DiagnosticCatalog silently hands out duplicate diagnostic IDs when a number is reused

[matt-edmondson]

What's wrong

DiagnosticCatalog.Add (SourceGeneratorToolkit/DiagnosticCatalog.cs:~56-68) formats {prefix}{number:000} without checking whether that number is already in use.

Reproduction (ran)

var catalog = new DiagnosticCatalog("X", "c");
catalog.Warning(1, "a", "a");
catalog.Error(1, "b", "b");

Both descriptors get the ID X001, with different titles, severities and messages, and both appear in Descriptors.

Why it matters

The catalog exists so that IDs can't drift or collide. With duplicates:

• a .editorconfig severity override or #pragma suppression meant for one diagnostic silently applies to the other;
• the docs and the analyzer release tracking see two different rules under one ID;
• a test that walks Descriptors doesn't flag it.

Suggested fix / acceptance criteria

• Track the numbers already used (a HashSet<int>) and throw ArgumentException from Add when a number is reused.
• Add a test: registering the same number twice throws.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. Two diagnostics under one ID break .editorconfig severity overrides and #pragma suppressions without any error. It only happens when a generator author reuses a number by mistake, and the fix is small and local.
• Area / suggested assignee: DiagnosticCatalog.Add in SourceGeneratorToolkit/DiagnosticCatalog.cs. Maintainer: @matt-edmondson.
• Duplicates / in progress: No duplicate and no open PR. Open PR Bump the ktsu group with 10 updates #18 is a dependency bump that doesn't touch this code.
• Next steps: Keep a HashSet<int> of the numbers already used and throw ArgumentException when one is reused. Add a test that registers the same number twice through Warning and Error and expects the throw.

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Generated by Claude Code