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Add flag to configure noalias on Box<T>

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To aid making an informed decision about the aliasing
rules of box, give users an option to remove `noalias`
from box.
This commit is contained in:
nils 2022-07-18 18:17:27 +02:00
parent 9ed0bf9f2b
commit 7c900c9b45
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 {