diff --git a/crates/oxc_span/src/source_type/error.rs b/crates/oxc_span/src/source_type/error.rs
new file mode 100644
index 0000000000000..72334364aaf2e
--- /dev/null
+++ b/crates/oxc_span/src/source_type/error.rs
@@ -0,0 +1,29 @@
+use std::{borrow::Cow, error::Error, fmt, ops::Deref};
+
+/// Error returned by [`SourceType::from_path`] when the file extension is not
+/// found or recognized.
+///
+/// [`SourceType::from_path`]: `crate::SourceType::from_path`
+#[derive(Debug)]
+pub struct UnknownExtension(/* msg */ pub(crate) Cow<'static, str>);
+
+impl Deref for UnknownExtension {
+    type Target = str;
+    fn deref(&self) -> &str {
+        &self.0
+    }
+}
+impl UnknownExtension {
+    #[inline]
+    pub(crate) fn new<S: Into<Cow<'static, str>>>(ext: S) -> Self {
+        Self(ext.into())
+    }
+}
+
+impl fmt::Display for UnknownExtension {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Unknown file extension: {}", self.0)
+    }
+}
+
+impl Error for UnknownExtension {}
diff --git a/crates/oxc_span/src/source_type/mod.rs b/crates/oxc_span/src/source_type/mod.rs
index 053268e987b48..d4caec37bc8fd 100644
--- a/crates/oxc_span/src/source_type/mod.rs
+++ b/crates/oxc_span/src/source_type/mod.rs
@@ -1,12 +1,11 @@
-use std::path::Path;
-
+mod error;
 mod types;
+
+pub use error::UnknownExtension;
 use oxc_allocator::{Allocator, CloneIn};
+use std::path::Path;
 pub use types::*;
 
-#[derive(Debug)]
-pub struct UnknownExtension(pub String);
-
 impl Default for SourceType {
     fn default() -> Self {
         Self {
@@ -160,7 +159,7 @@ impl SourceType {
             .as_ref()
             .file_name()
             .and_then(std::ffi::OsStr::to_str)
-            .ok_or_else(|| UnknownExtension("Please provide a valid file name.".to_string()))?;
+            .ok_or_else(|| UnknownExtension::new("Please provide a valid file name."))?;
 
         let extension = path
             .as_ref()
@@ -169,7 +168,7 @@ impl SourceType {
             .filter(|s| VALID_EXTENSIONS.contains(s))
             .ok_or_else(|| {
                 let path = path.as_ref().to_string_lossy();
-                UnknownExtension(
+                UnknownExtension::new(
                     format!("Please provide a valid file extension for {path}: .js, .mjs, .jsx or .cjs for JavaScript, or .ts, .d.ts, .mts, .cts or .tsx for TypeScript"),
                 )
             })?;
@@ -192,7 +191,7 @@ impl SourceType {
                 #[cfg(debug_assertions)]
                 unreachable!();
                 #[cfg(not(debug_assertions))]
-                return Err(UnknownExtension(format!("Unknown extension: {}", extension)));
+                return Err(UnknownExtension(format!("Unknown extension: {}", extension).into()));
             }
         };