Grow geometrically in CollectionsMarshal.SetCount

[SimonCropp]

The CollectionsMarshal.SetCount polyfill (the recreated type below net5 in CollectionsMarshal.cs, and the extension for net5-net7 in CollectionsMarshalSetCount.cs) grew the list with list.Capacity = count. That is an exact fit, so every later call that raises the count by a little reallocates and copies the whole backing array: raising the count one at a time is quadratic.

Both copies now grow the way List<T> itself does (and the way the BCL SetCount does through List<T>.Grow): double the current capacity (4 when empty), capped at the maximum array length, and use count when doubling is not enough. Counts, contents and the default-initialised tail are unchanged; the only observable difference is Capacity, which now matches what the BCL produces for the same calls.

Measurements

net48, Release, x64, best of 3, SetCount(list, i) for i = 1..40,000 on a List<int>:

before	after
141.57 ms	0.18 ms

Tests

Added to PolyfillTests_CollectionsMarshal.cs (polyfill below net8, BCL from net8):

• SetCount_GrowKeepsItemsAndDefaultsTheRest, SetCount_GrowFromEmpty, SetCount_GrowWithinCapacity (capacity untouched when it already fits), SetCount_ShrinkThenGrow
• SetCount_GrowsCapacityLikeList: pins the capacity sequence (exact fit when doubling is not enough, otherwise double, 4 from empty), which is the same on the BCL
• SetCount_IncrementalGrowth: 20,000 single-step increments, checking every element survives and the capacity stays within 2x
• SetCount_InvalidArguments