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Emit explanatory note for move errors in packed struct derives

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Derive expansions for packed structs cause move errors because
they prefer copying over borrowing since borrowing the fields of a
packed struct can result in unaligned access and therefore undefined
behaviour.

This underlying cause of the errors, however, is not apparent
to the user. We add a diagnostic note here to remedy that.
This commit is contained in:
Gurinder Singh 2023-11-03 07:32:10 +05:30
parent a395214a3a
commit 4b3ece475d
4 changed files with 290 additions and 1 deletions
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20
compiler/rustc_borrowck/src/diagnostics/move_errors.rs
View file

@ -2,7 +2,7 @@ use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed
use rustc_middle::mir::*;
use rustc_middle::ty::{self, Ty};
use rustc_mir_dataflow::move_paths::{LookupResult, MovePathIndex};
use rustc_span::{BytePos, Span};
use rustc_span::{BytePos, ExpnKind, MacroKind, Span};
use crate::diagnostics::CapturedMessageOpt;
use crate::diagnostics::{DescribePlaceOpt, UseSpans};
@ -488,6 +488,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
args_span,
}
});
self.add_note_for_packed_struct_derive(err, original_path.local);
}
}
}
@ -594,4 +596,20 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
);
}
}
/// Adds an explanatory note if the move error occurs in a derive macro
/// expansion of a packed struct.
/// Such errors happen because derive macro expansions shy away from taking
/// references to the struct's fields since doing so would be undefined behaviour
fn add_note_for_packed_struct_derive(&self, err: &mut Diagnostic, local: Local) {
let local_place: PlaceRef<'tcx> = local.into();
let local_ty = local_place.ty(self.body.local_decls(), self.infcx.tcx).ty.peel_refs();
if let Some(adt) = local_ty.ty_adt_def()
&& adt.repr().packed()
&& let ExpnKind::Macro(MacroKind::Derive, name) = self.body.span.ctxt().outer_expn_data().kind
{
err.note(format!("`#[derive({name})]` triggers a move because taking references to the fields of a packed struct is undefined behaviour"));
}
}
}