From a7633b8a71d163a7392f4ef2cb80ea7076302f71 Mon Sep 17 00:00:00 2001
From: Gary Guo <gary@garyguo.net>
Date: Thu, 17 Aug 2023 12:07:22 +0100
Subject: [PATCH] Add an (perma-)unstable option to disable vtable vptr
This flag is intended for evaluation of trait upcasting
space cost for embedded use cases.
---
compiler/rustc_interface/src/tests.rs | 1 +
compiler/rustc_session/src/options.rs | 2 ++
compiler/rustc_trait_selection/src/traits/vtable.rs | 2 +-
3 files changed, 4 insertions(+), 1 deletion(-)
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index e3d66d18388..b53ba251bcd 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -807,6 +807,7 @@ fn test_unstable_options_tracking_hash() {
tracked!(no_jump_tables, true);
tracked!(no_link, true);
tracked!(no_profiler_runtime, true);
+ tracked!(no_trait_vptr, true);
tracked!(no_unique_section_names, true);
tracked!(oom, OomStrategy::Panic);
tracked!(osx_rpath_install_name, true);
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 055ab2d9c15..40099de707b 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1631,6 +1631,8 @@ options! {
"run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"),
no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED],
"prevent automatic injection of the profiler_builtins crate"),
+ no_trait_vptr: bool = (false, parse_no_flag, [TRACKED],
+ "disable generation of trait vptr in vtable for upcasting"),
no_unique_section_names: bool = (false, parse_bool, [TRACKED],
"do not use unique names for text and data sections when -Z function-sections is used"),
normalize_docs: bool = (false, parse_bool, [TRACKED],
diff --git a/compiler/rustc_trait_selection/src/traits/vtable.rs b/compiler/rustc_trait_selection/src/traits/vtable.rs
index 427ac368432..e41073937be 100644
--- a/compiler/rustc_trait_selection/src/traits/vtable.rs
+++ b/compiler/rustc_trait_selection/src/traits/vtable.rs
@@ -152,7 +152,7 @@ fn prepare_vtable_segments_inner<'tcx, T>(
while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
segment_visitor(VtblSegment::TraitOwnEntries {
trait_ref: inner_most_trait_ref,
- emit_vptr,
+ emit_vptr: emit_vptr && !tcx.sess.opts.unstable_opts.no_trait_vptr,
})?;
// If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,