Auto merge of #104376 - compiler-errors:thin-metadata-implies-thin-ptr, r=wesleywiser

layout_of: `T: Thin` implies `sizeof(&T) == sizeof(usize)`

Use the `<T as Pointee>::Metadata` associated type to calculate the layout of a pointee's metadata, instead of hard-coding rules about certain types.

Maybe this approach is overkill -- we could instead hard-code this approach as a fallback, with the matching on `Slice`/`Dynamic`/etc. happening first

Fixes this issue here https://github.com/rust-lang/rust/pull/104338#issuecomment-1312595844 .. But is also useful with transmutes, for example, given the UI test I added below.

compiler/rustc_middle/src/ty/layout.rs

@@ -670,29 +670,50 @@ where
                         });
                     }
 
-                    match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).kind() {
-                        ty::Slice(_) | ty::Str => TyMaybeWithLayout::Ty(tcx.types.usize),
-                        ty::Dynamic(_, _, ty::Dyn) => {
-                            TyMaybeWithLayout::Ty(tcx.mk_imm_ref(
-                                tcx.lifetimes.re_static,
-                                tcx.mk_array(tcx.types.usize, 3),
-                            ))
-                            /* FIXME: use actual fn pointers
-                            Warning: naively computing the number of entries in the
-                            vtable by counting the methods on the trait + methods on
-                            all parent traits does not work, because some methods can
-                            be not object safe and thus excluded from the vtable.
-                            Increase this counter if you tried to implement this but
-                            failed to do it without duplicating a lot of code from
-                            other places in the compiler: 2
-                            tcx.mk_tup(&[
-                                tcx.mk_array(tcx.types.usize, 3),
-                                tcx.mk_array(Option<fn()>),
-                            ])
-                            */
+                    let mk_dyn_vtable = || {
+                        tcx.mk_imm_ref(tcx.lifetimes.re_static, tcx.mk_array(tcx.types.usize, 3))
+                        /* FIXME: use actual fn pointers
+                        Warning: naively computing the number of entries in the
+                        vtable by counting the methods on the trait + methods on
+                        all parent traits does not work, because some methods can
+                        be not object safe and thus excluded from the vtable.
+                        Increase this counter if you tried to implement this but
+                        failed to do it without duplicating a lot of code from
+                        other places in the compiler: 2
+                        tcx.mk_tup(&[
+                            tcx.mk_array(tcx.types.usize, 3),
+                            tcx.mk_array(Option<fn()>),
+                        ])
+                        */
+                    };
+
+                    let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type() {
+                        let metadata = tcx.normalize_erasing_regions(
+                            cx.param_env(),
+                            tcx.mk_projection(metadata_def_id, [pointee]),
+                        );
+
+                        // Map `Metadata = DynMetadata<dyn Trait>` back to a vtable, since it
+                        // offers better information than `std::ptr::metadata::VTable`,
+                        // and we rely on this layout information to trigger a panic in
+                        // `std::mem::uninitialized::<&dyn Trait>()`, for example.
+                        if let ty::Adt(def, substs) = metadata.kind()
+                            && Some(def.did()) == tcx.lang_items().dyn_metadata()
+                            && substs.type_at(0).is_trait()
+                        {
+                            mk_dyn_vtable()
+                        } else {
+                            metadata
                         }
-                        _ => bug!("TyAndLayout::field({:?}): not applicable", this),
-                    }
+                    } else {
+                        match tcx.struct_tail_erasing_lifetimes(pointee, cx.param_env()).kind() {
+                            ty::Slice(_) | ty::Str => tcx.types.usize,
+                            ty::Dynamic(_, _, ty::Dyn) => mk_dyn_vtable(),
+                            _ => bug!("TyAndLayout::field({:?}): not applicable", this),
+                        }
+                    };
+
+                    TyMaybeWithLayout::Ty(metadata)
                 }
 
                 // Arrays and slices.

compiler/rustc_ty_utils/src/layout.rs

@@ -155,17 +155,37 @@ fn layout_of_uncached<'tcx>(
             }
 
             let unsized_part = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
-            let metadata = match unsized_part.kind() {
-                ty::Foreign(..) => {
+
+            let metadata = if let Some(metadata_def_id) = tcx.lang_items().metadata_type() {
+                let metadata_ty = tcx.normalize_erasing_regions(
+                    param_env,
+                    tcx.mk_projection(metadata_def_id, [pointee]),
+                );
+                let metadata_layout = cx.layout_of(metadata_ty)?;
+                // If the metadata is a 1-zst, then the pointer is thin.
+                if metadata_layout.is_zst() && metadata_layout.align.abi.bytes() == 1 {
                     return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
                 }
-                ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
-                ty::Dynamic(..) => {
-                    let mut vtable = scalar_unit(Pointer);
-                    vtable.valid_range_mut().start = 1;
-                    vtable
+
+                let Abi::Scalar(metadata) = metadata_layout.abi else {
+                    return Err(LayoutError::Unknown(unsized_part));
+                };
+                metadata
+            } else {
+                match unsized_part.kind() {
+                    ty::Foreign(..) => {
+                        return Ok(tcx.intern_layout(LayoutS::scalar(cx, data_ptr)));
+                    }
+                    ty::Slice(_) | ty::Str => scalar_unit(Int(dl.ptr_sized_integer(), false)),
+                    ty::Dynamic(..) => {
+                        let mut vtable = scalar_unit(Pointer);
+                        vtable.valid_range_mut().start = 1;
+                        vtable
+                    }
+                    _ => {
+                        return Err(LayoutError::Unknown(unsized_part));
+                    }
                 }
-                _ => return Err(LayoutError::Unknown(unsized_part)),
             };
 
             // Effectively a (ptr, meta) tuple.

src/test/ui/layout/thin-meta-implies-thin-ptr.rs

@@ -0,0 +1,11 @@
+// check-pass
+
+#![feature(ptr_metadata)]
+
+use std::ptr::Thin;
+
+fn main() {}
+
+fn foo<T: ?Sized + Thin>(t: *const T) -> *const () {
+    unsafe { std::mem::transmute(t) }
+}