diff -r b2e41723f886 -r 52d40f8fb82d rust/hg-core/src/lib.rs
--- a/rust/hg-core/src/lib.rs	Tue Mar 10 16:04:30 2020 +0100
+++ b/rust/hg-core/src/lib.rs	Fri Jan 17 11:29:33 2020 +0100
@@ -126,6 +126,9 @@
     /// Needed a pattern that can be turned into a regex but got one that
     /// can't. This should only happen through programmer error.
     NonRegexPattern(IgnorePattern),
+    /// This is temporary, see `re2/mod.rs`.
+    /// This will cause a fallback to Python.
+    Re2NotInstalled,
 }
 
 impl ToString for PatternError {
@@ -148,6 +151,10 @@
             PatternError::NonRegexPattern(pattern) => {
                 format!("'{:?}' cannot be turned into a regex", pattern)
             }
+            PatternError::Re2NotInstalled => {
+                "Re2 is not installed, cannot use regex functionality."
+                    .to_string()
+            }
         }
     }
 }