Represent relative paths using a dedicated, separator-agnostic type (#38744)

Closes https://github.com/zed-industries/zed/issues/38690
Closes #37353

### Background

On Windows, paths are normally separated by `\`, unlike mac and linux
where they are separated by `/`. When editing code in a project that
uses a different path style than your local system (e.g. remoting from
Windows to Linux, using WSL, and collaboration between windows and unix
users), the correct separator for a path may differ from the "native"
separator.

Previously, to work around this, Zed converted paths' separators in
numerous places. This was applied to both absolute and relative paths,
leading to incorrect conversions in some cases.

### Solution

Many code paths in Zed use paths that are *relative* to either a
worktree root or a git repository. This PR introduces a dedicated type
for these paths called `RelPath`, which stores the path in the same way
regardless of host platform, and offers `Path`-like manipulation APIs.
RelPath supports *displaying* the path using either separator, so that
we can display paths in a style that is determined at runtime based on
the current project.

The representation of absolute paths is left untouched, for now.
Absolute paths are different from relative paths because (except in
contexts where we know that the path refers to the local filesystem)
they should generally be treated as opaque strings. Currently we use a
mix of types for these paths (std::path::Path, String, SanitizedPath).

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Peter Tripp <petertripp@gmail.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>
Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

crates/language/src/buffer.rs

@@ -41,13 +41,12 @@ use std::{
     cell::Cell,
     cmp::{self, Ordering, Reverse},
     collections::{BTreeMap, BTreeSet},
-    ffi::OsStr,
     future::Future,
     iter::{self, Iterator, Peekable},
     mem,
     num::NonZeroU32,
     ops::{Deref, Range},
-    path::{Path, PathBuf},
+    path::PathBuf,
     rc,
     sync::{Arc, LazyLock},
     time::{Duration, Instant},
@@ -65,7 +64,7 @@ pub use text::{
 use theme::{ActiveTheme as _, SyntaxTheme};
 #[cfg(any(test, feature = "test-support"))]
 use util::RandomCharIter;
-use util::{RangeExt, debug_panic, maybe};
+use util::{RangeExt, debug_panic, maybe, paths::PathStyle, rel_path::RelPath};
 
 #[cfg(any(test, feature = "test-support"))]
 pub use {tree_sitter_python, tree_sitter_rust, tree_sitter_typescript};
@@ -349,15 +348,18 @@ pub trait File: Send + Sync + Any {
     fn disk_state(&self) -> DiskState;
 
     /// Returns the path of this file relative to the worktree's root directory.
-    fn path(&self) -> &Arc<Path>;
+    fn path(&self) -> &Arc<RelPath>;
 
     /// Returns the path of this file relative to the worktree's parent directory (this means it
     /// includes the name of the worktree's root folder).
     fn full_path(&self, cx: &App) -> PathBuf;
 
+    /// Returns the path style of this file.
+    fn path_style(&self, cx: &App) -> PathStyle;
+
     /// Returns the last component of this handle's absolute path. If this handle refers to the root
     /// of its worktree, then this method will return the name of the worktree itself.
-    fn file_name<'a>(&'a self, cx: &'a App) -> &'a OsStr;
+    fn file_name<'a>(&'a self, cx: &'a App) -> &'a str;
 
     /// Returns the id of the worktree to which this file belongs.
     ///
@@ -4626,13 +4628,12 @@ impl BufferSnapshot {
         self.file.as_ref()
     }
 
-    /// Resolves the file path (relative to the worktree root) associated with the underlying file.
     pub fn resolve_file_path(&self, cx: &App, include_root: bool) -> Option<PathBuf> {
         if let Some(file) = self.file() {
             if file.path().file_name().is_none() || include_root {
                 Some(file.full_path(cx))
             } else {
-                Some(file.path().to_path_buf())
+                Some(file.path().as_std_path().to_owned())
             }
         } else {
             None
@@ -5117,19 +5118,19 @@ impl IndentSize {
 
 #[cfg(any(test, feature = "test-support"))]
 pub struct TestFile {
-    pub path: Arc<Path>,
+    pub path: Arc<RelPath>,
     pub root_name: String,
     pub local_root: Option<PathBuf>,
 }
 
 #[cfg(any(test, feature = "test-support"))]
 impl File for TestFile {
-    fn path(&self) -> &Arc<Path> {
+    fn path(&self) -> &Arc<RelPath> {
         &self.path
     }
 
     fn full_path(&self, _: &gpui::App) -> PathBuf {
-        PathBuf::from(&self.root_name).join(self.path.as_ref())
+        PathBuf::from(self.root_name.clone()).join(self.path.as_std_path())
     }
 
     fn as_local(&self) -> Option<&dyn LocalFile> {
@@ -5144,7 +5145,7 @@ impl File for TestFile {
         unimplemented!()
     }
 
-    fn file_name<'a>(&'a self, _: &'a gpui::App) -> &'a std::ffi::OsStr {
+    fn file_name<'a>(&'a self, _: &'a gpui::App) -> &'a str {
         self.path().file_name().unwrap_or(self.root_name.as_ref())
     }
 
@@ -5159,6 +5160,10 @@ impl File for TestFile {
     fn is_private(&self) -> bool {
         false
     }
+
+    fn path_style(&self, _cx: &App) -> PathStyle {
+        PathStyle::local()
+    }
 }
 
 #[cfg(any(test, feature = "test-support"))]
@@ -5166,7 +5171,7 @@ impl LocalFile for TestFile {
     fn abs_path(&self, _cx: &App) -> PathBuf {
         PathBuf::from(self.local_root.as_ref().unwrap())
             .join(&self.root_name)
-            .join(self.path.as_ref())
+            .join(self.path.as_std_path())
     }
 
     fn load(&self, _cx: &App) -> Task<Result<String>> {