rustScope errorMedium

String modification fails with Cow always borrowing issue

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What it should do

Modify a Cow<str> to uppercase its contents. Example: f(x) -> y

What happens instead

error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immutable

use std::borrow::Cow;

fn modify_string(s: &mut Cow<str>) -> Cow<str> {
    let borrowed = &s;
    s.to_mut().make_ascii_uppercase();
    borrowed.clone()
}

fn main() {
    let mut s = Cow::from("hello");
    let result = modify_string(&mut s);
    println!("{}", result);
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

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use std::borrow::Cow; fn modify_string(s: &mut Cow<str>) -> Cow<str> { let borrowed = &s; s.to_mut().make_ascii_uppercase(); borrowed.clone() } fn main() { let mut s = Cow::from("hello"); let result = modify_string(&mut s); println!("{}", result); }

String modification fails with Cow always borrowing issue

What it should do

What happens instead

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String modification fails with Cow always borrowing issue

What it should do

What happens instead

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