Auto merge of #42068 - petrochenkov:ustab, r=nikomatsakis

Stabilize unions with `Copy` fields and no destructor

What else is needed:
- [x] Documentation (rust-lang/reference#57).
- [x] Making assignments to `Copy` union fields safe (#42083).
- [ ] Backport? (The "stabilization decision" is from [Apr 13](#32836 (comment)), it's just this PR is late.)

cc #32836
r? @nikomatsakis

Author: bors

src/libcollections/lib.rs

@@ -65,7 +65,6 @@
 #![feature(trusted_len)]
 #![feature(unicode)]
 #![feature(unique)]
-#![feature(untagged_unions)]
 #![cfg_attr(not(test), feature(str_checked_slicing))]
 #![cfg_attr(test, feature(rand, test))]
 #![feature(offset_to)]

src/librustc/middle/stability.rs

@@ -625,6 +625,27 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                 }
             }
 
+            // There's no good place to insert stability check for non-Copy unions,
+            // so semi-randomly perform it here in stability.rs
+            hir::ItemUnion(..) if !self.tcx.sess.features.borrow().untagged_unions => {
+                let def_id = self.tcx.hir.local_def_id(item.id);
+                let adt_def = self.tcx.adt_def(def_id);
+                let ty = self.tcx.type_of(def_id);
+
+                if adt_def.has_dtor(self.tcx) {
+                    emit_feature_err(&self.tcx.sess.parse_sess,
+                                     "untagged_unions", item.span, GateIssue::Language,
+                                     "unions with `Drop` implementations are unstable");
+                } else {
+                    let param_env = self.tcx.param_env(def_id);
+                    if !param_env.can_type_implement_copy(self.tcx, ty, item.span).is_ok() {
+                        emit_feature_err(&self.tcx.sess.parse_sess,
+                                        "untagged_unions", item.span, GateIssue::Language,
+                                        "unions with non-`Copy` fields are unstable");
+                    }
+                }
+            }
+
             _ => (/* pass */)
         }
         intravisit::walk_item(self, item);

src/librustc_data_structures/lib.rs

@@ -29,7 +29,6 @@
 #![feature(nonzero)]
 #![feature(unboxed_closures)]
 #![feature(fn_traits)]
-#![feature(untagged_unions)]
 #![feature(associated_consts)]
 #![feature(unsize)]
 #![feature(i128_type)]

src/libsyntax/feature_gate.rs

@@ -1207,12 +1207,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemKind::Union(..) => {
-                gate_feature_post!(&self, untagged_unions,
-                                   i.span,
-                                   "unions are unstable and possibly buggy");
-            }
-
             ast::ItemKind::DefaultImpl(..) => {
                 gate_feature_post!(&self, optin_builtin_traits,
                                    i.span,

src/test/compile-fail/borrowck/borrowck-union-borrow-nested.rs

@@ -8,10 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-tidy-linelength
-
-#![feature(untagged_unions)]
-
 #[derive(Clone, Copy)]
 struct S {
     a: u8,

src/test/compile-fail/borrowck/borrowck-union-borrow.rs

@@ -10,8 +10,6 @@
 
 // ignore-tidy-linelength
 
-#![feature(untagged_unions)]
-
 #[derive(Clone, Copy)]
 union U {
     a: u8,

src/test/compile-fail/borrowck/borrowck-union-uninitialized.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 struct S {
     a: u8,
 }

src/test/compile-fail/privacy/union-field-privacy-1.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 mod m {
     pub union U {
         pub a: u8,

src/test/compile-fail/privacy/union-field-privacy-2.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 mod m {
     pub union U {
         pub a: u8,

src/test/compile-fail/union/union-const-eval.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(untagged_unions)]
-
 union U {
     a: usize,
     b: usize,