While working on bug 1894012 I wanted to add a WeakCached map where the keys were UniquePtrs and was surprised this wasn't possible. It turns out that our weak tracing for maps calls the copy constructors of keys and values and that doesn't work with something like UnqiuePtr where the copy constructor is deleted.
Bug 1895462 Comment 0 Edit History
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While working on bug 1894012 I wanted to add a WeakCached map where the keys were UniquePtrs and was surprised this wasn't possible. It turns out that our incremental sweeping for maps and sets calls the copy constructors of keys and values and that doesn't work with something like UniquePtr where the copy constructor is deleted.