produce the same generated class name in one namespace.
RSG1006	Warning	Generator	No (only the shared case’s helper is skipped)	A case type is shared with another union, so its helper is generated once to keep call sites unambiguous.
RSG1007	Error	Analyzer	Yes	The union uses an IUnionMembers member provider, which the first implementation does not support.

Case-level findings never block the union: the remaining cases are still generated and the skipped case is reported. Blocking is decided per rule in DiagnosticLibrary.IsBlocking, not derived from severity, because a finding may be an error the consumer must fix while usable output can still be produced.

Every rule is categorized as Purview.Results.Usage and tracked in src/src/SourceGenerator/AnalyzerReleases.Unshipped.md, which the compiler’s RS2008 rule catalogue validates.

Diagnostic suppressions

A union declaration gets no value equality from the compiler, so every union raises CA1815 (“Override equals and operator equals on value types”) unless it is silenced by hand with a #pragma warning disable CA1815 or a [SuppressMessage]. A [GenerateResult] union is the error type of a result and is read by matching its case type, not by comparing two unions by value, so the package answers that warning:

Suppression ID	Suppressed rule	Applies to
RSG2000	CA1815	A declaration that is a union and is opted in with [GenerateResult]

The suppression is narrow on purpose:

• A union without [GenerateResult] keeps the warning — the package only speaks for the unions it generates for.
• The union’s case types keep the warning, as does every other value type. A case declared as a plain struct still gets CA1815; a record struct case never gets it, because records synthesise equality.
• Nothing is hidden: a suppressor can only suppress non-error, configurable diagnostics, and RSG2000 is a suppression id rather than a rule, so RSG1000–RSG1007 remain the only diagnostics the package reports.

Every suppression is logged as an Info diagnostic against RSG2000, in the verbose build log and in an MSBuild binlog (and as a suppressed diagnostic in an /errorlog SARIF file), so a build can always be audited for what it suppressed.

Keeping CA1815

Add RSG2000 to the compiler’s warning suppressions, and the warning returns for every union declaration, including the opted-in ones:

<PropertyGroup>
  <NoWarn>$(NoWarn);RSG2000</NoWarn>
</PropertyGroup>

Declaring equality on the union is the other way to satisfy CA1815 — it is the code the warning asks for, and a union that has it never raises the warning, so the suppressor has nothing to do.

Related

• Union Errors — the union shapes the rules describe.
• Source Generator — where each host runs and how output is produced.
• Testing — how the analyzer, suppressor and code fix are verified.