Add user autoload directory (#14669)

# Description

Adds a user-level (non-vendor) autoload directory:

```
($nu.default-config-dir)/autoload
```

Currently this is the only directory. We can consider adding others if
needed.

Related: As a separate PR, I'm going to try to restore the ability to
set `$env.NU_AUTOLOAD_DIRS` during startup.

# User-Facing Changes

Files in `$nu.default-config-dir/autoload` will be autoload at startup.
These files will be loaded after any vendor autoloads, so that a user
can override the vendor settings.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

TODO; add a `$nu.user-autoload-dirs` constant.

Doc updates

crates/nu-cli/tests/completions/mod.rs

@@ -1357,7 +1357,7 @@ fn variables_completions() {
     // Test completions for $nu
     let suggestions = completer.complete("$nu.", 4);
 
-    assert_eq!(18, suggestions.len());
+    assert_eq!(19, suggestions.len());
 
     let expected: Vec<String> = vec![
         "cache-dir".into(),
@@ -1377,6 +1377,7 @@ fn variables_completions() {
         "plugin-path".into(),
         "startup-time".into(),
         "temp-path".into(),
+        "user-autoload-dirs".into(),
         "vendor-autoload-dirs".into(),
     ];
 

crates/nu-protocol/src/eval_const.rs

@@ -194,6 +194,17 @@ pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Valu
         ),
     );
 
+    record.push(
+        "user-autoload-dirs",
+        Value::list(
+            get_user_autoload_dirs(engine_state)
+                .iter()
+                .map(|path| Value::string(path.to_string_lossy(), span))
+                .collect(),
+            span,
+        ),
+    );
+
     record.push("temp-path", {
         let canon_temp_path = canonicalize_path(engine_state, &std::env::temp_dir());
         Value::string(canon_temp_path.to_string_lossy(), span)
@@ -306,10 +317,9 @@ pub fn get_vendor_autoload_dirs(_engine_state: &EngineState) -> Vec<PathBuf> {
         .map(into_autoload_path_fn)
         .for_each(&mut append_fn);
 
-    option_env!("NU_VENDOR_AUTOLOAD_DIR")
-        .into_iter()
-        .map(PathBuf::from)
-        .for_each(&mut append_fn);
+    if let Some(path) = option_env!("NU_VENDOR_AUTOLOAD_DIR") {
+        append_fn(PathBuf::from(path));
+    }
 
     #[cfg(target_os = "macos")]
     std::env::var("XDG_DATA_HOME")
@@ -326,15 +336,37 @@ pub fn get_vendor_autoload_dirs(_engine_state: &EngineState) -> Vec<PathBuf> {
         .into_iter()
         .for_each(&mut append_fn);
 
-    dirs::data_dir()
-        .into_iter()
-        .map(into_autoload_path_fn)
-        .for_each(&mut append_fn);
+    if let Some(data_dir) = dirs::data_dir() {
+        append_fn(into_autoload_path_fn(data_dir));
+    }
 
-    std::env::var_os("NU_VENDOR_AUTOLOAD_DIR")
-        .into_iter()
-        .map(PathBuf::from)
-        .for_each(&mut append_fn);
+    if let Some(path) = std::env::var_os("NU_VENDOR_AUTOLOAD_DIR") {
+        append_fn(PathBuf::from(path));
+    }
+
+    dirs
+}
+
+pub fn get_user_autoload_dirs(_engine_state: &EngineState) -> Vec<PathBuf> {
+    // User autoload directories - Currently just `autoload` in the default
+    // configuration directory
+    let into_autoload_path_fn = |mut path: PathBuf| {
+        path.push("nushell");
+        path.push("autoload");
+        path
+    };
+
+    let mut dirs = Vec::new();
+
+    let mut append_fn = |path: PathBuf| {
+        if !dirs.contains(&path) {
+            dirs.push(path)
+        }
+    };
+
+    if let Some(config_dir) = dirs::config_dir() {
+        append_fn(into_autoload_path_fn(config_dir));
+    }
 
     dirs
 }

src/config_files.rs

@@ -6,6 +6,7 @@ use nu_engine::convert_env_values;
 use nu_path::canonicalize_with;
 use nu_protocol::{
     engine::{EngineState, Stack, StateWorkingSet},
+    eval_const::{get_user_autoload_dirs, get_vendor_autoload_dirs},
     report_parse_error, report_shell_error, Config, ParseError, PipelineData, Spanned,
 };
 use nu_utils::{get_default_config, get_default_env, get_scaffold_config, get_scaffold_env, perf};
@@ -197,24 +198,29 @@ pub(crate) fn read_vendor_autoload_files(engine_state: &mut EngineState, stack:
 
     // The evaluation order is first determined by the semantics of `get_vendor_autoload_dirs`
     // to determine the order of directories to evaluate
-    for autoload_dir in nu_protocol::eval_const::get_vendor_autoload_dirs(engine_state) {
-        warn!("read_vendor_autoload_files: {}", autoload_dir.display());
-
-        if autoload_dir.exists() {
-            // on a second levels files are lexicographically sorted by the string of the filename
-            let entries = read_and_sort_directory(&autoload_dir);
-            if let Ok(entries) = entries {
-                for entry in entries {
-                    if !entry.ends_with(".nu") {
-                        continue;
+    get_vendor_autoload_dirs(engine_state)
+        .iter()
+        // User autoload directories are evaluated after vendor, which means that
+        // the user can override vendor autoload files
+        .chain(get_user_autoload_dirs(engine_state).iter())
+        .for_each(|autoload_dir| {
+            warn!("read_vendor_autoload_files: {}", autoload_dir.display());
+
+            if autoload_dir.exists() {
+                // on a second levels files are lexicographically sorted by the string of the filename
+                let entries = read_and_sort_directory(autoload_dir);
+                if let Ok(entries) = entries {
+                    for entry in entries {
+                        if !entry.ends_with(".nu") {
+                            continue;
+                        }
+                        let path = autoload_dir.join(entry);
+                        warn!("AutoLoading: {:?}", path);
+                        eval_config_contents(path, engine_state, stack);
                     }
-                    let path = autoload_dir.join(entry);
-                    warn!("AutoLoading: {:?}", path);
-                    eval_config_contents(path, engine_state, stack);
                 }
             }
-        }
-    }
+        });
 }
 
 fn eval_default_config(