std::fs::canonicalize for files that don't exist

Question

I'm writing a program in Rust that creates a file at a user-defined path. I need to be able to normalize intermediate components (~/ should become $HOME/, ../ should go up a directory, etc.) in order to create the file in the right place. std::fs::canonicalize does almost exactly what I want, but it panics if the path does not already exist.

Is there a function that normalizes componenets the same way as std::fs::canonicalize but doesn't panic if the file doesn't already exist?

Answer 1

There are good reasons such a function isn't standard:

1. there's no unique path when you're dealing with both links and non existing files. If a/b is a link to c/d/e, then a/b/../f could either mean a/f or c/d/f

2. the ~ shortcut is a shell feature. You may want to generalize it (I do), but that's a non obvious choice, especially when you consider ~ is a valid file name in most systems.

This being said, it's sometimes useful, in cases those ambiguities aren't a problem because of the nature of your application.

Here's what I do in such a case:

use {
    directories::UserDirs,
    lazy_regex::*,
    std::path::{Path, PathBuf},
};

/// build a usable path from a user input which may be absolute
/// (if it starts with / or ~) or relative to the supplied base_dir.
/// (we might want to try detect windows drives in the future, too)
pub fn path_from<P: AsRef<Path>>(
    base_dir: P,
    input: &str,
) -> PathBuf {
    let tilde = regex!(r"^~(/|$)");
    if input.starts_with('/') {
        // if the input starts with a `/`, we use it as is
        input.into()
    } else if tilde.is_match(input) {
        // if the input starts with `~` as first token, we replace
        // this `~` with the user home directory
        PathBuf::from(
            &*tilde
                .replace(input, |c: &Captures| {
                    if let Some(user_dirs) = UserDirs::new() {
                        format!(
                            "{}{}",
                            user_dirs.home_dir().to_string_lossy(),
                            &c[1],
                        )
                    } else {
                        warn!("no user dirs found, no expansion of ~");
                        c[0].to_string()
                    }
                })
        )
    } else {
        // we put the input behind the source (the selected directory
        // or its parent) and we normalize so that the user can type
        // paths with `../`
        normalize_path(base_dir.join(input))
    }
}


/// Improve the path to try remove and solve .. token.
///
/// This assumes that `a/b/../c` is `a/c` which might be different from
/// what the OS would have chosen when b is a link. This is OK
/// for broot verb arguments but can't be generally used elsewhere
///
/// This function ensures a given path ending with '/' still
/// ends with '/' after normalization.
pub fn normalize_path<P: AsRef<Path>>(path: P) -> PathBuf {
    let ends_with_slash = path.as_ref()
        .to_str()
        .map_or(false, |s| s.ends_with('/'));
    let mut normalized = PathBuf::new();
    for component in path.as_ref().components() {
        match &component {
            Component::ParentDir => {
                if !normalized.pop() {
                    normalized.push(component);
                }
            }
            _ => {
                normalized.push(component);
            }
        }
    }
    if ends_with_slash {
        normalized.push("");
    }
    normalized
}

(this uses the directories crate to get the home in a cross-platform way but other crates exist and you could also just read the $HOME env variable in most platforms)