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

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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
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Dylan DPC 2022-07-20 16:17:20 +05:30 • committed by GitHub
commit fffc6504bc
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6 changed files with 27 additions and 2 deletions
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9
compiler/rustc_middle/src/ty/layout.rs
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@ -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 {