Remove `-Zkeep-hygiene-data`.

It was added way back in #28585 under the name `-Zkeep-mtwt-tables`. The
justification was:

> This is so that the resolution results can be used after analysis,
> potentially for tool support.

There are no uses of significance in the code base, and various Google
searches for both option names (and variants) found nothing of interest.

@petrochenkov says removing this part (and it's only part) of the
hygiene data is dubious. It doesn't seem that big, so let's just keep it
around.

1 files changed, 0 insertions, 5 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index a88493acf98..55d50e5fe6a 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -443,11 +443,6 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
     drop(ast_index);
     sess.time("drop_ast", || drop(krate));
 
-    // Discard hygiene data, which isn't required after lowering to HIR.
-    if !sess.opts.unstable_opts.keep_hygiene_data {
-        rustc_span::hygiene::clear_syntax_context_map();
-    }
-
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =
         if tcx.needs_crate_hash() { Some(compute_hir_hash(tcx, &owners)) } else { None };