1 files changed, 108 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/check_packed_ref.rs b/compiler/rustc_mir_transform/src/check_packed_ref.rs
new file mode 100644
index 000000000..3b7ba3f9a
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/check_packed_ref.rs
@@ -0,0 +1,108 @@
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
+use rustc_middle::mir::*;
+use rustc_middle::ty::query::Providers;
+use rustc_middle::ty::{self, TyCtxt};
+use rustc_session::lint::builtin::UNALIGNED_REFERENCES;
+
+use crate::util;
+use crate::MirLint;
+
+pub(crate) fn provide(providers: &mut Providers) {
+    *providers = Providers { unsafe_derive_on_repr_packed, ..*providers };
+}
+
+pub struct CheckPackedRef;
+
+impl<'tcx> MirLint<'tcx> for CheckPackedRef {
+    fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
+        let param_env = tcx.param_env(body.source.def_id());
+        let source_info = SourceInfo::outermost(body.span);
+        let mut checker = PackedRefChecker { body, tcx, param_env, source_info };
+        checker.visit_body(&body);
+    }
+}
+
+struct PackedRefChecker<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    source_info: SourceInfo,
+}
+
+fn unsafe_derive_on_repr_packed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
+    let lint_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+
+    tcx.struct_span_lint_hir(UNALIGNED_REFERENCES, lint_hir_id, tcx.def_span(def_id), |lint| {
+        // FIXME: when we make this a hard error, this should have its
+        // own error code.
+        let extra = if tcx.generics_of(def_id).own_requires_monomorphization() {
+            "with type or const parameters"
+        } else {
+            "that does not derive `Copy`"
+        };
+        let message = format!(
+            "`{}` can't be derived on this `#[repr(packed)]` struct {}",
+            tcx.item_name(tcx.trait_id_of_impl(def_id.to_def_id()).expect("derived trait name")),
+            extra
+        );
+        lint.build(message).emit();
+    });
+}
+
+impl<'tcx> Visitor<'tcx> for PackedRefChecker<'_, 'tcx> {
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
+        // Make sure we know where in the MIR we are.
+        self.source_info = terminator.source_info;
+        self.super_terminator(terminator, location);
+    }
+
+    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
+        // Make sure we know where in the MIR we are.
+        self.source_info = statement.source_info;
+        self.super_statement(statement, location);
+    }
+
+    fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
+        if context.is_borrow() {
+            if util::is_disaligned(self.tcx, self.body, self.param_env, *place) {
+                let def_id = self.body.source.instance.def_id();
+                if let Some(impl_def_id) = self
+                    .tcx
+                    .impl_of_method(def_id)
+                    .filter(|&def_id| self.tcx.is_builtin_derive(def_id))
+                {
+                    // If a method is defined in the local crate,
+                    // the impl containing that method should also be.
+                    self.tcx.ensure().unsafe_derive_on_repr_packed(impl_def_id.expect_local());
+                } else {
+                    let source_info = self.source_info;
+                    let lint_root = self.body.source_scopes[source_info.scope]
+                        .local_data
+                        .as_ref()
+                        .assert_crate_local()
+                        .lint_root;
+                    self.tcx.struct_span_lint_hir(
+                        UNALIGNED_REFERENCES,
+                        lint_root,
+                        source_info.span,
+                        |lint| {
+                            lint.build("reference to packed field is unaligned")
+                                .note(
+                                    "fields of packed structs are not properly aligned, and creating \
+                                    a misaligned reference is undefined behavior (even if that \
+                                    reference is never dereferenced)",
+                                )
+                                .help(
+                                    "copy the field contents to a local variable, or replace the \
+                                    reference with a raw pointer and use `read_unaligned`/`write_unaligned` \
+                                    (loads and stores via `*p` must be properly aligned even when using raw pointers)"
+                                )
+                                .emit();
+                        },
+                    );
+                }
+            }
+        }
+    }
+}