Take R: Read and W: Write by value

[dtolnay]

The standard library contains these two impls:

impl<'a, R: Read + ?Sized> Read for &'a mut R
impl<'a, W: Write + ?Sized> Write for &'a mut W

That means any function that accepts R: Read or W: Write by value can be called with a mut reference if necessary.

Real examples:

// from flate2
impl<R: Read> DecoderReader<R> {
    pub fn new(r: R) -> Self
}

// from serde_json
pub fn from_reader<R, T>(rdr: R) -> Result<T> where R: Read, T: Deserialize

In the documentation of such functions, briefly remind users that a mut reference can be passed. New Rust users often struggle with this ("I want to deserialize two values from the same fs::File but I passed the file by value so I can't use it anymore").

[brson]

"Interop" section.

[dtolnay]

Written in eceb521.

[lfairy]

Can this be a Clippy lint (if it isn't already)?