I'm building a little Rust application used to make some statistics about programs and projects. My initial goal was to make it generic, so it can be modified and used as a library.
To do that I've created a Processor structure:
pub struct Processor {
handles: Vec<Box<dyn FileTypeHandle>>,
files_types: HashMap<String, Box<dyn FileType>>,
bytes_count: usize,
files_count: usize,
dirs_count: usize,
}
What I've named handle is an implementation of FileTypeHandle trait that checks, from the extension of a file, if it can handle it. If it can, the file is processed by it and statistics are made.
Secondly, file_types are implementations of FileType trait that implement methods to write final statistics in console.
Let's assume that my handles vector is filled with some handles.
First, I needed to implement a function in the Processor to search in handles vector for one that can handle the ext file extension:
pub fn handle_ext(&mut self, ext: &String) -> Option<&Box<dyn FileTypeHandle>> {
self.handles
.iter()
.find(move |handle: &&Box<dyn FileTypeHandle>| handle.can_handle(ext))
}
FileTypeHandle::can_handle is defined as:
fn can_handle(&self, ext: &String) -> bool;
I'm using the next function to get a file type in the processor, or create a default one:
pub fn get_file_type(&mut self, ext: &String) -> Option<&Box<dyn FileType>> {
if !self.files_types.contains_key(ext) {
let base = FileTypeBase::new(String::clone(ext));
if let Some(v) = self.handle_ext(ext) {
self.files_types.insert(String::clone(ext), v.handle(base));
}
}
self.files_types.get(ext)
}
I think you understand what I expect from this code, but:
error[E0499]: cannot borrow `self.files_types` as mutable more than once at a time
--> src\processor.rs:61:17
|
59 | if let Some(v) = self.handle_ext(ext) {
| ---- first mutable borrow occurs here
60 |
61 | self.files_types.insert(String::clone(ext), v.handle(base));
| ^^^^^^^^^^^^^^^^ - first borrow later used here
| |
| second mutable borrow occurs here
I've read lot of Rust documentation including the official book, but I didn't know why the static compilation check fails.
Okay, line 61 uses the self.file_types as said in the error message, but why is it the second borrow?! The first underlined word on line 59 is just self (or self.handle_ext?) which are not borrowing self.file_types.
