5.6 Loops

for u in users { … }
for (i, u) in users.enumerate() { … }
for i in 0..n where i % 2 == 0 { … }      // filter
for await msg in input { … }               // stream<T>
while cond { … }
while let x? = queue.pop() { … }
loop { if done { break result } }          // loop is an expression
'outer: for a in xs { for b in ys { if a == b { break 'outer } } }