make first component of dyn* use pointer layout+type, and adjust DynStar comment

Author: RalfJung

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};
@@ -273,13 +273,6 @@ 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
-    // a pointer, and really we should ensure that the value is a suitable
-    // pointer earlier in the compilation process.
-    let src = match src_ty_and_layout.pointee_info_at(bx.cx(), Size::ZERO) {
-        Some(_) => bx.ptrtoint(src, bx.cx().type_isize()),
-        None => bx.bitcast(src, bx.type_isize()),
-    };
     (src, unsized_info(bx, src_ty_and_layout.ty, dst_ty, old_info))
 }
 

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
-    /// (often a pointer to the real object, but not necessarily) and `vtable` is a pointer to
-    /// the vtable for `dyn* Trait`. The representation is essentially the same as `&dyn Trait`
-    /// 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
+    /// These objects are represented as a `(data, vtable)` pair where `data` is a value of some
+    /// ptr-sized and ptr-aligned dynamically determined type `T` and `vtable` is a pointer to the
+    /// vtable of `impl T for Trait`. This allows a `dyn*` object to be treated agnostically with
     /// respect to whether it points to a `Box<T>`, `Rc<T>`, etc.
     DynStar,
 }