What happens to a coroutine when its' coding finishes executing?

Depending on how and where the coroutine is created, the garbage collector may or may not automatically collect it.

Roblox automatically collects garbage at a set interval. For the purposes of simulation, I'm collecting garbage manually (via collectgarbage()) Since collectgarbage is disabled in Roblox Lua, you may use an online sandbox like this one.

• Coroutines created (but not stored in a variable) are marked for garbage collection.

collectgarbage()
print("Before coroutines:", collectgarbage("count"))

for i = 1, 1000 do
    -- I don't have to resume the coroutine for it to be marked for garbage collection
    coroutine.resume( coroutine.create(function() end) ) 
end

collectgarbage()
print("After coroutines:", collectgarbage("count"))

prints:

Before coroutines:	27.6806640625
After coroutines:	27.458984375

• Coroutines created and stored in a variable will be marked for garbage collection after it reaches the end of its lifetime (much like any other variable):

collectgarbage()
print("Before coroutines:", collectgarbage("count"))

for i = 1, 1000 do
    local cor = coroutine.create(function() end )
    -- Again, I don't have to run the coroutine for it to be marked for garbage collection
    coroutine.resume(cor)  
end

collectgarbage()
print("After coroutines:", collectgarbage("count")

prints:

Before coroutines:	27.7314453125
After coroutines:	27.509765625

• Coroutines that are stored in a variable that has not yet reached the end of its lifetime will NOT be marked for garbage collection, regardless of whether or not it has stopped.

collectgarbage()
print("Before coroutines:", collectgarbage("count"))

local t = {}

for i = 1, 1000 do
    table.insert(t, coroutine.create(function() end) )
    coroutine.resume(t[i])
end

collectgarbage()
print("After coroutines:", collectgarbage("count"))

prints:

Before coroutines:	27.7998046875
After coroutines:	856.1953125