bytes-no-mut.rs

use std::fs::File;
use std::io::Read;

fn validate_signature(_f: impl Read) {
}

struct Headers{}

fn read_database_headers(_f: impl Read) -> Headers {
    Headers{}
}

fn read_database(mut f: impl Read) {
    let mut buf = Vec::new();
    f.read_to_end(&mut buf).unwrap();
    let mut bytes = buf.as_slice();
    
    // using bytes here doesn't actually work, since the "cursor"/"position"
    // within the buffer isn't ever updated - all reads from bytes when it's
    // passed in start at the beginning (because the slice header can't get
    // updated).  What's surprising to me is that this *compiles*

    validate_signature(bytes);

    let _headers = read_database_headers(bytes);

    let mut cipher_text = Vec::new();
    bytes.read_to_end(&mut cipher_text).unwrap();

}

fn main() {
    let f = File::open("/dev/null").unwrap();
    read_database(f);
}

file-mut.rs

use std::fs::File;
use std::io::Read;

fn validate_signature(mut _f: impl Read) {
}

struct Headers{}

fn read_database_headers(mut _f: impl Read) -> Headers {
    Headers{}
}

fn read_database(mut f: impl Read) {
    validate_signature(&mut f);

    let _headers = read_database_headers(&mut f);

    let mut cipher_text = Vec::new();
    f.read_to_end(&mut cipher_text).unwrap();

}

fn main() {
    let f = File::open("/dev/null").unwrap();
    read_database(f); // it surprises me that I don't need to declare f as `let mut f`, nor do I need to pass in `&mut f`
}

file-no-mut-errors.txt

error[E0382]: use of moved value: `f`
  --> src/main.rs:16:42
   |
13 | fn read_database(f: impl Read) {
   |                  - move occurs because `f` has type `impl Read`, which does not implement the `Copy` trait
14 |     validate_signature(f);
   |                        - value moved here
15 | 
16 |     let _headers = read_database_headers(f);
   |                                          ^ value used here after move
   |
help: consider further restricting this bound
   |
13 | fn read_database(f: impl Read + Copy) {
   |                               ^^^^^^

error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
  --> src/main.rs:19:5
   |
13 | fn read_database(f: impl Read) {
   |                  - help: consider changing this to be mutable: `mut f`
...
19 |     f.read_to_end(&mut cipher_text).unwrap();
   |     ^ cannot borrow as mutable

error[E0382]: borrow of moved value: `f`
  --> src/main.rs:19:5
   |
13 | fn read_database(f: impl Read) {
   |                  - move occurs because `f` has type `impl Read`, which does not implement the `Copy` trait
...
16 |     let _headers = read_database_headers(f);
   |                                          - value moved here
...
19 |     f.read_to_end(&mut cipher_text).unwrap();
   |     ^ value borrowed here after move
   |
help: consider further restricting this bound
   |
13 | fn read_database(f: impl Read + Copy) {
   |                               ^^^^^^

Some errors have detailed explanations: E0382, E0596.
For more information about an error, try `rustc --explain E0382`.

file-no-mut.rs

use std::fs::File;
use std::io::Read;

fn validate_signature(_f: impl Read) {
}

struct Headers{}

fn read_database_headers(_f: impl Read) -> Headers {
    Headers{}
}

fn read_database(f: impl Read) {
    validate_signature(f);

    let _headers = read_database_headers(f);

    let mut cipher_text = Vec::new();
    f.read_to_end(&mut cipher_text).unwrap();

}

fn main() {
    let f = File::open("/dev/null").unwrap();
    read_database(f);
}