I want to know if the folder foo
exists in my current directory, so I wrote a function to do so:
use std::env;
use std::fs;
use std::io;
fn does_folder_foo_exist_in_current_directory() -> Result<bool, io::Error> {
let cur_path_buf = env::current_dir()?;
let cur_dir = cur_path_buf.as_path();
Ok(fs::read_dir(cur_dir)?.find(|ref x| {
let x = x.unwrap();
x.file_type().unwrap().is_dir() && x.file_name().to_str().unwrap() == "foo"
}).is_some())
}
However, the compiler says that I cannot move out of borrowed content here: let x = x.unwrap();
.
Why is this moving out of borrowed content since I ref x
?
ref
in patterns is used to construct a reference. If the pattern x
would have type T
, then the pattern ref x
will have type &T
instead. However, it's not valid to move out of a reference, so you definitely don't want to construct a reference! ( unwrap
takes self
by value, which is why the code is trying to do a move in the first place.)
Here, the type of the parameter on the closure is a reference, because that's what Iterator::find
wants to pass as an argument. If you want to deconstruct a reference, you want to use &
instead. However, if you write the pattern &x
here, you'll still get the error cannot move out of borrowed content
, but this time directly on &x
.
What can we do instead? DirEntry
doesn't implement Clone
, therefore we can't clone x
(which is an &std::io::Result<DirEntry>
). Instead, we could turn the &Result<DirEntry>
into a Result<&DirEntry>
. There's a method in the standard library to do just that: as_ref
.
fn does_folder_foo_exist_in_current_directory() -> Result<bool, io::Error> {
let cur_path_buf = env::current_dir()?;
let cur_dir = cur_path_buf.as_path();
Ok(fs::read_dir(cur_dir)?.find(|x| {
let x = x.as_ref().unwrap();
x.file_type().unwrap().is_dir() && x.file_name().to_str().unwrap() == "foo"
}).is_some())
}
By the way, instead of doing find(...).is_some()
, you can use any(...)
, which is shorter and perhaps slightly more efficient. any
also passes ownership of each iterated value to the closure, so we don't actually need to use as_ref
with it!
fn does_folder_foo_exist_in_current_directory() -> Result<bool, io::Error> {
let cur_path_buf = env::current_dir()?;
let cur_dir = cur_path_buf.as_path();
Ok(fs::read_dir(cur_dir)?.any(|x| {
let x = x.unwrap();
x.file_type().unwrap().is_dir() && x.file_name().to_str().unwrap() == "foo"
}))
}
There's no reason to iterate over all the entries in a directory to check if a single item exists. Just check for the specific item:
use std::{env, fs, io};
fn does_folder_foo_exist_in_current_directory() -> io::Result<bool> {
let mut path = env::current_dir()?;
path.push("foo");
let metadata = fs::metadata(path)?;
Ok(metadata.is_dir())
}
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