Rollup merge of #77547 - RalfJung:stable-union-drop, r=matthewjasper

stabilize union with 'ManuallyDrop' fields and 'impl Drop for Union'

As [discussed by @SimonSapin and @withoutboats](#55149 (comment)), this PR proposes to stabilize parts of the `untagged_union` feature gate:

* It will be possible to have a union with field type `ManuallyDrop<T>` for any `T`.
* While at it I propose we also stabilize `impl Drop for Union`; to my knowledge, there are no open concerns around this feature.

In the RFC discussion, we also talked about allowing `&mut T` as another non-`Copy` non-dropping type, but that felt to me like an overly specific exception so I figured we'd wait if there is actually any use for such a special case.

Some things remain unstable and still require the `untagged_union` feature gate:
* Union with fields that do not drop, are not `Copy`, and are not `ManuallyDrop<_>`. The reason to not stabilize this is to avoid semver concerns around libraries adding `Drop` implementations later. (This is already not fully semver compatible as, to my knowledge, the borrow checker will exploit the non-dropping nature of any type, but it seems prudent to avoid further increasing the amount of trouble adding an `impl Drop` can cause.)

Due to this, quite a few tests still need the `untagged_union` feature, but I think the ones where I could remove the feature flag provide good test coverage for the stable part.

Cc @rust-lang/lang

Author: JohnTitor

compiler/rustc_passes/src/stability.rs

@@ -13,15 +13,13 @@ use rustc_hir::{Generics, HirId, Item, StructField, TraitRef, Ty, TyKind, Varian
 use rustc_middle::hir::map::Map;
 use rustc_middle::middle::privacy::AccessLevels;
 use rustc_middle::middle::stability::{DeprecationEntry, Index};
-use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::{self, query::Providers, TyCtxt};
 use rustc_session::lint;
 use rustc_session::lint::builtin::INEFFECTIVE_UNSTABLE_TRAIT_IMPL;
 use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
-use rustc_span::Span;
-use rustc_trait_selection::traits::misc::can_type_implement_copy;
+use rustc_span::{Span, DUMMY_SP};
 
 use std::cmp::Ordering;
 use std::mem::replace;
@@ -711,27 +709,35 @@ impl Visitor<'tcx> for Checker<'tcx> {
             // so semi-randomly perform it here in stability.rs
             hir::ItemKind::Union(..) if !self.tcx.features().untagged_unions => {
                 let def_id = self.tcx.hir().local_def_id(item.hir_id);
-                let adt_def = self.tcx.adt_def(def_id);
                 let ty = self.tcx.type_of(def_id);
+                let (adt_def, substs) = match ty.kind() {
+                    ty::Adt(adt_def, substs) => (adt_def, substs),
+                    _ => bug!(),
+                };
 
-                if adt_def.has_dtor(self.tcx) {
-                    feature_err(
-                        &self.tcx.sess.parse_sess,
-                        sym::untagged_unions,
-                        item.span,
-                        "unions with `Drop` implementations are unstable",
-                    )
-                    .emit();
-                } else {
-                    let param_env = self.tcx.param_env(def_id);
-                    if can_type_implement_copy(self.tcx, param_env, ty).is_err() {
-                        feature_err(
-                            &self.tcx.sess.parse_sess,
-                            sym::untagged_unions,
-                            item.span,
-                            "unions with non-`Copy` fields are unstable",
-                        )
-                        .emit();
+                // Non-`Copy` fields are unstable, except for `ManuallyDrop`.
+                let param_env = self.tcx.param_env(def_id);
+                for field in &adt_def.non_enum_variant().fields {
+                    let field_ty = field.ty(self.tcx, substs);
+                    if !field_ty.ty_adt_def().map_or(false, |adt_def| adt_def.is_manually_drop())
+                        && !field_ty.is_copy_modulo_regions(self.tcx.at(DUMMY_SP), param_env)
+                    {
+                        if field_ty.needs_drop(self.tcx, param_env) {
+                            // Avoid duplicate error: This will error later anyway because fields
+                            // that need drop are not allowed.
+                            self.tcx.sess.delay_span_bug(
+                                item.span,
+                                "union should have been rejected due to potentially dropping field",
+                            );
+                        } else {
+                            feature_err(
+                                &self.tcx.sess.parse_sess,
+                                sym::untagged_unions,
+                                self.tcx.def_span(field.did),
+                                "unions with non-`Copy` fields other than `ManuallyDrop<T>` are unstable",
+                            )
+                            .emit();
+                        }
                     }
                 }
             }

compiler/rustc_typeck/src/check/check.rs

@@ -348,8 +348,7 @@ pub(super) fn check_union(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) {
     check_packed(tcx, span, def);
 }
 
-/// When the `#![feature(untagged_unions)]` gate is active,
-/// check that the fields of the `union` does not contain fields that need dropping.
+/// Check that the fields of the `union` do not need dropping.
 pub(super) fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: LocalDefId) -> bool {
     let item_type = tcx.type_of(item_def_id);
     if let ty::Adt(def, substs) = item_type.kind() {

library/core/src/lib.rs

@@ -131,7 +131,7 @@
 #![feature(transparent_unions)]
 #![feature(unboxed_closures)]
 #![feature(unsized_locals)]
-#![feature(untagged_unions)]
+#![cfg_attr(bootstrap, feature(untagged_unions))]
 #![feature(unwind_attributes)]
 #![feature(variant_count)]
 #![feature(tbm_target_feature)]

library/core/src/ptr/mod.rs

@@ -229,6 +229,16 @@ pub(crate) struct FatPtr<T> {
     pub(crate) len: usize,
 }
 
+// Manual impl needed to avoid `T: Clone` bound.
+impl<T> Clone for FatPtr<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+// Manual impl needed to avoid `T: Copy` bound.
+impl<T> Copy for FatPtr<T> {}
+
 /// Forms a raw slice from a pointer and a length.
 ///
 /// The `len` argument is the number of **elements**, not the number of bytes.

library/std/src/lib.rs

@@ -319,7 +319,7 @@
 #![feature(unsafe_block_in_unsafe_fn)]
 #![feature(unsafe_cell_get_mut)]
 #![feature(unsafe_cell_raw_get)]
-#![feature(untagged_unions)]
+#![cfg_attr(bootstrap, feature(untagged_unions))]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]
 #![feature(wake_trait)]

src/test/ui/did_you_mean/bad-assoc-ty.rs

@@ -68,7 +68,6 @@ enum N<F> where F: Fn() -> _ {
 
 union O<F> where F: Fn() -> _ {
 //~^ ERROR the type placeholder `_` is not allowed within types on item signatures
-//~| ERROR unions with non-`Copy` fields are unstable
     foo: F,
 }
 

src/test/ui/did_you_mean/bad-assoc-ty.stderr

@@ -57,19 +57,6 @@ LL | type J = ty!(u8);
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0658]: unions with non-`Copy` fields are unstable
-  --> $DIR/bad-assoc-ty.rs:69:1
-   |
-LL | / union O<F> where F: Fn() -> _ {
-LL | |
-LL | |
-LL | |     foo: F,
-LL | | }
-   | |_^
-   |
-   = note: see issue #55149 <https://github.com/rust-lang/rust/issues/55149> for more information
-   = help: add `#![feature(untagged_unions)]` to the crate attributes to enable
-
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:1:10
    |
@@ -215,7 +202,7 @@ LL | union O<F, T> where F: Fn() -> T {
    |          ^^^                   ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/bad-assoc-ty.rs:75:29
+  --> $DIR/bad-assoc-ty.rs:74:29
    |
 LL | trait P<F> where F: Fn() -> _ {
    |                             ^ not allowed in type signatures
@@ -226,7 +213,7 @@ LL | trait P<F, T> where F: Fn() -> T {
    |          ^^^                   ^
 
 error[E0121]: the type placeholder `_` is not allowed within types on item signatures
-  --> $DIR/bad-assoc-ty.rs:80:38
+  --> $DIR/bad-assoc-ty.rs:79:38
    |
 LL |     fn foo<F>(_: F) where F: Fn() -> _ {}
    |                                      ^ not allowed in type signatures
@@ -236,7 +223,7 @@ help: use type parameters instead
 LL |     fn foo<F, T>(_: F) where F: Fn() -> T {}
    |             ^^^                         ^
 
-error: aborting due to 29 previous errors
+error: aborting due to 28 previous errors
 
-Some errors have detailed explanations: E0121, E0223, E0658.
+Some errors have detailed explanations: E0121, E0223.
 For more information about an error, try `rustc --explain E0121`.

src/test/ui/drop/dynamic-drop.rs

@@ -1,7 +1,7 @@
 // run-pass
 // ignore-wasm32-bare compiled with panic=abort by default
 
-#![feature(generators, generator_trait, untagged_unions)]
+#![feature(generators, generator_trait)]
 #![feature(bindings_after_at)]
 
 #![allow(unused_assignments)]

src/test/ui/dropck/dropck-union.rs

@@ -1,5 +1,3 @@
-#![feature(untagged_unions)]
-
 use std::cell::Cell;
 use std::ops::Deref;
 use std::mem::ManuallyDrop;

src/test/ui/dropck/dropck-union.stderr

@@ -1,5 +1,5 @@
 error[E0597]: `v` does not live long enough
-  --> $DIR/dropck-union.rs:39:18
+  --> $DIR/dropck-union.rs:37:18
    |
 LL |     v.0.set(Some(&v));
    |                  ^^ borrowed value does not live long enough