Moving local data out of function

Question

I'm pretty new to Rust and struggling with the concepts of ownership. In this example, I'm using SIMD Json for Rust library to parse JSON. It has a concept of parsing "borrowed values" - JSON objects referencing the original JSON bytes.

To make this example self-sufficient, let's use these signatures:

// Emulating simd_json::BorrowedValue
pub enum BorrowedValue<'value> {
    SomeEnumOption(&'value str)
}

// Emulating simd_json::to_borrowed_value parsing function
pub fn to_value(buf: &mut [u8]) -> Result<BorrowedValue, String> {
    todo!()
}

Now I want to make a self-sufficient struct that would carry both the original JSON as well as the parsed structue:

pub struct ParsedJson<'value> {
    file_content: Vec<u8>,
    pub parsed: BorrowedValue<'value>,
}

However, I fail to create such struct:

fn transform<'value>(mut buf: Vec<u8>) -> Result<ParsedJson<'value>, String> {
    let mut result = ParsedJson {
        file_content: buf,
        parsed: BorrowedValue::SomeEnumOption("nothing here yet"),
    };

    match to_value(&mut result.file_content) {
        Ok(bw) => {
            result.parsed = bw;
            Ok(result)
        }
        Err(e) => Err(e)
    }
}

This results in an error:

error[E0515]: cannot return value referencing local data `result.file_content`
  --> src/json.rs:39:13
   |
36 |     match to_value(&mut result.file_content) {
   |                    ------------------------ `result.file_content` is borrowed here
...
39 |             Ok(result)
   |             ^^^^^^^^^^ returns a value referencing data owned by the current function

Why is result.file_content considered local data? I expected I could move it out.

To add to the confusion, it seems to not be considered a local data anymore if I don't pass a mutable reference to to_value:

fn transform<'value>(mut buf: Vec<u8>) -> Result<ParsedJson<'value>, String> {
    let mut result = ParsedJson {
        file_content: buf,
        parsed: BorrowedValue::SomeEnumOption("nothing here yet"),
    };

    Ok(result) // This compiles just fine!
}

What is the problem and what is the proper way to implement that? I'm completely lost here, any pointers would be appreciated.