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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.
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Use FnOnce as a bound when you want to accept a parameter of function-like type and only need to call it once. If you need to call the parameter repeatedly, use FnMut as a bound; if you also need it to not mutate state, use Fn .
Rust's closures are anonymous functions you can save in a variable or pass as arguments to other functions. You can create the closure in one place and then call the closure elsewhere to evaluate it in a different context. Unlike functions, closures can capture values from the scope in which they're defined.
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.
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