Do not report MA0078 when the selector does more than casting its element

[meziantou]

Problem

MA0078 took the first return operation and the first lambda found anywhere under the Select selector argument, without checking that the cast was the whole selector body. The code fix then replaced Select with Cast<T>(), which could change behavior:

static int count;

var values = new object[] { "a" }
    .Select(x => { count++; return (string)x; }).ToArray();
// Fixed to: (new object[] { "a" }).Cast<string>().ToArray();  -> count++ is silently removed

The same logic also reported:

• selectors with several return paths, e.g. dt => { if (dt.Name is not null) return (BaseType)dt; return null; }
• a cast in a lambda that is not the selector itself, e.g. source.Select(Wrap(dt => (BaseType)dt))

Fix

The selector must now be a lambda passed directly as the argument (IDelegateCreationOperation targeting an IAnonymousFunctionOperation), whose body is exactly one return of a cast of its first parameter. The existing checks (no as, no numeric/user-defined conversions, enum handling) are unchanged.

x => (T)x and x => { return (T)x; } are still reported and fixed.

Note for reviewers: a lambda explicitly cast to a delegate type, such as Select((Func<A, B>)(x => (B)x)), may no longer be reported. This errs on the safe side, and no existing test covered it.

Tests

• Added no-diagnostic cases for the side effect, the multiple return paths, and the nested lambda. All three fail with the previous analyzer.
• Added a code fix case for a statement-bodied selector with a single return.
• OptimizeLinqUsageAnalyzerTests (157 tests) pass on Roslyn 4.8, 4.14, 5.0, 5.6 and 5.9. The full suite was not run locally.

Docs

docs/Rules/MA0078.md now states which selectors are reported, and its compliant example (Cast<object>(str), which did not compile) is fixed.