Replace unreachable! with match

Summary: Make more clear why this code is unreachable.

Reviewed By: JakobDegen

Differential Revision: D63786763

fbshipit-source-id: da313c6940c13a05d77d41ac35ad58185a7c73ac

Author: stepancheg

starlark/src/values/typing/ty.rs

@@ -43,10 +43,9 @@ impl<'v> StarlarkValue<'v> for AbstractType {
     }
 
     fn eval_type(&self) -> Option<Ty> {
-        unreachable!(
-            "This is unreachable, but this function is needed \
-            so `TyStarlarkValue` could think this is a type"
-        )
+        // This is unreachable, but this function is needed
+        // so `TyStarlarkValue` could think this is a type".
+        match *self {}
     }
 }