Rust: What the fuck is a borrow checker and why is it yelling at me?

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Rust: What the fuck is a borrow checker and why is it yelling at me?

pseudorandom name: May 6, 2007

Firefox has been shipping a Rust MP4 container parser for a while now.

# ¿ Oct 2, 2017 02:33

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# ¿ Apr 29, 2024 11:47

pseudorandom name: May 6, 2007

const test: f64 = 1. * 10.; works because you aren't trying to call a method on an unknown type.

const ELECTRON_MASS: f64 = 9.109390 * (10.).powi(-31); fails because you're trying to do a method lookup on some kind of floating point type, but the compiler doesn't know which one yet, hence the error.

If you change it to something that isn't ambiguous, like const ELECTRON_MASS: f64 = 9.109390 * 10.0f64.powi(-31);, the error becomes

code:

error[E0015]: calls in constants are limited to tuple structs and tuple variants
 --> src/lib.rs:1:39
  |
1 | const ELECTRON_MASS: f64 = 9.109390 * 10.0f64.powi(-31);
  |                                       ^^^^^^^^^^^^^^^^^
  |
note: a limited form of compile-time function evaluation is available on a nightly compiler via `const fn`
 --> src/lib.rs:1:39
  |
1 | const ELECTRON_MASS: f64 = 9.109390 * 10.0f64.powi(-31);
  |                                       ^^^^^^^^^^^^^^^^^

error: aborting due to previous error
For more information about this error, try `rustc --explain E0015`.

# ¿ Sep 28, 2018 00:28

pseudorandom name: May 6, 2007

std::vec::Vec::into_boxed_slice()

# ¿ May 20, 2022 01:28

pseudorandom name: May 6, 2007

notwithoutmyanus posted:

I'm trying to wrap my head around this:
code:
fn main() {
    let mut a: [i32; 6] = [10, 20, 30, 40, 50, 60];
    println!("a: {a:?}");

    let s: &[i32] = &a[2..4];

    println!("s: {s:?}");
}
That part that throws me off is: let s: &[i32] = &a[2..4]; . Wouldn't specifying that you're borrowing cells 2-4 print out 20 30 and 40, not just 30 and 40? I know I'm wrong/missing something here, so why am I wrong?

Arrays are zero-based, the element at offset 2 is the third element in the array.

# ¿ Oct 26, 2023 02:37

pseudorandom name: May 6, 2007

BitTorrent uses Bencode and Bencode already has a serde implementation.

It�s really easy to use a serialization library with a serialization format, especially when the work has already been done for you. Not so much for ad hoc bit twiddling.

# ¿ Dec 16, 2023 21:27

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > Rust: What the fuck is a borrow checker and why is it yelling at me?