verbose-file-reads false positive

[MikailBag]

Reproducer on playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a06d5c34e0ec3cd9eafbd42af458028f

std::fs::read only supports reading from path, not from raw handle.

[MikailBag]

Second bug: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8e49e12c99d43fe69dbfdbe375f785bf
If this code will be rewritten using std::fs::read, it won't be able to reuse one allocation, because std::fs::read allocates and returns new buffer

[MikailBag]

Third bug: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9f18b7ce2a3758ff5d37cd4e08ca4afe
Current behavior: all file are read into same buffer, so their contents are concatenated easily.
With std::fs::read: I need manually append.
It is less efficient and it is not less verbose

[flip1995]

I'd move this lint to nursery, but since these bugs are hard to fix, and therefore this lint probably won't get out of nursery any time soon. But since I think the lint idea is good, I'll move it to restriction, so it can be enabled if someone wants to check their code about this.

[vorner]

I would like to add another false positive to this lint. Here the provided hint doesn't really make sense at all:

use std::fs::File;
use std::io::Read;
fn main() {
    let mut f = File::open("example.txt").unwrap();
    let mut header = [0; 4];
    f.read_exact(&mut header).unwrap();
    let mut rest = Vec::new();
    f.read_to_end(&mut rest).unwrap();
}

Further similar ones would include seeking around the file and reading it multiple times, opening the file sooner, then deleting it on the FS but keeping it open and similar cases.

Clippy version:

clippy 0.0.212 (23549a8 2020-03-16)