Auto merge of #107772 - compiler-errors:dyn-star-backend-is-ptr, r=eholk

Make `dyn*`'s value backend type a pointer

One tweak on top of Ralf's commit should fix using `usize` as a `dyn*`-coercible type, and should fix when we're using various other pointer types when LLVM opaque pointers is disabled.

r? `@eholk` but feel free to reassign
cc https://github.com/rust-lang/rust/pull/107728#issuecomment-1421231823 `@RalfJung`

compiler/rustc_codegen_llvm/src/type_of.rs

@@ -329,7 +329,7 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
     ) -> &'a Type {
         // HACK(eddyb) special-case fat pointers until LLVM removes
         // pointee types, to avoid bitcasting every `OperandRef::deref`.
-        match self.ty.kind() {
+        match *self.ty.kind() {
             ty::Ref(..) | ty::RawPtr(_) => {
                 return self.field(cx, index).llvm_type(cx);
             }
@@ -339,6 +339,11 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
                 let ptr_ty = cx.tcx.mk_mut_ptr(self.ty.boxed_ty());
                 return cx.layout_of(ptr_ty).scalar_pair_element_llvm_type(cx, index, immediate);
             }
+            // `dyn* Trait` has the same ABI as `*mut dyn Trait`
+            ty::Dynamic(bounds, region, ty::DynStar) => {
+                let ptr_ty = cx.tcx.mk_mut_ptr(cx.tcx.mk_dynamic(bounds, region, ty::Dyn));
+                return cx.layout_of(ptr_ty).scalar_pair_element_llvm_type(cx, index, immediate);
+            }
             _ => {}
         }
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -39,7 +39,7 @@ use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::Symbol;
 use rustc_span::{DebuggerVisualizerFile, DebuggerVisualizerType};
-use rustc_target::abi::{Align, Size, VariantIdx};
+use rustc_target::abi::{Align, VariantIdx};
 
 use std::collections::BTreeSet;
 use std::time::{Duration, Instant};
@@ -273,12 +273,13 @@ pub fn cast_to_dyn_star<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
         "destination type must be a dyn*"
     );
-    // FIXME(dyn-star): this is probably not the best way to check if this is
+    // FIXME(dyn-star): We can remove this when all supported LLVMs use opaque ptrs only.
-    // a pointer, and really we should ensure that the value is a suitable
+    let unit_ptr = bx.cx().type_ptr_to(bx.cx().type_struct(&[], false));
-    // pointer earlier in the compilation process.
+    let src = match bx.cx().type_kind(bx.cx().backend_type(src_ty_and_layout)) {
-    let src = match src_ty_and_layout.pointee_info_at(bx.cx(), Size::ZERO) {
+        TypeKind::Pointer => bx.pointercast(src, unit_ptr),
-        Some(_) => bx.ptrtoint(src, bx.cx().type_isize()),
+        TypeKind::Integer => bx.inttoptr(src, unit_ptr),
-        None => bx.bitcast(src, bx.type_isize()),
+        // FIXME(dyn-star): We probably have to do a bitcast first, then inttoptr.
+        kind => bug!("unexpected TypeKind for left-hand side of `dyn*` cast: {kind:?}"),
     };
     (src, unsized_info(bx, src_ty_and_layout.ty, dst_ty, old_info))
 }

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -452,7 +452,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             args1 = [place.llval];
             &args1[..]
         };
-        let (drop_fn, fn_abi) = match ty.kind() {
+        let (drop_fn, fn_abi) =
+            match ty.kind() {
                 // FIXME(eddyb) perhaps move some of this logic into
                 // `Instance::resolve_drop_in_place`?
                 ty::Dynamic(_, _, ty::Dyn) => {
@@ -516,17 +517,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     debug!("drop_fn = {:?}", drop_fn);
                     debug!("args = {:?}", args);
                     let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
-                let data = args[0];
+                    let meta_ptr = place.project_field(bx, 1);
-                let data_ty = bx.cx().backend_type(place.layout);
+                    let meta = bx.load_operand(meta_ptr);
-                let vtable_ptr =
-                    bx.gep(data_ty, data, &[bx.cx().const_i32(0), bx.cx().const_i32(1)]);
-                let vtable = bx.load(bx.type_i8p(), vtable_ptr, abi::Align::ONE);
                     // Truncate vtable off of args list
                     args = &args[..1];
                     debug!("args' = {:?}", args);
                     (
                         meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
-                        .get_fn(bx, vtable, ty, &fn_abi),
+                            .get_fn(bx, meta.immediate(), ty, &fn_abi),
                         fn_abi,
                     )
                 }

compiler/rustc_middle/src/ty/layout.rs

@@ -770,7 +770,7 @@ where
 
                 ty::Dynamic(_, _, ty::DynStar) => {
                     if i == 0 {
-                        TyMaybeWithLayout::Ty(tcx.types.usize)
+                        TyMaybeWithLayout::Ty(tcx.mk_mut_ptr(tcx.types.unit))
                     } else if i == 1 {
                         // FIXME(dyn-star) same FIXME as above applies here too
                         TyMaybeWithLayout::Ty(

compiler/rustc_ty_utils/src/layout.rs

@@ -193,7 +193,7 @@ fn layout_of_uncached<'tcx>(
         }
 
         ty::Dynamic(_, _, ty::DynStar) => {
-            let mut data = scalar_unit(Int(dl.ptr_sized_integer(), false));
+            let mut data = scalar_unit(Pointer(AddressSpace::DATA));
             data.valid_range_mut().start = 0;
             let mut vtable = scalar_unit(Pointer(AddressSpace::DATA));
             vtable.valid_range_mut().start = 1;

compiler/rustc_type_ir/src/sty.rs

@@ -26,11 +26,9 @@ pub enum DynKind {
     Dyn,
     /// A sized `dyn* Trait` object
     ///
-    /// These objects are represented as a `(data, vtable)` pair where `data` is a ptr-sized value
+    /// These objects are represented as a `(data, vtable)` pair where `data` is a value of some
-    /// (often a pointer to the real object, but not necessarily) and `vtable` is a pointer to
+    /// ptr-sized and ptr-aligned dynamically determined type `T` and `vtable` is a pointer to the
-    /// the vtable for `dyn* Trait`. The representation is essentially the same as `&dyn Trait`
+    /// vtable of `impl T for Trait`. This allows a `dyn*` object to be treated agnostically with
-    /// or similar, but the drop function included in the vtable is responsible for freeing the
-    /// underlying storage if needed. This allows a `dyn*` object to be treated agnostically with
     /// respect to whether it points to a `Box<T>`, `Rc<T>`, etc.
     DynStar,
 }

tests/codegen/function-arguments.rs

@@ -1,6 +1,7 @@
 // compile-flags: -O -C no-prepopulate-passes
 
 #![crate_type = "lib"]
+#![feature(dyn_star)]
 
 use std::mem::MaybeUninit;
 use std::num::NonZeroU64;
@@ -279,3 +280,11 @@ pub fn enum_id_1(x: Option<Result<u16, u16>>) -> Option<Result<u16, u16>> {
 pub fn enum_id_2(x: Option<u8>) -> Option<u8> {
   x
 }
+
+// CHECK: { {{\{\}\*|ptr}}, {{.+}} } @dyn_star({{\{\}\*|ptr}} noundef %x.0, {{.+}} noalias noundef readonly align {{.*}} dereferenceable({{.*}}) %x.1)
+// Expect an ABI something like `{ {}*, [3 x i64]* }`, but that's hard to match on generically,
+// so do like the `trait_box` test and just match on `{{.+}}` for the vtable.
+#[no_mangle]
+pub fn dyn_star(x: dyn* Drop) -> dyn* Drop {
+  x
+}

tests/ui/dyn-star/llvm-old-style-ptrs.rs

@@ -0,0 +1,23 @@
+// run-pass
+// compile-flags: -Copt-level=0 -Cllvm-args=-opaque-pointers=0
+
+// (opaque-pointers flag is called force-opaque-pointers in LLVM 13...)
+// min-llvm-version: 14.0
+
+// This test can be removed once non-opaque pointers are gone from LLVM, maybe.
+
+#![feature(dyn_star, pointer_like_trait)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+use std::marker::PointerLike;
+
+fn make_dyn_star<'a>(t: impl PointerLike + Debug + 'a) -> dyn* Debug + 'a {
+    t as _
+}
+
+fn main() {
+    println!("{:?}", make_dyn_star(Box::new(1i32)));
+    println!("{:?}", make_dyn_star(2usize));
+    println!("{:?}", make_dyn_star((3usize,)));
+}