Rollup merge of #99436 - Nilstrieb:toggle-box-noalias, r=fee1-dead

Add flag to configure `noalias` on `Box<T>`

The aliasing rules of `Box<T>` are still not decided, but currently, `Box<T>` is unique and gets `noalias`. To aid making an informed decision about the future of `Box<T>`, this PR adds a flag `-Zbox-noalias` to configure `noalias` for `Box<T>` (for example, for benchmarking). The same flag already exists for `&mut T` `noalias`, where it was added because it was the problem of various miscompilations in LLVM.

For more information, see rust-lang/unsafe-code-guidelines#326

3 files changed, 17 insertions, 0 deletions

diff --git a/src/test/codegen/noalias-box-off.rs b/src/test/codegen/noalias-box-off.rs
new file mode 100644
index 00000000000..afd17c7c160
--- /dev/null
+++ b/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>) {}
diff --git a/src/test/codegen/noalias-box.rs b/src/test/codegen/noalias-box.rs
new file mode 100644
index 00000000000..a3d1f093d8b
--- /dev/null
+++ b/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>) {}
diff --git a/src/test/rustdoc-ui/z-help.stdout b/src/test/rustdoc-ui/z-help.stdout
index 7296b35788a..6f5248f5b18 100644
--- a/src/test/rustdoc-ui/z-help.stdout
+++ b/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