Borrow checked treat value as borrowed after using `to_owned`

[ppoliani]

I've got the following code snippet which is part of a bigger function and I have borrow checker complaining for (obviously valid) reason but I don't quite understand why.

 async fn func() -> Result<()> { 
  let mut file_mime_sub_type = None;

  while let Some(item) = payload.try_next().await? {
    let mut field = item;
    let mut content = vec![];
    
    // Field in turn is stream of *Bytes* object
    while let Some(chunk) = field.next().await {
      let chunk = chunk?;
      content.push(chunk);
    }

    let field_name = field.name();
    let content_type = field.content_type().unwrap_or(&mime::TEXT_PLAIN).to_owned();
    let mime_subtype = content_type.subtype();
    let content = content.concat();

    if mime_subtype.eq(&mime::TEXT_PLAIN.subtype()) {
      
    } else if is_supported_media_type(mime_subtype) {
      file_mime_sub_type = Some(mime_subtype);
    }
  }

  if let Some(mime_subtype) = file_mime_sub_type {
    // do something
  }
  ...
}

The error I see is:

   |
63 |     let content_type = field.content_type().unwrap_or(&mime::TEXT_PLAIN).to_owned();
   |         ------------ binding `content_type` declared here
64 |     let mime_subtype = content_type.subtype();
   |                                        ^^^^^^^^^^^^ borrowed value does not live long enough
...
79 |   }
   |   - `content_type` dropped here while still borrowed
...
93 |   if let Some(mime_subtype) = file_mime_sub_type {
   |                                                    ------------------ borrow later used here

I'm struggling to understand why content_type is a borrowed value. I did use to_owned and then assigned it to file_mime_sub_type. Doesn't this move the owned value to that variable?

Below I add the fn signature of a couple of function. Hopefully that helps.

pub fn content_type(&self) -> Option<&mime::Mime> {
    self.ct.as_ref()
}


pub fn subtype(&self) -> Name {}
``

[jofas]

@ppoliani

I'm struggling to understand why content_type is a borrowed value. I did use to_owned and then assigned it to file_mime_sub_type. Doesn't this move the owned value to that variable?

The content_type is indeed owned. But the Name returned by calling content_type.subtype() borrows from the content_type. You don't assign the owned content_type to file_mime_sub_type thus transferring ownership, but the borrowing mime_subtype. That is causing the lifetime error. To fix this I would suggest assigning file_mime_sub_type^[1] to content_type instead of mime_subtype. You can always retrieve the subtype when you need it and there is no convenient way to convert a Name<'a> to Name<'static> and make Name own the underlying data.

---

1. Renaming the variable to file_mime_type instead I guess. ↩︎

[ppoliani]

Thanks for the prompt reply mate, much appreciated. However, if Name is copy so shouldn't it simply copy it?

#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]

pub struct Name<'a> {}

Also why wouldn't the following work either (fails with the same exact error)

let content_type = field.content_type().unwrap_or(&mime::TEXT_PLAIN).to_owned();
let mime_subtype = content_type.subtype().to_owned();

[SkiFire13]

.to_owned() for a type that implements Clone is simply a .clone(). For Name this means you're simply copying the reference it contains (but not the referenced value!). Thus the value you get is still borrowing the same thing.

[jofas]

The Copy implementation of Name just copies the reference inside of Name to the underlying string slice, it doesn't copy the content of the str and makes it owned data.

[ppoliani]

Oh ok that does make sense indeed. Thanks buddy Basically, Name still has the lifetime of content_type. Got it now.