Ensure currently_parsed_cwd is set for config files (nushell#12338)

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# Description
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Fixes nushell#7849, nushell#11465 based on @kubouch's suggestion in
nushell#11465 (comment).

# User-Facing Changes
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helps us keep track of breaking changes. -->

Can source files relative to `env.nu` or `config.nu` like in nushell#6150.

# Tests + Formatting
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Adds test that previously failed.

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crates/nu-cli/src/config_files.rs

@@ -91,6 +91,10 @@ pub fn eval_config_contents(
         let config_filename = config_path.to_string_lossy();
 
         if let Ok(contents) = std::fs::read(&config_path) {
+            // Change currently parsed directory
+            let prev_currently_parsed_cwd = engine_state.currently_parsed_cwd.clone();
+            engine_state.start_in_file(Some(&config_filename));
+
             eval_source(
                 engine_state,
                 stack,
@@ -100,6 +104,9 @@ pub fn eval_config_contents(
                 false,
             );
 
+            // Restore the currently parsed directory back
+            engine_state.currently_parsed_cwd = prev_currently_parsed_cwd;
+
             // Merge the environment in case env vars changed in the config
             match nu_engine::env::current_dir(engine_state, stack) {
                 Ok(cwd) => {

crates/nu-protocol/src/engine/engine_state.rs

@@ -99,7 +99,7 @@ pub struct EngineState {
     pub history_enabled: bool,
     pub history_session_id: i64,
     // If Nushell was started, e.g., with `nu spam.nu`, the file's parent is stored here
-    pub(super) currently_parsed_cwd: Option<PathBuf>,
+    pub currently_parsed_cwd: Option<PathBuf>,
     pub regex_cache: Arc<Mutex<LruCache<String, Regex>>>,
     pub is_interactive: bool,
     pub is_login: bool,

tests/parsing/mod.rs

@@ -13,6 +13,15 @@ fn source_file_relative_to_file() {
     assert_eq!(actual.out, "5");
 }
 
+#[test]
+fn source_file_relative_to_config() {
+    let actual = nu!("
+        nu --config tests/parsing/samples/source_file_relative.nu --commands ''
+        ");
+
+    assert_eq!(actual.out, "5");
+}
+
 #[test]
 fn source_const_file() {
     let actual = nu!(cwd: "tests/parsing/samples",