Draft: ✨ Add FD_CONFIG_PATH environment variable

Summary: Add FD_CONFIG_PATH environment variable which points to a file with config options that are prepended to any `fd` command.
This has the same behavior as `RG_CONFIG_PATH` from ripgrep

Test Plan: `cargo test`

TODO:
- add flag `--no-config` to not respect FD_CONFIG_PATH, `.fdignore`, or global
- add documentation
fd ignore
- make parsing config file blazingly fast
- if FD_CONFIG_PATH not set, or file does not exist, just use `args_os()`
directly
- fix `test_invalid_cwd`

src/main.rs

@@ -18,7 +18,7 @@ use std::time;
 
 use anyhow::{anyhow, bail, Context, Result};
 use atty::Stream;
-use clap::{CommandFactory, Parser};
+use clap::{CommandFactory, FromArgMatches};
 use globset::GlobBuilder;
 use lscolors::LsColors;
 use regex::bytes::{Regex, RegexBuilder, RegexSetBuilder};
@@ -67,7 +67,7 @@ fn main() {
 }
 
 fn run() -> Result<ExitCode> {
-    let opts = Opts::parse();
+    let opts = parse_opts();
 
     #[cfg(feature = "completions")]
     if let Some(shell) = opts.gen_completions()? {
@@ -105,6 +105,40 @@ fn run() -> Result<ExitCode> {
     walk::scan(&search_paths, Arc::new(regexps), Arc::new(config))
 }
 
+fn parse_opts() -> Opts {
+    let mut cli_args = std::env::args_os(); // [/usr/bin/fd, --hidden, --etc]
+    let mut args: Vec<std::ffi::OsString> = args_from_config_file(); // [--no-require-git]
+
+    // TODO(vegerot): If `args` is empty we can just skip the next two lines and not pay the cost
+    // of moving things around
+    args.insert(0, cli_args.next().unwrap()); // [/usr/bin/fd, --no-require-git]
+
+    // this way, CLI arguments override config-file arguments
+    args.extend(cli_args.skip(1)); // [/usr/bin/fd, --no-require-git, --hidden, --etc]
+
+    let mut matches = Opts::command().get_matches_from(args);
+    let res =
+        Opts::from_arg_matches_mut(&mut matches).map_err(|err| err.format(&mut Opts::command()));
+
+    match res {
+        Ok(s) => s,
+        Err(e) => e.exit(),
+    }
+}
+
+fn args_from_config_file() -> Vec<std::ffi::OsString> {
+    let config_path = match std::env::var_os("FD_CONFIG_PATH") {
+        Some(path) => path,
+        None => return vec![],
+    };
+
+    // TODO(vegerot): 🚀
+    match std::fs::read_to_string(config_path) {
+        Ok(config) => config.split_whitespace().map(|conf| conf.into()).collect(),
+        Err(_) => vec![],
+    }
+}
+
 #[cfg(feature = "completions")]
 #[cold]
 fn print_completions(shell: clap_complete::Shell) -> Result<ExitCode> {

tests/testenv/mod.rs

@@ -161,6 +161,14 @@ impl TestEnv {
         }
     }
 
+    pub fn set_environment_variable(&self, key: &str, value: &str) {
+        env::set_var(key, value);
+    }
+
+    pub fn remove_environment_variable(&self, key: &str) {
+        env::remove_var(key);
+    }
+
     /// Create a broken symlink at the given path in the temp_dir.
     pub fn create_broken_symlink<P: AsRef<Path>>(
         &mut self,

tests/tests.rs

@@ -2500,3 +2500,32 @@ fn test_invalid_cwd() {
         panic!("{:?}", output);
     }
 }
+
+/// Read config file from FD_CONFIG_PATH environment variable if present
+#[test]
+#[cfg(unix)]
+fn test_fd_config_path() {
+    let te = TestEnv::new(DEFAULT_DIRS, DEFAULT_FILES);
+
+    te.set_environment_variable("FD_CONFIG_PATH", "fd.conf");
+
+    fs::File::create(te.test_root().join("fd.conf"))
+        .unwrap()
+        // test flags separated by spaces and newlines
+        .write_all(
+            b"--hidden --ignore-case
+              --no-ignore-vcs
+              --maxdepth=2",
+        )
+        .unwrap();
+
+    te.assert_output(
+        &["foo"],
+        ".hidden.foo
+         a.foo
+         gitignored.foo
+         one/b.foo",
+    );
+
+    te.remove_environment_variable("FD_CONFIG_PATH");
+}