compiler/rustc_mir/src/transform/check_packed_ref.rs

use rustc_middle::mir::visit::{PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::{self, TyCtxt};
use rustc_session::lint::builtin::UNALIGNED_REFERENCES;

use crate::transform::{MirPass, MirSource};
use crate::util;

pub struct CheckPackedRef;

impl<'tcx> MirPass<'tcx> for CheckPackedRef {
    fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
        let param_env = tcx.param_env(src.instance.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,
}

impl<'a, 'tcx> Visitor<'tcx> for PackedRefChecker<'a, '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 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)",
                            )
                            .emit()
                    },
                );
            }
        }
    }
}