Optimization of tilde expansion (#9709)

* Use next and avoid a redundant prefix strip

* Avoid allocations

Especially when `expand_tilde` is claled on a path
that doesn't contain a tilde.

* Add a test

* Use Into<Cow<…>>

* Put the expand_tilde test at the end of the file

* Remove unused importsw

helix-stdx/src/path.rs

@@ -1,6 +1,9 @@
 pub use etcetera::home_dir;
 
-use std::path::{Component, Path, PathBuf};
+use std::{
+    borrow::Cow,
+    path::{Component, Path, PathBuf},
+};
 
 use crate::env::current_working_dir;
 
@@ -19,19 +22,22 @@ pub fn fold_home_dir(path: &Path) -> PathBuf {
 /// Expands tilde `~` into users home directory if available, otherwise returns the path
 /// unchanged. The tilde will only be expanded when present as the first component of the path
 /// and only slash follows it.
-pub fn expand_tilde(path: impl AsRef<Path>) -> PathBuf {
-    let path = path.as_ref();
-    let mut components = path.components().peekable();
-    if let Some(Component::Normal(c)) = components.peek() {
-        if c == &"~" {
-            if let Ok(home) = home_dir() {
-                // it's ok to unwrap, the path starts with `~`
-                return home.join(path.strip_prefix("~").unwrap());
+pub fn expand_tilde<'a, P>(path: P) -> Cow<'a, Path>
+where
+    P: Into<Cow<'a, Path>>,
+{
+    let path = path.into();
+    let mut components = path.components();
+    if let Some(Component::Normal(c)) = components.next() {
+        if c == "~" {
+            if let Ok(mut buf) = home_dir() {
+                buf.push(components);
+                return Cow::Owned(buf);
             }
         }
     }
 
-    path.to_path_buf()
+    path
 }
 
 /// Normalize a path without resolving symlinks.
@@ -109,9 +115,9 @@ pub fn normalize(path: impl AsRef<Path>) -> PathBuf {
 /// This function is used instead of [`std::fs::canonicalize`] because we don't want to verify
 /// here if the path exists, just normalize it's components.
 pub fn canonicalize(path: impl AsRef<Path>) -> PathBuf {
-    let path = expand_tilde(path);
+    let path = expand_tilde(path.as_ref());
     let path = if path.is_relative() {
-        current_working_dir().join(path)
+        Cow::Owned(current_working_dir().join(path))
     } else {
         path
     };
@@ -183,3 +189,32 @@ pub fn get_truncated_path(path: impl AsRef<Path>) -> PathBuf {
     ret.push(file);
     ret
 }
+
+#[cfg(test)]
+mod tests {
+    use std::{
+        ffi::OsStr,
+        path::{Component, Path},
+    };
+
+    use crate::path;
+
+    #[test]
+    fn expand_tilde() {
+        for path in ["~", "~/foo"] {
+            let expanded = path::expand_tilde(Path::new(path));
+
+            let tilde = Component::Normal(OsStr::new("~"));
+
+            let mut component_count = 0;
+            for component in expanded.components() {
+                // No tilde left.
+                assert_ne!(component, tilde);
+                component_count += 1;
+            }
+
+            // The path was at least expanded to something.
+            assert_ne!(component_count, 0);
+        }
+    }
+}