2 files changed, 35 insertions, 2 deletions
diff --git a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
index 0f5bdc8d768..94ee34c8b7b 100644
--- a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
+++ b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
@@ -111,6 +111,11 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
         }
     }
 
+    // Zero-sized types are dropped in both ABIs, so they're safe
+    if arg.layout.is_zst() {
+        return true;
+    }
+
     false
 }
 
diff --git a/compiler/rustc_monomorphize/src/partitioning/autodiff.rs b/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
index ebe0b258c1b..22d593b80b8 100644
--- a/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
@@ -2,7 +2,7 @@ use rustc_ast::expand::autodiff_attrs::{AutoDiffItem, DiffActivity};
 use rustc_hir::def_id::LOCAL_CRATE;
 use rustc_middle::bug;
 use rustc_middle::mir::mono::MonoItem;
-use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
+use rustc_middle::ty::{self, Instance, PseudoCanonicalInput, Ty, TyCtxt, TypingEnv};
 use rustc_symbol_mangling::symbol_name_for_instance_in_crate;
 use tracing::{debug, trace};
 
@@ -22,23 +22,51 @@ fn adjust_activity_to_abi<'tcx>(tcx: TyCtxt<'tcx>, fn_ty: Ty<'tcx>, da: &mut Vec
     for (i, ty) in sig.inputs().iter().enumerate() {
         if let Some(inner_ty) = ty.builtin_deref(true) {
             if inner_ty.is_slice() {
+                // Now we need to figure out the size of each slice element in memory to allow
+                // safety checks and usability improvements in the backend.
+                let sty = match inner_ty.builtin_index() {
+                    Some(sty) => sty,
+                    None => {
+                        panic!("slice element type unknown");
+                    }
+                };
+                let pci = PseudoCanonicalInput {
+                    typing_env: TypingEnv::fully_monomorphized(),
+                    value: sty,
+                };
+
+                let layout = tcx.layout_of(pci);
+                let elem_size = match layout {
+                    Ok(layout) => layout.size,
+                    Err(_) => {
+                        bug!("autodiff failed to compute slice element size");
+                    }
+                };
+                let elem_size: u32 = elem_size.bytes() as u32;
+
                 // We know that the length will be passed as extra arg.
                 if !da.is_empty() {
                     // We are looking at a slice. The length of that slice will become an
                     // extra integer on llvm level. Integers are always const.
                     // However, if the slice get's duplicated, we want to know to later check the
                     // size. So we mark the new size argument as FakeActivitySize.
+                    // There is one FakeActivitySize per slice, so for convenience we store the
+                    // slice element size in bytes in it. We will use the size in the backend.
                     let activity = match da[i] {
                         DiffActivity::DualOnly
                         | DiffActivity::Dual
+                        | DiffActivity::Dualv
                         | DiffActivity::DuplicatedOnly
-                        | DiffActivity::Duplicated => DiffActivity::FakeActivitySize,
+                        | DiffActivity::Duplicated => {
+                            DiffActivity::FakeActivitySize(Some(elem_size))
+                        }
                         DiffActivity::Const => DiffActivity::Const,
                         _ => bug!("unexpected activity for ptr/ref"),
                     };
                     new_activities.push(activity);
                     new_positions.push(i + 1);
                 }
+
                 continue;
             }
         }