Auto merge of #97729 - Dylan-DPC:rollup-dv43xo9, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - #96642 (Avoid zero-sized allocs in ThinBox if T and H are both ZSTs.)
 - #97647 (Lazily allocate and initialize pthread locks.)
 - #97715 (Support the `#[expect]` attribute on fn parameters (RFC-2383))
 - #97716 (Fix reachability analysis for const methods)
 - #97722 (Tighten spans for bad fields in struct deriving `Copy`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -420,7 +420,15 @@ impl<'a> AstValidator<'a> {
             .iter()
             .flat_map(|i| i.attrs.as_ref())
             .filter(|attr| {
-                let arr = [sym::allow, sym::cfg, sym::cfg_attr, sym::deny, sym::forbid, sym::warn];
+                let arr = [
+                    sym::allow,
+                    sym::cfg,
+                    sym::cfg_attr,
+                    sym::deny,
+                    sym::expect,
+                    sym::forbid,
+                    sym::warn,
+                ];
                 !arr.contains(&attr.name_or_empty()) && rustc_attr::is_builtin_attr(attr)
             })
             .for_each(|attr| {
@@ -435,7 +443,7 @@ impl<'a> AstValidator<'a> {
                 } else {
                     self.err_handler().span_err(
                         attr.span,
-                        "allow, cfg, cfg_attr, deny, \
+                        "allow, cfg, cfg_attr, deny, expect, \
                 forbid, and warn are the only allowed built-in attributes in function parameters",
                     );
                 }

compiler/rustc_passes/src/reachable.rs

@@ -148,32 +148,15 @@ impl<'tcx> ReachableContext<'tcx> {
                 hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
                 | hir::TraitItemKind::Type(..) => false,
             },
-            Some(Node::ImplItem(impl_item)) => {
-                match impl_item.kind {
-                    hir::ImplItemKind::Const(..) => true,
-                    hir::ImplItemKind::Fn(..) => {
-                        let attrs = self.tcx.codegen_fn_attrs(def_id);
-                        let generics = self.tcx.generics_of(def_id);
-                        if generics.requires_monomorphization(self.tcx) || attrs.requests_inline() {
-                            true
-                        } else {
-                            let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
-                            let impl_did = self.tcx.hir().get_parent_item(hir_id);
-                            // Check the impl. If the generics on the self
-                            // type of the impl require inlining, this method
-                            // does too.
-                            match self.tcx.hir().expect_item(impl_did).kind {
-                                hir::ItemKind::Impl { .. } => {
-                                    let generics = self.tcx.generics_of(impl_did);
-                                    generics.requires_monomorphization(self.tcx)
-                                }
-                                _ => false,
-                            }
-                        }
-                    }
-                    hir::ImplItemKind::TyAlias(_) => false,
+            Some(Node::ImplItem(impl_item)) => match impl_item.kind {
+                hir::ImplItemKind::Const(..) => true,
+                hir::ImplItemKind::Fn(..) => {
+                    let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
+                    let impl_did = self.tcx.hir().get_parent_item(hir_id);
+                    method_might_be_inlined(self.tcx, impl_item, impl_did)
                 }
-            }
+                hir::ImplItemKind::TyAlias(_) => false,
+            },
             Some(_) => false,
             None => false, // This will happen for default methods.
         }

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -20,7 +20,7 @@ pub fn can_type_implement_copy<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     self_type: Ty<'tcx>,
-    cause: ObligationCause<'tcx>,
+    parent_cause: ObligationCause<'tcx>,
 ) -> Result<(), CopyImplementationError<'tcx>> {
     // FIXME: (@jroesch) float this code up
     tcx.infer_ctxt().enter(|infcx| {
@@ -59,7 +59,7 @@ pub fn can_type_implement_copy<'tcx>(
                     .ty(tcx, traits::InternalSubsts::identity_for_item(tcx, adt.did()))
                     .has_param_types_or_consts()
                 {
-                    cause.clone()
+                    parent_cause.clone()
                 } else {
                     ObligationCause::dummy_with_span(span)
                 };

compiler/rustc_typeck/src/coherence/builtin.rs

@@ -107,6 +107,10 @@ fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
 
             for (field, ty) in fields {
                 let field_span = tcx.def_span(field.did);
+                let field_ty_span = match tcx.hir().get_if_local(field.did) {
+                    Some(hir::Node::Field(field_def)) => field_def.ty.span,
+                    _ => field_span,
+                };
                 err.span_label(field_span, "this field does not implement `Copy`");
                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
                 // why this field does not implement Copy. This is useful because sometimes
@@ -119,7 +123,7 @@ fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
                         param_env,
                         ty,
                         tcx.lang_items().copy_trait().unwrap(),
-                        traits::ObligationCause::dummy_with_span(field_span),
+                        traits::ObligationCause::dummy_with_span(field_ty_span),
                     );
                     for error in fulfill_cx.select_all_or_error(&infcx) {
                         let error_predicate = error.obligation.predicate;

library/alloc/src/boxed/thin.rs

@@ -138,7 +138,11 @@ impl<T: ?Sized> ThinBox<T> {
     }
 }
 
-/// A pointer to type-erased data, guaranteed to have a header `H` before the pointed-to location.
+/// A pointer to type-erased data, guaranteed to either be:
+/// 1. `NonNull::dangling()`, in the case where both the pointee (`T`) and
+///    metadata (`H`) are ZSTs.
+/// 2. A pointer to a valid `T` that has a header `H` directly before the
+///    pointed-to location.
 struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
 
 impl<H> WithHeader<H> {
@@ -156,16 +160,27 @@ impl<H> WithHeader<H> {
         };
 
         unsafe {
-            let ptr = alloc::alloc(layout);
-
-            if ptr.is_null() {
-                alloc::handle_alloc_error(layout);
-            }
-            //  Safety:
-            //  -   The size is at least `aligned_header_size`.
-            let ptr = ptr.add(value_offset) as *mut _;
-
-            let ptr = NonNull::new_unchecked(ptr);
+            // Note: It's UB to pass a layout with a zero size to `alloc::alloc`, so
+            // we use `layout.dangling()` for this case, which should have a valid
+            // alignment for both `T` and `H`.
+            let ptr = if layout.size() == 0 {
+                // Some paranoia checking, mostly so that the ThinBox tests are
+                // more able to catch issues.
+                debug_assert!(
+                    value_offset == 0 && mem::size_of::<T>() == 0 && mem::size_of::<H>() == 0
+                );
+                layout.dangling()
+            } else {
+                let ptr = alloc::alloc(layout);
+                if ptr.is_null() {
+                    alloc::handle_alloc_error(layout);
+                }
+                // Safety:
+                // - The size is at least `aligned_header_size`.
+                let ptr = ptr.add(value_offset) as *mut _;
+
+                NonNull::new_unchecked(ptr)
+            };
 
             let result = WithHeader(ptr, PhantomData);
             ptr::write(result.header(), header);
@@ -175,18 +190,28 @@ impl<H> WithHeader<H> {
         }
     }
 
-    //  Safety:
-    //  -   Assumes that `value` can be dereferenced.
+    // Safety:
+    // - Assumes that either `value` can be dereferenced, or is the
+    //   `NonNull::dangling()` we use when both `T` and `H` are ZSTs.
     unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
         unsafe {
+            let value_layout = Layout::for_value_raw(value);
             // SAFETY: Layout must have been computable if we're in drop
-            let (layout, value_offset) =
-                Self::alloc_layout(Layout::for_value_raw(value)).unwrap_unchecked();
+            let (layout, value_offset) = Self::alloc_layout(value_layout).unwrap_unchecked();
 
-            ptr::drop_in_place::<T>(value);
             // We only drop the value because the Pointee trait requires that the metadata is copy
-            // aka trivially droppable
-            alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
+            // aka trivially droppable.
+            ptr::drop_in_place::<T>(value);
+
+            // Note: Don't deallocate if the layout size is zero, because the pointer
+            // didn't come from the allocator.
+            if layout.size() != 0 {
+                alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
+            } else {
+                debug_assert!(
+                    value_offset == 0 && mem::size_of::<H>() == 0 && value_layout.size() == 0
+                );
+            }
         }
     }
 
@@ -198,7 +223,9 @@ impl<H> WithHeader<H> {
         //    needed to align the header. Subtracting the header size from the aligned data pointer
         //    will always result in an aligned header pointer, it just may not point to the
         //    beginning of the allocation.
-        unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H }
+        let hp = unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H };
+        debug_assert!(hp.is_aligned());
+        hp
     }
 
     fn value(&self) -> *mut u8 {

library/alloc/src/lib.rs

@@ -169,6 +169,7 @@
 #![feature(nll)] // Not necessary, but here to test the `nll` feature.
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
+#![feature(pointer_is_aligned)]
 #![feature(slice_internals)]
 #![feature(staged_api)]
 #![feature(stmt_expr_attributes)]

library/alloc/tests/lib.rs

@@ -42,6 +42,9 @@
 #![feature(panic_update_hook)]
 #![feature(slice_flatten)]
 #![feature(thin_box)]
+#![feature(bench_black_box)]
+#![feature(strict_provenance)]
+#![feature(once_cell)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};