bjoernager/rust
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Auto merge of #83605 - RalfJung:unaligned, r=petrochenkov

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unaligned_references: align(N) fields in packed(N) structs are fine

This removes some false positives from the unaligned_references lint: in a `repr(packed(2))` struct, fields of alignment 2 (and less) are guaranteed to be properly aligned, so we do not have to consider them "disaligned".
This commit is contained in:
bors 2021-03-29 00:17:23 +00:00
commit cc4103089f
7 changed files with 87 additions and 34 deletions
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35
compiler/rustc_mir/src/util/alignment.rs
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@ -1,5 +1,6 @@
use rustc_middle::mir::*;
use rustc_middle::ty::{self, TyCtxt};
use rustc_target::abi::Align;
/// Returns `true` if this place is allowed to be less aligned
/// than its containing struct (because it is within a packed
@ -14,17 +15,25 @@ where
L: HasLocalDecls<'tcx>,
{
debug!("is_disaligned({:?})", place);
if !is_within_packed(tcx, local_decls, place) {
debug!("is_disaligned({:?}) - not within packed", place);
return false;
}
let pack = match is_within_packed(tcx, local_decls, place) {
None => {
debug!("is_disaligned({:?}) - not within packed", place);
return false;
}
Some(pack) => pack,
};
let ty = place.ty(local_decls, tcx).ty;
match tcx.layout_raw(param_env.and(ty)) {
Ok(layout) if layout.align.abi.bytes() == 1 => {
// if the alignment is 1, the type can't be further
// disaligned.
debug!("is_disaligned({:?}) - align = 1", place);
Ok(layout) if layout.align.abi <= pack => {
// If the packed alignment is greater or equal to the field alignment, the type won't be
// further disaligned.
debug!(
"is_disaligned({:?}) - align = {}, packed = {}; not disaligned",
place,
layout.align.abi.bytes(),
pack.bytes()
);
false
}
_ => {
@ -34,7 +43,11 @@ where
}
}
fn is_within_packed<'tcx, L>(tcx: TyCtxt<'tcx>, local_decls: &L, place: Place<'tcx>) -> bool
fn is_within_packed<'tcx, L>(
tcx: TyCtxt<'tcx>,
local_decls: &L,
place: Place<'tcx>,
) -> Option<Align>
where
L: HasLocalDecls<'tcx>,
{
@ -45,7 +58,7 @@ where
ProjectionElem::Field(..) => {
let ty = place_base.ty(local_decls, tcx).ty;
match ty.kind() {
ty::Adt(def, _) if def.repr.packed() => return true,
ty::Adt(def, _) => return def.repr.pack,
_ => {}
}
}
@ -53,5 +66,5 @@ where
}
}
false
None
}