Re-introduce nu_expand_path for polars open/save (nushell#16900)

## Release notes summary - What our users need to know
Fixes a regression that causes absolute paths to be required when
opening files with the polars plugin.

Co-authored-by: Jack Wright <[email protected]>

Author: ayax79

crates/nu_plugin_polars/src/dataframe/command/core/open.rs

@@ -154,7 +154,7 @@ fn command(
     let spanned_file: Spanned<String> = call.req(0)?;
     debug!("file: {}", spanned_file.item);
 
-    let resource = Resource::new(plugin, &spanned_file)?;
+    let resource = Resource::new(plugin, engine, &spanned_file)?;
     let type_option: Option<(String, Span)> = call
         .get_flag("type")?
         .map(|t: Spanned<String>| (t.item, t.span))

crates/nu_plugin_polars/src/dataframe/command/core/resource.rs

@@ -1,10 +1,12 @@
 use std::path::PathBuf;
 
 use crate::{PolarsPlugin, cloud::build_cloud_options};
+use nu_path::expand_path_with;
+use nu_plugin::EngineInterface;
 use nu_protocol::{ShellError, Span, Spanned};
 use polars::{
     io::cloud::CloudOptions,
-    prelude::{PlPath, SinkTarget},
+    prelude::{PlPath, PlPathRef, SinkTarget},
 };
 
 #[derive(Clone)]
@@ -29,11 +31,12 @@ impl std::fmt::Debug for Resource {
 impl Resource {
     pub(crate) fn new(
         plugin: &PolarsPlugin,
+        engine: &EngineInterface,
         spanned_path: &Spanned<String>,
     ) -> Result<Self, ShellError> {
         let path = PlPath::from_str(&spanned_path.item);
 
-        let cloud_options: Option<CloudOptions> = if path.is_cloud_url() {
+        let (path, cloud_options): (PlPath, Option<CloudOptions>) = if path.is_cloud_url() {
             let options = build_cloud_options(plugin, &path)?;
             if options.is_none() {
                 return Err(ShellError::GenericError {
@@ -47,9 +50,10 @@ impl Resource {
                     inner: vec![],
                 });
             }
-            options
+            (path, options)
         } else {
-            None
+            let new_path = expand_path_with(&spanned_path.item, engine.get_current_dir()?, true);
+            (PlPathRef::from_local_path(&new_path).into_owned(), None)
         };
 
         Ok(Self {

crates/nu_plugin_polars/src/dataframe/command/core/save/mod.rs

@@ -138,12 +138,12 @@ impl PluginCommand for SaveDF {
 
 fn command(
     plugin: &PolarsPlugin,
-    _engine: &EngineInterface,
+    engine: &EngineInterface,
     call: &EvaluatedCall,
     polars_object: PolarsPluginObject,
     spanned_file: Spanned<String>,
 ) -> Result<PipelineData, ShellError> {
-    let resource = Resource::new(plugin, &spanned_file)?;
+    let resource = Resource::new(plugin, engine, &spanned_file)?;
     let type_option: Option<(String, Span)> = call
         .get_flag("type")?
         .map(|t: Spanned<String>| (t.item, t.span))