+= and friends do not count as mutated without reading

[Kampfkarren]

local function test(x)
    x += 1
end

This counts x as used

[chriscerie]

It's good to note fixing this could false positive if x implements some metamethod magic, but it's such a niche case it can probably be ignored.

Some more cases

Should remain marked as used

local function test(x)
    x.y += 1
end

Should not be marked as used

local x = {}
x.y += 1

Should remain marked as used

local x = a()
x.y += 1