Add MA0242: Use a value factory with ConcurrentDictionary.GetOrAdd/AddOrUpdate when the value is expensive

[meziantou]

Fixes #1610

What

New rule MA0242 (Performance, Info, enabled by default) with a code fix.

ConcurrentDictionary<TKey, TValue>.GetOrAdd(key, value) and AddOrUpdate(key, addValue, updateValueFactory) evaluate the value before the call, even when the key is already in the dictionary. The rule reports the value arguments that are not cheap to evaluate, and suggests the overloads taking a Func<TKey, TValue>.

dict.GetOrAdd("1", value);                                   // ok
dict.GetOrAdd("2", (int)floatValue);                         // ok
dict.GetOrAdd("3", (int)Math.Ceiling(Math.Pow(value, 2)));   // MA0242
lists.GetOrAdd("key", new List<int>());                      // MA0242

• Reported: invocations, object creations, collection expressions, string concatenations, non-constant interpolated strings, indexers, await, struct creation with a user-defined constructor.
• Not reported: constants, locals, parameters, this, fields, properties without arguments, array elements, default, typeof, sizeof, lambdas / method groups, throw expressions, non-user-defined conversions, and built-in value-type operators / ?: / ?? / ?. / tuples whose operands are themselves cheap.

Code fix

Wraps the expression in a lambda: GetOrAdd(key, obj.ToString()) → GetOrAdd(key, _ => obj.ToString()).

• Named arguments value: / addValue: are renamed to valueFactory: / addValueFactory:.
• If _ is already in scope, the parameter is named _1, _2, ...
• The fix is only registered after checking that the rewritten document binds to the factory overload and does not introduce new compilation errors (e.g. await, ref structs, out var used after the call).

Reviewer notes

• Properties without arguments are considered cheap, which is broader than the "simple variable" wording in the issue, to avoid noise on obj.Name-style arguments.
• The fix changes when the value is evaluated (side effects are skipped when the key exists); this is documented in docs/Rules/MA0242.md.
• Tests pass on all Roslyn versions (roslyn4.8, 4.14, 5.0, 5.6, 5.9); full roslyn5.9 suite passes (5409 tests). dotnet run --project src/DocumentationGenerator produces no further changes.