fold's param init func should be FnOnce instead of FnMut

[sify21]

Take this for example

nom/tests/arithmetic.rs

Line 60 in 90af84b

move || init,

You can't move the captured variable init out of the "init" func if it is a non-Copy type, even though init variable has been moved into the capturing "init" func. That's because FnMut means "init" can be called multiple times. This only works as a matter of fact that init variable is i64 in this case.

Since this func is called only once, FnOnce is more suitable here.

[Geal]

with the way the Parser trait is designed, it cannot be a FnOnce, because there's no guarantee that the parser would be called only once: you can take the result of fold and call parse on it with various inputs

[sify21]

Got it! A Parser is reusable

[TennyZhuang]

Mirroring my reply here, how about make Init: Parser<Input, Result, Error>?

winnow-rs/winnow#513 (comment)