rust - When does a closure implement Fn, FnMut and FnOnce? -

what specific conditions closure implement fn, fnmut , fnonce traits?

that is:

• when closure not implement fnonce trait?
• when closure not implement fnmut trait?
• when closure not implement fn trait?

for instance, mutating state of closure on it's body makes compiler not implement fn on it.

the traits each represent more , more restrictive properties closures/functions, indicated signatures of call_... method, , particularly type of self:

• fnonce (self) functions can called once,
• fnmut (&mut self) functions can called if have &mut access environment
• fn (&self) functions can still called if have & access environment.

a closure |...| ... automatically implement many of can.

• all closures implement fnonce: closure can't called once doesn't deserve name. note if closure implements fnonce, can called once.
• closures don't move out of captures implement fnmut, allowing them called more once (if there unaliased access function object).
• closures don't need unique/mutable access captures implement fn, allowing them called everywhere.

these restrictions follow directly type of self , "desugaring" of closures structs (described in finding closure in rust).