perf: reduce IO

crates/erg_common/lib.rs

@@ -1,6 +1,6 @@
 //! provides utilities for parser, compiler, and vm crate.
 use std::fmt;
-use std::path::PathBuf;
+use std::path::{Component, Path, PathBuf};
 
 pub mod cache;
 pub mod config;
@@ -192,6 +192,33 @@ pub fn normalize_path(path: PathBuf) -> PathBuf {
     PathBuf::from(lower)
 }
 
+pub fn cheap_canonicalize_path(path: &Path) -> PathBuf {
+    let mut components = path.components().peekable();
+    let mut ret = if let Some(c @ Component::Prefix(..)) = components.peek().cloned() {
+        components.next();
+        PathBuf::from(c.as_os_str())
+    } else {
+        PathBuf::new()
+    };
+
+    for component in components {
+        match component {
+            Component::Prefix(..) => unreachable!(),
+            Component::RootDir => {
+                ret.push(component.as_os_str());
+            }
+            Component::CurDir => {}
+            Component::ParentDir => {
+                ret.pop();
+            }
+            Component::Normal(c) => {
+                ret.push(c);
+            }
+        }
+    }
+    ret
+}
+
 /// ```
 /// use erg_common::trim_eliminate_top_indent;
 /// let code = r#"