I’m not sure if this is a simpler way to look at it, but to quote the docs, “X .+= Y etcetera is equivalent to X .= X .+ Y”. So v[1:i] .+= @view w[end-i+1:end] is equivalent to v[1:i] .= v[1:i] .+ @view w[end-i+1:end]. Just a hazard of the shorthand +='s left side actually representing duplicate subexpressions on the left and right side of the actual assignment.
I am actually surprised here, because the code was in a local let block to begin with, and all the benchmarks are first-call @time. I can’t spot any globals, and the fact that it’s always 4 allocations and 152 MiB suggests to me there weren’t any type instabilities being fixed.