Unnecessary Fn bound for SizeError::map_src.

[conradludgate]

I am implementing a small wrapper over read_from_bytes that allows working over types like Bytes instead of &[u8]. As such I want to change the Src bound to be Bytes instead of &[u8].

However, the bound on map_src is unnecessarily a Fn(Src) -> NewSrc which implies it would be callable multiple times. This is inconsistent with other forms of map which use FnOnce. This is a problem for me as I cannot simply map_src(|_| bytes_buf) as that moves ownership, giving me the error

error[E0507]: cannot move out of `buf`, a captured variable in an `Fn` closure
   --> proxy/src/protocol2.rs:371:40
    |
363 |         let buf = if self.len() < len {
    |             --- captured outer variable
...
371 |         res.map_err(|e| e.map_src(|()| buf))
    |                                   ---- ^^^ move occurs because `buf` has type `bytes::BytesMut`, which does not implement the `Copy` trait
    |                                   |
    |                                   captured by this `Fn` closure
    |
help: consider cloning the value if the performance cost is acceptable
    |
371 |         res.map_err(|e| e.map_src(|()| buf.clone()))
    |                                           ++++++++

Easy to fix here with a clone, sure. But it's not ideal.

I have a feeling this can be fixed without a breaking change, but I'm not 100% certain.

[joshlf]

This is fixed in 0.8.9.