Prevent double encoding when the filename wasn't remapped at all

This is done by making the `local` part of `RealFileName` none.

This works because `maybe_remapped` is equal to `local` when no
remapping happened.

Author: Urgau

compiler/rustc_span/src/lib.rs

@@ -420,9 +420,14 @@ impl RealFileName {
     /// Update the filename for encoding in the crate metadata.
     ///
     /// Currently it's about removing the local part when the filename
-    /// is fully remapped.
+    /// is either fully remapped or not remapped at all.
+    #[inline]
     pub fn update_for_crate_metadata(&mut self) {
-        if self.scopes.is_all() {
+        if self.scopes.is_all() || self.scopes.is_empty() {
+            // NOTE: This works because when the filename is fully
+            // remapped, we don't care about the `local` part,
+            // and when the filename is not remapped at all,
+            // `maybe_remapped` and `local` are equal.
             self.local = None;
         }
     }