interpret/memory: explain why we check is_thread_local_static

1 files changed, 6 insertions, 0 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/memory.rs b/compiler/rustc_const_eval/src/interpret/memory.rs
index 3fde6ae9b8e..7ff970661d6 100644
--- a/compiler/rustc_const_eval/src/interpret/memory.rs
+++ b/compiler/rustc_const_eval/src/interpret/memory.rs
@@ -165,6 +165,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // We need to handle `extern static`.
         match self.tcx.try_get_global_alloc(alloc_id) {
             Some(GlobalAlloc::Static(def_id)) if self.tcx.is_thread_local_static(def_id) => {
+                // Thread-local statics do not have a constant address. They *must* be accessed via
+                // `ThreadLocalRef`; we can never have a pointer to them as a regular constant value.
                 bug!("global memory cannot point to thread-local static")
             }
             Some(GlobalAlloc::Static(def_id)) if self.tcx.is_foreign_item(def_id) => {
@@ -539,6 +541,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             None => throw_ub!(PointerUseAfterFree(id, CheckInAllocMsg::MemoryAccessTest)),
             Some(GlobalAlloc::Static(def_id)) => {
                 assert!(self.tcx.is_static(def_id));
+                // Thread-local statics do not have a constant address. They *must* be accessed via
+                // `ThreadLocalRef`; we can never have a pointer to them as a regular constant value.
                 assert!(!self.tcx.is_thread_local_static(def_id));
                 // Notice that every static has two `AllocId` that will resolve to the same
                 // thing here: one maps to `GlobalAlloc::Static`, this is the "lazy" ID,
@@ -740,6 +744,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match self.tcx.try_get_global_alloc(id) {
             Some(GlobalAlloc::Static(def_id)) => {
                 assert!(self.tcx.is_static(def_id));
+                // Thread-local statics do not have a constant address. They *must* be accessed via
+                // `ThreadLocalRef`; we can never have a pointer to them as a regular constant value.
                 assert!(!self.tcx.is_thread_local_static(def_id));
                 // Use size and align of the type.
                 let ty = self