What are the specific conditions for a closure to implement the Fn
, FnMut
and FnOnce
traits?
That is:
FnOnce
trait?FnMut
trait?Fn
trait?For instance, mutating the state of the closure on it's body makes the compiler not implement Fn
on it.
The traits each represent more and more restrictive properties about closures/functions, indicated by the signatures of their call_...
method, and particularly the type of self
:
FnOnce
(self
) are functions that can be called onceFnMut
(&mut self
) are functions that can be called if they have &mut
access to their environmentFn
(&self
) are functions that can be called if they only have &
access to their environmentA closure |...| ...
will automatically implement as many of those as it can.
FnOnce
: a closure that can't be called once doesn't deserve the name. Note that if a closure only implements FnOnce
, it can be called only once.FnMut
, allowing them to be called more than once (if there is unaliased access to the function object).Fn
, allowing them to be called essentially everywhere.These restrictions follow directly from the type of self
and the "desugaring" of closures into structs; described in my blog post Finding Closure in Rust.
For information on closures, see Closures: Anonymous Functions that Can Capture Their Environment in The Rust Programming Language.