Checking Path Existence in Rust: Speed or Accuracy?

Checking Path Existence in Rust: The Race for Performance

Imagine: you are writing a Rust utility that needs to process hundreds of thousands of files. Or perhaps you are creating a web server that checks for the presence of a configuration file on every request. In such scenarios, even a microsecond of delay can turn into hours of waiting. The question that plagues many developers (and has gathered thousands of views on Stack Overflow): «How to check whether a path exists?».

It would seem, what could be simpler? Call Path::exists() and be happy. But the devil, as always, is in the details. In Rust, there are two main ways to check if a path exists, and the choice between them can seriously impact the performance and reliability of your code. Today, we will not just give an answer, but conduct a real investigation: we will break down exists() and try_exists(), look under the hood of the file system, and create a checklist for optimization.

This material is for those who write in Rust and want their code to be not only correct but also fast. Let's go!

Two Acrobatic Brothers: exists() and try_exists()

In the Rust standard library, the std::path::Path trait is responsible for checking path existence. It has two methods:

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• exists(&self) -> bool — the classic, well-known method.
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• try_exists(&self) -> Result<bool> — a newer one, introduced in Rust 1.40.
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At first glance, the difference is only in the return type: one returns just a bool, the other returns a Result. But behind this lies a fundamental difference in the philosophy of error handling.

The exists() Method: Simplicity That Can Deceive

The exists() method is a convenient wrapper that says: «Yes, the file exists» or «No, the file does not exist.» But what happens if an error occurs during the check? For example, you do not have permission to read the parent directory, or the file system is temporarily unavailable?

Herein lies the catch. exists() swallows all I/O errors. If something goes wrong, the method simply returns false. From a program logic perspective, this can be a disaster: you think the file does not exist and create it, but in reality, it does exist, and you just cannot read it.

Let's look at the code:

use std::path::Path;\n\nfn main() {\n    let path = Path::new("/etc/shadow"); // File accessible only by root\n    if path.exists() {\n        println!("File exists!");\n    } else {\n        println!("File does NOT exist or an error occurred.");\n    }\n}\n

If you run this code as a regular user, exists() will return false, even though technically the file exists. The program will think the file is missing and might try to create it — which will lead to an error in the next step. Unpleasant, isn't it?

The try_exists() Method: Honesty and Control

The try_exists() method is an evolution. It does not hide problems but honestly reports them via Result. If the file exists, you get Ok(true). If it does not exist — Ok(false). And if an error occurs (no permissions, bad disk, network folder unavailable) — you get Err(...).

Here is what correct usage looks like:

use std::path::Path;\n\nfn main() {\n    let path = Path::new("/etc/shadow");\n\n    match path.try_exists() {\n        Ok(true) => println!("File exists and is accessible for checking."),\n        Ok(false) => println!("File definitely does not exist."),\n        Err(e) => eprintln!("Error checking path: {}", e),\n    }\n}\n

Now you are in control of the situation. In case of an error, you can:

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