bump index version

and add constant for lowest supported version

src/directory/footer.rs

@@ -7,7 +7,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::directory::error::Incompatibility;
 use crate::directory::{AntiCallToken, FileSlice, TerminatingWrite};
-use crate::{Version, INDEX_FORMAT_VERSION};
+use crate::{Version, INDEX_FORMAT_OLDEST_SUPPORTED_VERSION, INDEX_FORMAT_VERSION};
 
 const FOOTER_MAX_LEN: u32 = 50_000;
 
@@ -103,7 +103,7 @@ impl Footer {
     /// Has to be called after `extract_footer` to make sure it's not accessing uninitialised memory
     pub fn is_compatible(&self) -> Result<(), Incompatibility> {
         let library_version = crate::version();
-        if self.version.index_format_version < 4
+        if self.version.index_format_version < INDEX_FORMAT_OLDEST_SUPPORTED_VERSION
             || self.version.index_format_version > INDEX_FORMAT_VERSION
         {
             return Err(Incompatibility::IndexMismatch {

src/lib.rs

@@ -238,7 +238,9 @@ pub use crate::schema::DatePrecision;
 pub use crate::schema::{DateOptions, DateTimePrecision, Document, TantivyDocument, Term};
 
 /// Index format version.
-const INDEX_FORMAT_VERSION: u32 = 5;
+const INDEX_FORMAT_VERSION: u32 = 6;
+/// Oldest index format version this tantivy version can read.
+const INDEX_FORMAT_OLDEST_SUPPORTED_VERSION: u32 = 4;
 
 /// Structure version for the index.
 #[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]