Do not report MA0028 on StringBuilder.Insert with a Substring argument

[meziantou]

What

OptimizeStringBuilderUsageAnalyzer reported MA0028 on StringBuilder.Insert when the inserted string came from Substring:

new StringBuilder().Insert(0, s.Substring(1));

Use Insert(string, int, int) or Insert(ReadOnlySpan<char>) instead of Substring

StringBuilder.Insert(string, int, int) does not exist. The closest real overload is Insert(int index, string value, int count), which repeats the value rather than slicing it — so there is no Insert counterpart to suggest here at all.

Why

The Substring branch of IsOptimizable was the only one of the four IInvocationOperation branches missing the methodName != "Insert" guard. The three siblings — ToString, string.Format and string.Join — all begin with it, and the comment above them explains exactly why Insert has to be excluded: those overloads are Append's, and Insert has no counterpart for some of them.

Change

One line, matching the neighbouring branches:

else if (methodName != "Insert" && string.Equals(targetMethod.Name, nameof(string.Substring), ...))

Plus two Insert_NoDiagnostic theory cases ("abc".Substring(2) and "abc".Substring(0, 1)) with a comment recording why Insert is excluded.

Notes for the reviewer

The code fixer was not throwing on these. Its pattern Arguments: [{ Value: IInvocationOperation ... }, ..] fails to match, because argument 0 of Insert is the int index, so it returned before calling RegisterCodeFix and no lightbulb was ever offered. The user-visible defect was the diagnostic message naming a non-existent API, so the fixer needed no change and is untouched.

docs/Rules/MA0028.md never described the Substring/Insert behaviour, so there was nothing to update there.

Verification

• Analyzer builds clean, 0 warnings.
• Reverted the analyzer fix while keeping the new tests: both failed with exactly the reported message, confirming the tests are a real regression guard rather than vacuous. Restored the fix and they pass.
• MA0028 tests pass on all five Roslyn versions (4.8, 4.14, 5.0, 5.6, 5.9) — 107/107 each, up from 105 with the two added cases.
• dotnet run --project src/DocumentationGenerator exits 0 with no markdown changes.