Does `MutableDiffResult` have to exist at all?

Preliminary: DiffResult objects must always be realiased when updated, as mentioned in the docs

https://juliadiff.org/DiffResults.jl/stable/#DiffResults.value!

In most cases it is unnecessary, because the result object itself gets mutated. However, it causes a heap allocation to create the mutable struct defined here:

https://github.com/JuliaDiff/DiffResults.jl/blob/724a23d326353353aa78ae9b9e770685119492dc/src/DiffResults.jl#L19-L25

Here's a proof of concept: instead of the current code

https://github.com/JuliaDiff/DiffResults.jl/blob/724a23d326353353aa78ae9b9e770685119492dc/src/DiffResults.jl#L156-L157

we could instead return a new object every time

value!(r::MutableDiffResult, x::Number) = MutableDiffResult(x, r.derivs)
value!(r::MutableDiffResult, x::AbstractArray) = (copyto!(value(r), x); return r)

• Would it decrease or increase the performance of the update? Don't know
• Would it be considered a breaking change? Perhaps (see below)
• Would it break stuff in practice? Definitely (see below)

Start with the value! documentation and the MutableDiffResult definition in src/DiffResults.jl at lines 19–25, then inspect the update code at lines 156–157. Compare the proposed behavior for scalar and array updates, and establish whether the change affects performance or compatibility. Done means a maintainer-approved decision on whether MutableDiffResult should remain and what API behavior is supported.