I'd like to achieve something similar to the code below, but without incurring the costs of String (UTF-8 validation mainly), trying to use &[u8] instead:
fn main() {
let mut outer: Vec<String> = Vec::new();
{
for _ in 0..1 {
let mut inner: Vec<&str> = Vec::new();
inner.push("hello");
inner.push("world");
outer.push(inner.join(",").to_string());
}
}
}
But my &[u8]-ified attempt below does not compile (https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=20130b6377241f1566a724440522443e)
fn main() {
let mut outer: Vec<&[u8]> = Vec::new();
{
for _ in 0..1 {
let mut inner: Vec<&[u8]> = Vec::new();
inner.push(b"hello");
inner.push(b"world");
outer.push(&inner.join(&b','));
}
}
}
Rustc complains that:
error[E0716]: temporary value dropped while borrowed
--> src/lib.rs:8:25
|
8 | outer.push(&inner.join(&b'!'));
| ----- ^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| | |
| | creates a temporary which is freed while still in use
| borrow later used here
|
= note: consider using a `let` binding to create a longer lived value
While I do understand why the Rust compiler rejects this code, I do not know how to actually fix this, while keeping the concept intact. I basically want to transfer ownership of the result of inner.join(&b',') into the outer Vec, More generally, creating temporary [u8] for text in a loop using join(), that are then to be pushed into a outer Vec, without running into ownership issues.
Background: parsing an enormous quantity (as in hundreds of terabytes) of csv files using the csv crate, which has variants working with ByteBuffers and with Strings, the larger having a measurable negative performance impact. In the full code, I loop though some fields of each csv record, sometimes modify the value. The selected fields are added to the "inner" Vec and then want to create a csv record again with join() to add Vec of selected records. To avoid the noise of the full code, including CSV parsing etc, I created this MRP above to depict the kind of problem I am facing.
Disclaimer: Still learning Rust, so your understanding and some didactic feedback would be appreciated.
