Rollup merge of #65448 - eddyb:codegen-box-less-special, r=oli-obk

rustc_codegen_ssa: remove some unnecessary Box special-casing.

Could help simplify #60900.

r? @nikomatsakis

Author: Centril

src/librustc_codegen_ssa/base.rs

@@ -168,12 +168,6 @@ pub fn unsize_thin_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             let ptr_ty = bx.cx().type_ptr_to(bx.cx().backend_type(bx.cx().layout_of(b)));
             (bx.pointercast(src, ptr_ty), unsized_info(bx.cx(), a, b, None))
         }
-        (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
-            let (a, b) = (src_ty.boxed_ty(), dst_ty.boxed_ty());
-            assert!(bx.cx().type_is_sized(a));
-            let ptr_ty = bx.cx().type_ptr_to(bx.cx().backend_type(bx.cx().layout_of(b)));
-            (bx.pointercast(src, ptr_ty), unsized_info(bx.cx(), a, b, None))
-        }
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 
@@ -196,6 +190,8 @@ pub fn unsize_thin_ptr<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             }
             let (lldata, llextra) = result.unwrap();
             // HACK(eddyb) have to bitcast pointers until LLVM removes pointee types.
+            // FIXME(eddyb) move these out of this `match` arm, so they're always
+            // applied, uniformly, no matter the source/destination types.
             (bx.bitcast(lldata, bx.cx().scalar_pair_element_backend_type(dst_layout, 0, true)),
              bx.bitcast(llextra, bx.cx().scalar_pair_element_backend_type(dst_layout, 1, true)))
         }
@@ -212,31 +208,27 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 ) {
     let src_ty = src.layout.ty;
     let dst_ty = dst.layout.ty;
-    let mut coerce_ptr = || {
-        let (base, info) = match bx.load_operand(src).val {
-            OperandValue::Pair(base, info) => {
-                // fat-ptr to fat-ptr unsize preserves the vtable
-                // i.e., &'a fmt::Debug+Send => &'a fmt::Debug
-                // So we need to pointercast the base to ensure
-                // the types match up.
-                let thin_ptr = dst.layout.field(bx.cx(), FAT_PTR_ADDR);
-                (bx.pointercast(base, bx.cx().backend_type(thin_ptr)), info)
-            }
-            OperandValue::Immediate(base) => {
-                unsize_thin_ptr(bx, base, src_ty, dst_ty)
-            }
-            OperandValue::Ref(..) => bug!()
-        };
-        OperandValue::Pair(base, info).store(bx, dst);
-    };
     match (&src_ty.kind, &dst_ty.kind) {
         (&ty::Ref(..), &ty::Ref(..)) |
         (&ty::Ref(..), &ty::RawPtr(..)) |
         (&ty::RawPtr(..), &ty::RawPtr(..)) => {
-            coerce_ptr()
-        }
-        (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
-            coerce_ptr()
+            let (base, info) = match bx.load_operand(src).val {
+                OperandValue::Pair(base, info) => {
+                    // fat-ptr to fat-ptr unsize preserves the vtable
+                    // i.e., &'a fmt::Debug+Send => &'a fmt::Debug
+                    // So we need to pointercast the base to ensure
+                    // the types match up.
+                    // FIXME(eddyb) use `scalar_pair_element_backend_type` here,
+                    // like `unsize_thin_ptr` does.
+                    let thin_ptr = dst.layout.field(bx.cx(), FAT_PTR_ADDR);
+                    (bx.pointercast(base, bx.cx().backend_type(thin_ptr)), info)
+                }
+                OperandValue::Immediate(base) => {
+                    unsize_thin_ptr(bx, base, src_ty, dst_ty)
+                }
+                OperandValue::Ref(..) => bug!()
+            };
+            OperandValue::Pair(base, info).store(bx, dst);
         }
 
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {