Add flag to configure noalias on Box<T>

To aid making an informed decision about the aliasing
rules of box, give users an option to remove `noalias`
from box.

compiler/rustc_interface/src/tests.rs

@@ -718,6 +718,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(asm_comments, true);
     tracked!(assume_incomplete_release, true);
     tracked!(binary_dep_depinfo, true);
+    tracked!(box_noalias, Some(false));
     tracked!(
         branch_protection,
         Some(BranchProtection {

compiler/rustc_middle/src/ty/layout.rs

@@ -3266,7 +3266,12 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     // this attribute doesn't make it UB for the pointed-to data to be undef.
                     attrs.set(ArgAttribute::NoUndef);
 
-                    // `Box` pointer parameters never alias because ownership is transferred
+                    // The aliasing rules for `Box<T>` are still not decided, but currently we emit
+                    // `noalias` for it. This can be turned off using an unstable flag.
+                    // See https://github.com/rust-lang/unsafe-code-guidelines/issues/326
+                    let noalias_for_box =
+                        self.tcx().sess.opts.unstable_opts.box_noalias.unwrap_or(true);
+
                     // `&mut` pointer parameters never alias other parameters,
                     // or mutable global data
                     //
@@ -3281,7 +3286,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     // `-Zmutable-noalias` debugging option.
                     let no_alias = match kind {
                         PointerKind::Shared | PointerKind::UniqueBorrowed => false,
-                        PointerKind::UniqueOwned => true,
+                        PointerKind::UniqueOwned => noalias_for_box,
                         PointerKind::Frozen => !is_return,
                     };
                     if no_alias {

compiler/rustc_session/src/options.rs

@@ -1209,6 +1209,8 @@ options! {
     binary_dep_depinfo: bool = (false, parse_bool, [TRACKED],
         "include artifacts (sysroot, crate dependencies) used during compilation in dep-info \
         (default: no)"),
+    box_noalias: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "emit noalias metadata for box (default: yes)"),
     branch_protection: Option<BranchProtection> = (None, parse_branch_protection, [TRACKED],
         "set options for branch target identification and pointer authentication on AArch64"),
     cf_protection: CFProtection = (CFProtection::None, parse_cfprotection, [TRACKED],

src/test/codegen/noalias-box-off.rs

@@ -0,0 +1,8 @@
+// compile-flags: -O -Z box-noalias=no
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @box_should_not_have_noalias_if_disabled(
+// CHECK-NOT: noalias
+#[no_mangle]
+pub fn box_should_not_have_noalias_if_disabled(_b: Box<u8>) {}

src/test/codegen/noalias-box.rs

@@ -0,0 +1,8 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @box_should_have_noalias_by_default(
+// CHECK: noalias
+#[no_mangle]
+pub fn box_should_have_noalias_by_default(_b: Box<u8>) {}

src/test/rustdoc-ui/z-help.stdout

@@ -4,6 +4,7 @@
     -Z                            asm-comments=val -- generate comments into the assembly (may change behavior) (default: no)
     -Z                       assert-incr-state=val -- assert that the incremental cache is in given state: either `loaded` or `not-loaded`.
     -Z                      binary-dep-depinfo=val -- include artifacts (sysroot, crate dependencies) used during compilation in dep-info (default: no)
+    -Z                             box-noalias=val -- emit noalias metadata for box (default: yes)
     -Z                       branch-protection=val -- set options for branch target identification and pointer authentication on AArch64
     -Z                           cf-protection=val -- instrument control-flow architecture protection
     -Z               cgu-partitioning-strategy=val -- the codegen unit partitioning strategy to use