Remove unused LoadResult::DecodeIncrCache variant

compiler/rustc_incremental/messages.ftl

@@ -30,8 +30,6 @@ incremental_create_lock =
     incremental compilation: could not create session directory lock file: {$lock_err}
 incremental_create_new = failed to create {$name} at `{$path}`: {$err}
 
-incremental_decode_incr_cache = could not decode incremental cache: {$err}
-
 incremental_delete_full = error deleting incremental compilation session directory `{$path}`: {$err}
 
 incremental_delete_incompatible =

compiler/rustc_incremental/src/errors.rs

@@ -270,12 +270,6 @@ pub struct LoadDepGraph {
     pub err: std::io::Error,
 }
 
-#[derive(Diagnostic)]
-#[diag(incremental_decode_incr_cache)]
-pub struct DecodeIncrCache {
-    pub err: String,
-}
-
 #[derive(Diagnostic)]
 #[diag(incremental_write_dep_graph)]
 pub struct WriteDepGraph<'a> {

compiler/rustc_incremental/src/persist/load.rs

@@ -30,8 +30,6 @@ pub enum LoadResult<T> {
     DataOutOfDate,
     /// Loading the dep graph failed.
     LoadDepGraph(PathBuf, std::io::Error),
-    /// Decoding loaded incremental cache failed.
-    DecodeIncrCache(Box<dyn std::any::Any + Send>),
 }
 
 impl<T: Default> LoadResult<T> {
@@ -44,9 +42,7 @@ impl<T: Default> LoadResult<T> {
             }
             (
                 Some(IncrementalStateAssertion::Loaded),
-                LoadResult::LoadDepGraph(..)
-                | LoadResult::DecodeIncrCache(..)
-                | LoadResult::DataOutOfDate,
+                LoadResult::LoadDepGraph(..) | LoadResult::DataOutOfDate,
             ) => {
                 sess.emit_fatal(errors::AssertLoaded);
             }
@@ -58,10 +54,6 @@ impl<T: Default> LoadResult<T> {
                 sess.emit_warning(errors::LoadDepGraph { path, err });
                 Default::default()
             }
-            LoadResult::DecodeIncrCache(err) => {
-                sess.emit_warning(errors::DecodeIncrCache { err: format!("{err:?}") });
-                Default::default()
-            }
             LoadResult::DataOutOfDate => {
                 if let Err(err) = delete_all_session_dir_contents(sess) {
                     sess.emit_err(errors::DeleteIncompatible { path: dep_graph_path(sess), err });
@@ -150,7 +142,6 @@ fn load_dep_graph(sess: &Session) -> LoadResult<(SerializedDepGraph, WorkProduct
     match load_data(&path, sess) {
         LoadResult::DataOutOfDate => LoadResult::DataOutOfDate,
         LoadResult::LoadDepGraph(path, err) => LoadResult::LoadDepGraph(path, err),
-        LoadResult::DecodeIncrCache(err) => LoadResult::DecodeIncrCache(err),
         LoadResult::Ok { data: (bytes, start_pos) } => {
             let mut decoder = MemDecoder::new(&bytes, start_pos);
             let prev_commandline_args_hash = u64::decode(&mut decoder);