rustc_hir_typeck/

op.rs

//! Code related to processing overloaded binary and unary operators.

use rustc_ast::{self as ast, AssignOp, BinOp};
use rustc_data_structures::packed::Pu128;
use rustc_errors::codes::*;
use rustc_errors::{Applicability, Diag, struct_span_code_err};
use rustc_hir::def_id::DefId;
use rustc_hir::{self as hir, AssignOpKind, BinOpKind, Expr, ExprKind};
use rustc_infer::traits::ObligationCauseCode;
use rustc_middle::bug;
use rustc_middle::ty::adjustment::{
    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
use rustc_session::errors::ExprParenthesesNeeded;
use rustc_span::{Span, Spanned, Symbol, sym};
use rustc_trait_selection::infer::InferCtxtExt;
use rustc_trait_selection::traits::{FulfillmentError, Obligation, ObligationCtxt};
use tracing::debug;

use super::FnCtxt;
use super::method::MethodCallee;
use crate::method::TreatNotYetDefinedOpaques;
use crate::{Expectation, errors};

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    /// Checks a `a <op>= b`
    pub(crate) fn check_expr_assign_op(
        &self,
        expr: &'tcx Expr<'tcx>,
        op: hir::AssignOp,
        lhs: &'tcx Expr<'tcx>,
        rhs: &'tcx Expr<'tcx>,
        expected: Expectation<'tcx>,
    ) -> Ty<'tcx> {
        let (lhs_ty, rhs_ty, return_ty) =
            self.check_overloaded_binop(expr, lhs, rhs, Op::AssignOp(op), expected);

        let category = BinOpCategory::from(op.node);
        let ty = if !lhs_ty.is_ty_var()
            && !rhs_ty.is_ty_var()
            && is_builtin_binop(lhs_ty, rhs_ty, category)
        {
            self.enforce_builtin_binop_types(lhs.span, lhs_ty, rhs.span, rhs_ty, category);
            self.tcx.types.unit
        } else {
            return_ty
        };

        self.check_lhs_assignable(lhs, E0067, op.span, |err| {
            if let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty) {
                if self
                    .lookup_op_method(
                        (lhs, lhs_deref_ty),
                        Some((rhs, rhs_ty)),
                        lang_item_for_binop(self.tcx, Op::AssignOp(op)),
                        op.span,
                        expected,
                    )
                    .is_ok()
                {
                    // If LHS += RHS is an error, but *LHS += RHS is successful, then we will have
                    // emitted a better suggestion during error handling in check_overloaded_binop.
                    if self
                        .lookup_op_method(
                            (lhs, lhs_ty),
                            Some((rhs, rhs_ty)),
                            lang_item_for_binop(self.tcx, Op::AssignOp(op)),
                            op.span,
                            expected,
                        )
                        .is_err()
                    {
                        err.downgrade_to_delayed_bug();
                    } else {
                        // Otherwise, it's valid to suggest dereferencing the LHS here.
                        err.span_suggestion_verbose(
                            lhs.span.shrink_to_lo(),
                            "consider dereferencing the left-hand side of this operation",
                            "*",
                            Applicability::MaybeIncorrect,
                        );
                    }
                }
            }
        });

        ty
    }

    /// Checks a potentially overloaded binary operator.
    pub(crate) fn check_expr_binop(
        &self,
        expr: &'tcx Expr<'tcx>,
        op: hir::BinOp,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        expected: Expectation<'tcx>,
    ) -> Ty<'tcx> {
        let tcx = self.tcx;

        debug!(
            "check_binop(expr.hir_id={}, expr={:?}, op={:?}, lhs_expr={:?}, rhs_expr={:?})",
            expr.hir_id, expr, op, lhs_expr, rhs_expr
        );

        match BinOpCategory::from(op.node) {
            BinOpCategory::Shortcircuit => {
                // && and || are a simple case.
                self.check_expr_coercible_to_type(lhs_expr, tcx.types.bool, None);
                let lhs_diverges = self.diverges.get();
                self.check_expr_coercible_to_type(rhs_expr, tcx.types.bool, None);

                // Depending on the LHS' value, the RHS can never execute.
                self.diverges.set(lhs_diverges);

                tcx.types.bool
            }
            _ => {
                // Otherwise, we always treat operators as if they are
                // overloaded. This is the way to be most flexible w/r/t
                // types that get inferred.
                let (lhs_ty, rhs_ty, return_ty) =
                    self.check_overloaded_binop(expr, lhs_expr, rhs_expr, Op::BinOp(op), expected);

                // Supply type inference hints if relevant. Probably these
                // hints should be enforced during select as part of the
                // `consider_unification_despite_ambiguity` routine, but this
                // more convenient for now.
                //
                // The basic idea is to help type inference by taking
                // advantage of things we know about how the impls for
                // scalar types are arranged. This is important in a
                // scenario like `1_u32 << 2`, because it lets us quickly
                // deduce that the result type should be `u32`, even
                // though we don't know yet what type 2 has and hence
                // can't pin this down to a specific impl.
                let category = BinOpCategory::from(op.node);
                if !lhs_ty.is_ty_var()
                    && !rhs_ty.is_ty_var()
                    && is_builtin_binop(lhs_ty, rhs_ty, category)
                {
                    let builtin_return_ty = self.enforce_builtin_binop_types(
                        lhs_expr.span,
                        lhs_ty,
                        rhs_expr.span,
                        rhs_ty,
                        category,
                    );
                    self.demand_eqtype(expr.span, builtin_return_ty, return_ty);
                    builtin_return_ty
                } else {
                    return_ty
                }
            }
        }
    }

    fn enforce_builtin_binop_types(
        &self,
        lhs_span: Span,
        lhs_ty: Ty<'tcx>,
        rhs_span: Span,
        rhs_ty: Ty<'tcx>,
        category: BinOpCategory,
    ) -> Ty<'tcx> {
        debug_assert!(is_builtin_binop(lhs_ty, rhs_ty, category));

        // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
        // (See https://github.com/rust-lang/rust/issues/57447.)
        let (lhs_ty, rhs_ty) = (deref_ty_if_possible(lhs_ty), deref_ty_if_possible(rhs_ty));

        let tcx = self.tcx;
        match category {
            BinOpCategory::Shortcircuit => {
                self.demand_suptype(lhs_span, tcx.types.bool, lhs_ty);
                self.demand_suptype(rhs_span, tcx.types.bool, rhs_ty);
                tcx.types.bool
            }

            // result type is same as LHS always
            BinOpCategory::Shift => lhs_ty,

            BinOpCategory::Math | BinOpCategory::Bitwise => {
                // both LHS and RHS and result will have the same type
                self.demand_suptype(rhs_span, lhs_ty, rhs_ty);
                lhs_ty
            }

            BinOpCategory::Comparison => {
                // both LHS and RHS and result will have the same type
                self.demand_suptype(rhs_span, lhs_ty, rhs_ty);
                tcx.types.bool
            }
        }
    }

    fn check_overloaded_binop(
        &self,
        expr: &'tcx Expr<'tcx>,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
    ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) {
        debug!("check_overloaded_binop(expr.hir_id={}, op={:?})", expr.hir_id, op);

        let lhs_ty = match op {
            Op::BinOp(_) => {
                // Find a suitable supertype of the LHS expression's type, by coercing to
                // a type variable, to pass as the `Self` to the trait, avoiding invariant
                // trait matching creating lifetime constraints that are too strict.
                // e.g., adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
                // in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
                let lhs_ty = self.check_expr(lhs_expr);
                let fresh_var = self.next_ty_var(lhs_expr.span);
                self.demand_coerce(lhs_expr, lhs_ty, fresh_var, Some(rhs_expr), AllowTwoPhase::No)
            }
            Op::AssignOp(_) => {
                // rust-lang/rust#52126: We have to use strict
                // equivalence on the LHS of an assign-op like `+=`;
                // overwritten or mutably-borrowed places cannot be
                // coerced to a supertype.
                self.check_expr(lhs_expr)
            }
        };
        let lhs_ty = self.resolve_vars_with_obligations(lhs_ty);

        // N.B., as we have not yet type-checked the RHS, we don't have the
        // type at hand. Make a variable to represent it. The whole reason
        // for this indirection is so that, below, we can check the expr
        // using this variable as the expected type, which sometimes lets
        // us do better coercions than we would be able to do otherwise,
        // particularly for things like `String + &String`.
        let rhs_ty_var = self.next_ty_var(rhs_expr.span);
        let result = self.lookup_op_method(
            (lhs_expr, lhs_ty),
            Some((rhs_expr, rhs_ty_var)),
            lang_item_for_binop(self.tcx, op),
            op.span(),
            expected,
        );

        // see `NB` above
        let rhs_ty = self.check_expr_coercible_to_type_or_error(
            rhs_expr,
            rhs_ty_var,
            Some(lhs_expr),
            |err, ty| {
                self.err_ctxt().note_field_shadowed_by_private_candidate(
                    err,
                    rhs_expr.hir_id,
                    self.param_env,
                );
                if let Op::BinOp(binop) = op
                    && binop.node == hir::BinOpKind::Eq
                {
                    self.suggest_swapping_lhs_and_rhs(err, ty, lhs_ty, rhs_expr, lhs_expr);
                }
            },
        );
        let rhs_ty = self.resolve_vars_with_obligations(rhs_ty);

        let return_ty = self.overloaded_binop_ret_ty(
            expr, lhs_expr, rhs_expr, op, expected, lhs_ty, result, rhs_ty,
        );

        (lhs_ty, rhs_ty, return_ty)
    }

    fn overloaded_binop_ret_ty(
        &self,
        expr: &'tcx Expr<'tcx>,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
        lhs_ty: Ty<'tcx>,
        result: Result<MethodCallee<'tcx>, Vec<FulfillmentError<'tcx>>>,
        rhs_ty: Ty<'tcx>,
    ) -> Ty<'tcx> {
        match result {
            Ok(method) => {
                let by_ref_binop = !op.is_by_value();

                if matches!(op, Op::AssignOp(_)) || by_ref_binop {
                    if let ty::Ref(_, _, mutbl) = method.sig.inputs()[0].kind() {
                        let mutbl = AutoBorrowMutability::new(*mutbl, AllowTwoPhase::Yes);
                        let autoref = Adjustment {
                            kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
                            target: method.sig.inputs()[0],
                        };
                        self.apply_adjustments(lhs_expr, vec![autoref]);
                    }

                    if let ty::Ref(_, _, mutbl) = method.sig.inputs()[1].kind() {
                        // Allow two-phase borrows for binops in initial deployment
                        // since they desugar to methods
                        let mutbl = AutoBorrowMutability::new(*mutbl, AllowTwoPhase::Yes);
                        let autoref = Adjustment {
                            kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
                            target: method.sig.inputs()[1],
                        };
                        // HACK(eddyb) Bypass checks due to reborrows being in
                        // some cases applied on the RHS, on top of which we need
                        // to autoref, which is not allowed by apply_adjustments.
                        // self.apply_adjustments(rhs_expr, vec![autoref]);
                        self.typeck_results
                            .borrow_mut()
                            .adjustments_mut()
                            .entry(rhs_expr.hir_id)
                            .or_default()
                            .push(autoref);
                    }
                }

                self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
                method.sig.output()
            }
            // error types are considered "builtin"
            Err(_) if lhs_ty.references_error() || rhs_ty.references_error() => {
                Ty::new_misc_error(self.tcx)
            }
            Err(errors) => self.report_binop_fulfillment_errors(
                expr, lhs_expr, rhs_expr, op, expected, lhs_ty, rhs_ty, errors,
            ),
        }
    }

    fn report_binop_fulfillment_errors(
        &self,
        expr: &'tcx Expr<'tcx>,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
        lhs_ty: Ty<'tcx>,
        rhs_ty: Ty<'tcx>,
        errors: Vec<FulfillmentError<'tcx>>,
    ) -> Ty<'tcx> {
        let (_, trait_def_id) = lang_item_for_binop(self.tcx, op);

        let mut path = None;
        let lhs_ty_str = self.tcx.short_string(lhs_ty, &mut path);
        let rhs_ty_str = self.tcx.short_string(rhs_ty, &mut path);

        let (mut err, output_def_id) = match op {
            // Try and detect when `+=` was incorrectly
            // used instead of `==` in a let-chain
            Op::AssignOp(assign_op) => {
                if let Err(e) =
                    errors::maybe_emit_plus_equals_diagnostic(&self, assign_op, lhs_expr)
                {
                    (e, None)
                } else {
                    let s = assign_op.node.as_str();
                    let mut err = struct_span_code_err!(
                        self.dcx(),
                        expr.span,
                        E0368,
                        "binary assignment operation `{}` cannot be applied to type `{}`",
                        s,
                        lhs_ty_str,
                    );
                    err.span_label(
                        lhs_expr.span,
                        format!("cannot use `{}` on type `{}`", s, lhs_ty_str),
                    );
                    let err_ctxt = self.err_ctxt();
                    err_ctxt.note_field_shadowed_by_private_candidate(
                        &mut err,
                        lhs_expr.hir_id,
                        self.param_env,
                    );
                    err_ctxt.note_field_shadowed_by_private_candidate(
                        &mut err,
                        rhs_expr.hir_id,
                        self.param_env,
                    );
                    self.note_unmet_impls_on_type(&mut err, &errors, false);
                    (err, None)
                }
            }
            Op::BinOp(bin_op) => {
                use hir::BinOpKind;
                let message = match bin_op.node {
                    BinOpKind::Add => {
                        format!("cannot add `{rhs_ty_str}` to `{lhs_ty_str}`")
                    }
                    BinOpKind::Sub => {
                        format!("cannot subtract `{rhs_ty_str}` from `{lhs_ty_str}`")
                    }
                    BinOpKind::Mul => {
                        format!("cannot multiply `{lhs_ty_str}` by `{rhs_ty_str}`")
                    }
                    BinOpKind::Div => {
                        format!("cannot divide `{lhs_ty_str}` by `{rhs_ty_str}`")
                    }
                    BinOpKind::Rem => format!(
                        "cannot calculate the remainder of `{lhs_ty_str}` divided by `{rhs_ty_str}`"
                    ),
                    BinOpKind::BitAnd
                    | BinOpKind::BitXor
                    | BinOpKind::BitOr
                    | BinOpKind::Shl
                    | BinOpKind::Shr => format!(
                        "no implementation for `{lhs_ty_str} {} {rhs_ty_str}`",
                        bin_op.node.as_str()
                    ),
                    _ => format!(
                        "binary operation `{}` cannot be applied to type `{lhs_ty_str}`",
                        bin_op.node.as_str()
                    ),
                };

                let output_def_id = trait_def_id.and_then(|def_id| {
                    self.tcx
                        .associated_item_def_ids(def_id)
                        .iter()
                        .find(|&&item_def_id| {
                            self.tcx.associated_item(item_def_id).name() == sym::Output
                        })
                        .cloned()
                });
                let mut err = struct_span_code_err!(self.dcx(), bin_op.span, E0369, "{message}");
                if !lhs_expr.span.eq(&rhs_expr.span) {
                    err.span_label(lhs_expr.span, lhs_ty_str.clone());
                    err.span_label(rhs_expr.span, rhs_ty_str);
                }
                let err_ctxt = self.err_ctxt();
                err_ctxt.note_field_shadowed_by_private_candidate(
                    &mut err,
                    lhs_expr.hir_id,
                    self.param_env,
                );
                err_ctxt.note_field_shadowed_by_private_candidate(
                    &mut err,
                    rhs_expr.hir_id,
                    self.param_env,
                );
                let suggest_derive = self.can_eq(self.param_env, lhs_ty, rhs_ty);
                self.note_unmet_impls_on_type(&mut err, &errors, suggest_derive);
                (err, output_def_id)
            }
        };
        *err.long_ty_path() = path;

        // Try to suggest a semicolon if it's `A \n *B` where `B` is a place expr
        let maybe_missing_semi = self.check_for_missing_semi(expr, &mut err);

        // We defer to the later error produced by `check_lhs_assignable`.
        // We only downgrade this if it's the LHS, though, and if this is a
        // valid assignment statement.
        if maybe_missing_semi && self.is_lhs_of_assign_stmt(expr) {
            err.downgrade_to_delayed_bug();
        }

        let is_compatible_after_call = |lhs_ty, rhs_ty| {
            let op_ok = self
                .lookup_op_method(
                    (lhs_expr, lhs_ty),
                    Some((rhs_expr, rhs_ty)),
                    lang_item_for_binop(self.tcx, op),
                    op.span(),
                    expected,
                )
                .is_ok();

            op_ok || self.can_eq(self.param_env, lhs_ty, rhs_ty)
        };

        // We should suggest `a + b` => `*a + b` if `a` is copy, and suggest
        // `a += b` => `*a += b` if a is a mut ref.
        self.suggest_deref_or_call_for_binop_error(
            lhs_expr,
            rhs_expr,
            op,
            expected,
            lhs_ty,
            rhs_ty,
            &mut err,
            is_compatible_after_call,
        );

        if let Some(missing_trait) =
            trait_def_id.map(|def_id| with_no_trimmed_paths!(self.tcx.def_path_str(def_id)))
        {
            if matches!(
                op,
                Op::BinOp(BinOp { node: BinOpKind::Add, .. })
                    | Op::AssignOp(AssignOp { node: AssignOpKind::AddAssign, .. })
            ) && self.check_str_addition(lhs_expr, rhs_expr, lhs_ty, rhs_ty, &mut err, op)
            {
                // This has nothing here because it means we did string
                // concatenation (e.g., "Hello " + "World!"). This means
                // we don't want the note in the else clause to be emitted
            } else if lhs_ty.has_non_region_param() {
                if !errors.is_empty() {
                    for error in errors {
                        if let Some(trait_pred) = error.obligation.predicate.as_trait_clause() {
                            let output_associated_item = if let ObligationCauseCode::BinOp {
                                output_ty: Some(output_ty),
                                ..
                            } = error.obligation.cause.code()
                            {
                                output_def_id
                                    .zip(trait_def_id)
                                    .filter(|(output_def_id, trait_def_id)| {
                                        self.tcx.parent(*output_def_id) == *trait_def_id
                                    })
                                    .and_then(|_| output_ty.make_suggestable(self.tcx, false, None))
                                    .map(|output_ty| ("Output", output_ty))
                            } else {
                                None
                            };

                            self.err_ctxt().suggest_restricting_param_bound(
                                &mut err,
                                trait_pred,
                                output_associated_item,
                                self.body_id,
                            );
                        }
                    }
                } else {
                    // When we know that a missing bound is responsible, we don't show
                    // this note as it is redundant.
                    err.note(format!(
                        "the trait `{missing_trait}` is not implemented for `{lhs_ty_str}`"
                    ));
                }
            }
        }

        // Suggest using `add`, `offset` or `offset_from` for pointer - {integer},
        // pointer + {integer} or pointer - pointer.
        self.suggest_raw_ptr_binop_arithmetic(lhs_expr, rhs_expr, op, lhs_ty, rhs_ty, &mut err);

        let lhs_name_str = match lhs_expr.kind {
            ExprKind::Path(hir::QPath::Resolved(_, path)) => {
                path.segments.last().map_or("_".to_string(), |s| s.ident.to_string())
            }
            _ => self
                .tcx
                .sess
                .source_map()
                .span_to_snippet(lhs_expr.span)
                .unwrap_or_else(|_| "_".to_string()),
        };

        self.suggest_raw_ptr_assign_arithmetic(
            lhs_expr,
            rhs_expr,
            op,
            lhs_ty,
            rhs_ty,
            &lhs_name_str,
            &mut err,
        );

        Ty::new_error(self.tcx, err.emit())
    }

    fn suggest_deref_or_call_for_binop_error(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
        lhs_ty: Ty<'tcx>,
        rhs_ty: Ty<'tcx>,
        err: &mut Diag<'_>,
        is_compatible_after_call: impl Fn(Ty<'tcx>, Ty<'tcx>) -> bool,
    ) {
        // Suppress suggestions when lhs and rhs are not in the same span as the error
        if !op.span().can_be_used_for_suggestions() {
            return;
        }

        if let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty)
            && matches!(op, Op::AssignOp(_))
        {
            self.suggest_deref_binop(lhs_expr, rhs_expr, op, expected, rhs_ty, err, lhs_deref_ty);
        } else if let ty::Ref(region, lhs_deref_ty, mutbl) = lhs_ty.kind()
            && matches!(op, Op::BinOp(_))
        {
            if self.type_is_copy_modulo_regions(self.param_env, *lhs_deref_ty) {
                self.suggest_deref_binop(
                    lhs_expr,
                    rhs_expr,
                    op,
                    expected,
                    rhs_ty,
                    err,
                    *lhs_deref_ty,
                );
            } else {
                let lhs_inv_mutbl = mutbl.invert();
                let lhs_inv_mutbl_ty = Ty::new_ref(self.tcx, *region, *lhs_deref_ty, lhs_inv_mutbl);

                self.suggest_different_borrow(
                    lhs_expr,
                    rhs_expr,
                    op,
                    expected,
                    err,
                    lhs_inv_mutbl_ty,
                    Some(lhs_inv_mutbl),
                    rhs_ty,
                    None,
                );

                if let ty::Ref(region, rhs_deref_ty, mutbl) = rhs_ty.kind() {
                    let rhs_inv_mutbl = mutbl.invert();
                    let rhs_inv_mutbl_ty =
                        Ty::new_ref(self.tcx, *region, *rhs_deref_ty, rhs_inv_mutbl);

                    self.suggest_different_borrow(
                        lhs_expr,
                        rhs_expr,
                        op,
                        expected,
                        err,
                        lhs_ty,
                        None,
                        rhs_inv_mutbl_ty,
                        Some(rhs_inv_mutbl),
                    );
                    self.suggest_different_borrow(
                        lhs_expr,
                        rhs_expr,
                        op,
                        expected,
                        err,
                        lhs_inv_mutbl_ty,
                        Some(lhs_inv_mutbl),
                        rhs_inv_mutbl_ty,
                        Some(rhs_inv_mutbl),
                    );
                }
            }
        } else {
            let suggested = self.suggest_fn_call(err, lhs_expr, lhs_ty, |lhs_ty| {
                is_compatible_after_call(lhs_ty, rhs_ty)
            }) || self.suggest_fn_call(err, rhs_expr, rhs_ty, |rhs_ty| {
                is_compatible_after_call(lhs_ty, rhs_ty)
            });

            if !suggested {
                self.suggest_two_fn_call(
                    err,
                    rhs_expr,
                    rhs_ty,
                    lhs_expr,
                    lhs_ty,
                    is_compatible_after_call,
                );
            }
        }
    }

    fn suggest_raw_ptr_binop_arithmetic(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        lhs_ty: Ty<'tcx>,
        rhs_ty: Ty<'tcx>,
        err: &mut Diag<'_>,
    ) {
        if !op.span().can_be_used_for_suggestions() {
            return;
        }

        match op {
            Op::BinOp(BinOp { node: BinOpKind::Add, .. })
                if lhs_ty.is_raw_ptr() && rhs_ty.is_integral() =>
            {
                err.multipart_suggestion(
                    "consider using `wrapping_add` or `add` for pointer + {integer}",
                    vec![
                        (lhs_expr.span.between(rhs_expr.span), ".wrapping_add(".to_owned()),
                        (rhs_expr.span.shrink_to_hi(), ")".to_owned()),
                    ],
                    Applicability::MaybeIncorrect,
                );
            }
            Op::BinOp(BinOp { node: BinOpKind::Sub, .. }) => {
                if lhs_ty.is_raw_ptr() && rhs_ty.is_integral() {
                    err.multipart_suggestion(
                        "consider using `wrapping_sub` or `sub` for pointer - {integer}",
                        vec![
                            (lhs_expr.span.between(rhs_expr.span), ".wrapping_sub(".to_owned()),
                            (rhs_expr.span.shrink_to_hi(), ")".to_owned()),
                        ],
                        Applicability::MaybeIncorrect,
                    );
                }
                if lhs_ty.is_raw_ptr() && rhs_ty.is_raw_ptr() {
                    err.multipart_suggestion(
                        "consider using `offset_from` for pointer - pointer if the \
                     pointers point to the same allocation",
                        vec![
                            (lhs_expr.span.shrink_to_lo(), "unsafe { ".to_owned()),
                            (lhs_expr.span.between(rhs_expr.span), ".offset_from(".to_owned()),
                            (rhs_expr.span.shrink_to_hi(), ") }".to_owned()),
                        ],
                        Applicability::MaybeIncorrect,
                    );
                }
            }
            _ => {}
        }
    }

    fn suggest_raw_ptr_assign_arithmetic(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        lhs_ty: Ty<'tcx>,
        rhs_ty: Ty<'tcx>,
        lhs_name_str: &str,
        err: &mut Diag<'_>,
    ) {
        if !op.span().can_be_used_for_suggestions()
            || !matches!(op, Op::AssignOp(_))
            || !lhs_ty.is_raw_ptr()
            || !rhs_ty.is_integral()
        {
            return;
        }

        let (msg, method) = match op {
            Op::AssignOp(AssignOp { node: AssignOpKind::AddAssign, .. }) => {
                ("consider using `add` or `wrapping_add` to do pointer arithmetic", "wrapping_add")
            }
            Op::AssignOp(AssignOp { node: AssignOpKind::SubAssign, .. }) => {
                ("consider using `sub` or `wrapping_sub` to do pointer arithmetic", "wrapping_sub")
            }
            _ => return,
        };

        err.multipart_suggestion(
            msg,
            vec![
                (lhs_expr.span.shrink_to_lo(), format!("{} = ", lhs_name_str)),
                (lhs_expr.span.between(rhs_expr.span), format!(".{method}(")),
                (rhs_expr.span.shrink_to_hi(), ")".to_owned()),
            ],
            Applicability::MaybeIncorrect,
        );
    }

    fn suggest_different_borrow(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
        err: &mut Diag<'_>,
        lhs_adjusted_ty: Ty<'tcx>,
        lhs_new_mutbl: Option<ty::Mutability>,
        rhs_adjusted_ty: Ty<'tcx>,
        rhs_new_mutbl: Option<ty::Mutability>,
    ) {
        if self
            .lookup_op_method(
                (lhs_expr, lhs_adjusted_ty),
                Some((rhs_expr, rhs_adjusted_ty)),
                lang_item_for_binop(self.tcx, op),
                op.span(),
                expected,
            )
            .is_ok()
        {
            let lhs = self.tcx.short_string(lhs_adjusted_ty, err.long_ty_path());
            let rhs = self.tcx.short_string(rhs_adjusted_ty, err.long_ty_path());
            let op = op.as_str();
            err.note(format!("an implementation for `{lhs} {op} {rhs}` exists"));

            if lhs_new_mutbl.is_some_and(|lhs_mutbl| lhs_mutbl.is_not())
                && rhs_new_mutbl.is_some_and(|rhs_mutbl| rhs_mutbl.is_not())
            {
                err.multipart_suggestion(
                    "consider reborrowing both sides",
                    vec![
                        (lhs_expr.span.shrink_to_lo(), "&*".to_string()),
                        (rhs_expr.span.shrink_to_lo(), "&*".to_string()),
                    ],
                    rustc_errors::Applicability::MachineApplicable,
                );
            } else {
                let mut suggest_new_borrow = |new_mutbl: ast::Mutability, sp: Span| {
                    // Can reborrow (&mut -> &)
                    if new_mutbl.is_not() {
                        err.span_suggestion_verbose(
                            sp.shrink_to_lo(),
                            "consider reborrowing this side",
                            "&*",
                            rustc_errors::Applicability::MachineApplicable,
                        );
                    // Works on &mut but have &
                    } else {
                        err.span_help(sp, "consider making this expression a mutable borrow");
                    }
                };

                if let Some(lhs_new_mutbl) = lhs_new_mutbl {
                    suggest_new_borrow(lhs_new_mutbl, lhs_expr.span);
                }
                if let Some(rhs_new_mutbl) = rhs_new_mutbl {
                    suggest_new_borrow(rhs_new_mutbl, rhs_expr.span);
                }
            }
        }
    }

    fn is_lhs_of_assign_stmt(&self, expr: &Expr<'_>) -> bool {
        let hir::Node::Expr(parent) = self.tcx.parent_hir_node(expr.hir_id) else { return false };
        let ExprKind::Assign(lhs, _, _) = parent.kind else { return false };
        let hir::Node::Stmt(stmt) = self.tcx.parent_hir_node(parent.hir_id) else { return false };
        matches!(stmt.kind, hir::StmtKind::Expr(_) | hir::StmtKind::Semi(_))
            && lhs.hir_id == expr.hir_id
    }

    fn suggest_deref_binop(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        op: Op,
        expected: Expectation<'tcx>,
        rhs_ty: Ty<'tcx>,
        err: &mut Diag<'_>,
        lhs_deref_ty: Ty<'tcx>,
    ) {
        if self
            .lookup_op_method(
                (lhs_expr, lhs_deref_ty),
                Some((rhs_expr, rhs_ty)),
                lang_item_for_binop(self.tcx, op),
                op.span(),
                expected,
            )
            .is_ok()
        {
            let msg = format!(
                "`{}` can be used on `{}` if you dereference the left-hand side",
                op.as_str(),
                self.tcx.short_string(lhs_deref_ty, err.long_ty_path()),
            );
            err.span_suggestion_verbose(
                lhs_expr.span.shrink_to_lo(),
                msg,
                "*",
                rustc_errors::Applicability::MachineApplicable,
            );
        }
    }

    /// Provide actionable suggestions when trying to add two strings with incorrect types,
    /// like `&str + &str`, `String + String` and `&str + &String`.
    ///
    /// If this function returns `true` it means a note was printed, so we don't need
    /// to print the normal "implementation of `std::ops::Add` might be missing" note
    fn check_str_addition(
        &self,
        lhs_expr: &'tcx Expr<'tcx>,
        rhs_expr: &'tcx Expr<'tcx>,
        lhs_ty: Ty<'tcx>,
        rhs_ty: Ty<'tcx>,
        err: &mut Diag<'_>,
        op: Op,
    ) -> bool {
        let str_concat_note = "string concatenation requires an owned `String` on the left";
        let rm_borrow_msg = "remove the borrow to obtain an owned `String`";
        let to_owned_msg = "create an owned `String` from a string reference";

        let string_type = self.tcx.lang_items().string();
        let is_std_string =
            |ty: Ty<'tcx>| ty.ty_adt_def().is_some_and(|def| Some(def.did()) == string_type);
        let is_str_like = |ty: Ty<'tcx>| *ty.kind() == ty::Str || is_std_string(ty);

        // Returns (suggestion_span, Some(replacement)) or (span, None) if lhs is a borrow to remove.
        let lhs_owned_sugg = |lhs_expr: &Expr<'_>| {
            if let ExprKind::AddrOf(_, _, inner) = lhs_expr.kind {
                (lhs_expr.span.until(inner.span), None)
            } else {
                (lhs_expr.span.shrink_to_hi(), Some(".to_owned()".to_owned()))
            }
        };

        let (&ty::Ref(_, l_ty, _), rhs_kind) = (lhs_ty.kind(), rhs_ty.kind()) else {
            return false;
        };
        if !is_str_like(l_ty) {
            return false;
        }

        match rhs_kind {
            // &str or &String + &str, &String, or &&str
            &ty::Ref(_, r_ty, _)
                if is_str_like(r_ty)
                    || matches!(r_ty.kind(), ty::Ref(_, inner, _) if *inner.kind() == ty::Str) =>
            {
                // Do not supply this message if `&str += &str`
                if let Op::BinOp(_) = op {
                    err.span_label(
                        op.span(),
                        "`+` cannot be used to concatenate two `&str` strings",
                    );
                    err.note(str_concat_note);
                    let (span, replacement) = lhs_owned_sugg(lhs_expr);
                    let (msg, replacement) = match replacement {
                        None => (rm_borrow_msg, "".to_owned()),
                        Some(r) => (to_owned_msg, r),
                    };
                    err.span_suggestion_verbose(
                        span,
                        msg,
                        replacement,
                        Applicability::MachineApplicable,
                    );
                }
                true
            }
            // &str or &String + String
            ty::Adt(..) if is_std_string(rhs_ty) => {
                err.span_label(
                    op.span(),
                    "`+` cannot be used to concatenate a `&str` with a `String`",
                );
                if matches!(op, Op::BinOp(_)) {
                    let (lhs_span, lhs_replacement) = lhs_owned_sugg(lhs_expr);
                    let (sugg_msg, lhs_replacement) = match lhs_replacement {
                        None => (
                            "remove the borrow on the left and add one on the right",
                            "".to_owned(),
                        ),
                        Some(r) => (
                            "create an owned `String` on the left and add a borrow on the right",
                            r,
                        ),
                    };
                    err.multipart_suggestion(
                        sugg_msg,
                        vec![
                            (lhs_span, lhs_replacement),
                            (rhs_expr.span.shrink_to_lo(), "&".to_owned()),
                        ],
                        Applicability::MachineApplicable,
                    );
                } else if matches!(op, Op::AssignOp(_)) {
                    err.note(str_concat_note);
                }
                true
            }
            _ => false,
        }
    }

    pub(crate) fn check_user_unop(
        &self,
        ex: &'tcx Expr<'tcx>,
        operand_ty: Ty<'tcx>,
        op: hir::UnOp,
        expected: Expectation<'tcx>,
    ) -> Ty<'tcx> {
        assert!(op.is_by_value());
        match self.lookup_op_method(
            (ex, operand_ty),
            None,
            lang_item_for_unop(self.tcx, op),
            ex.span,
            expected,
        ) {
            Ok(method) => {
                self.write_method_call_and_enforce_effects(ex.hir_id, ex.span, method);
                method.sig.output()
            }
            Err(errors) => {
                let actual = self.resolve_vars_if_possible(operand_ty);
                let guar = actual.error_reported().err().unwrap_or_else(|| {
                    let mut file = None;
                    let ty_str = self.tcx.short_string(actual, &mut file);
                    let mut err = struct_span_code_err!(
                        self.dcx(),
                        ex.span,
                        E0600,
                        "cannot apply unary operator `{}` to type `{ty_str}`",
                        op.as_str(),
                    );
                    *err.long_ty_path() = file;
                    err.span_label(
                        ex.span,
                        format!("cannot apply unary operator `{}`", op.as_str()),
                    );

                    if operand_ty.has_non_region_param() {
                        let predicates = errors
                            .iter()
                            .filter_map(|error| error.obligation.predicate.as_trait_clause());
                        for pred in predicates {
                            self.err_ctxt().suggest_restricting_param_bound(
                                &mut err,
                                pred,
                                None,
                                self.body_id,
                            );
                        }
                    }

                    let sp = self.tcx.sess.source_map().start_point(ex.span).with_parent(None);
                    if let Some(sp) =
                        self.tcx.sess.psess.ambiguous_block_expr_parse.borrow().get(&sp)
                    {
                        // If the previous expression was a block expression, suggest parentheses
                        // (turning this into a binary subtraction operation instead.)
                        // for example, `{2} - 2` -> `({2}) - 2` (see src\test\ui\parser\expr-as-stmt.rs)
                        err.subdiagnostic(ExprParenthesesNeeded::surrounding(*sp));
                    } else {
                        match actual.kind() {
                            ty::Uint(_) if op == hir::UnOp::Neg => {
                                err.note("unsigned values cannot be negated");

                                if let ExprKind::Unary(
                                    _,
                                    Expr {
                                        kind:
                                            ExprKind::Lit(Spanned {
                                                node: ast::LitKind::Int(Pu128(1), _),
                                                ..
                                            }),
                                        ..
                                    },
                                ) = ex.kind
                                {
                                    let span = if let hir::Node::Expr(parent) =
                                        self.tcx.parent_hir_node(ex.hir_id)
                                        && let ExprKind::Cast(..) = parent.kind
                                    {
                                        // `-1 as usize` -> `usize::MAX`
                                        parent.span
                                    } else {
                                        ex.span
                                    };
                                    err.span_suggestion_verbose(
                                        span,
                                        format!(
                                            "you may have meant the maximum value of `{actual}`",
                                        ),
                                        format!("{actual}::MAX"),
                                        Applicability::MaybeIncorrect,
                                    );
                                }
                            }
                            ty::Str | ty::Never | ty::Char | ty::Tuple(_) | ty::Array(_, _) => {}
                            ty::Ref(_, lty, _) if *lty.kind() == ty::Str => {}
                            _ => {
                                self.note_unmet_impls_on_type(&mut err, &errors, true);
                            }
                        }
                    }
                    err.emit()
                });
                Ty::new_error(self.tcx, guar)
            }
        }
    }

    fn lookup_op_method(
        &self,
        (lhs_expr, lhs_ty): (&'tcx Expr<'tcx>, Ty<'tcx>),
        opt_rhs: Option<(&'tcx Expr<'tcx>, Ty<'tcx>)>,
        (opname, trait_did): (Symbol, Option<hir::def_id::DefId>),
        span: Span,
        expected: Expectation<'tcx>,
    ) -> Result<MethodCallee<'tcx>, Vec<FulfillmentError<'tcx>>> {
        let Some(trait_did) = trait_did else {
            // Bail if the operator trait is not defined.
            return Err(vec![]);
        };

        debug!(
            "lookup_op_method(lhs_ty={:?}, opname={:?}, trait_did={:?})",
            lhs_ty, opname, trait_did
        );

        let (opt_rhs_expr, opt_rhs_ty) = opt_rhs.unzip();
        let cause = self.cause(
            span,
            match opt_rhs_expr {
                Some(rhs) => ObligationCauseCode::BinOp {
                    lhs_hir_id: lhs_expr.hir_id,
                    rhs_hir_id: rhs.hir_id,
                    rhs_span: rhs.span,
                    rhs_is_lit: matches!(rhs.kind, ExprKind::Lit(_)),
                    output_ty: expected.only_has_type(self),
                },
                None => ObligationCauseCode::UnOp { hir_id: lhs_expr.hir_id },
            },
        );

        // We don't consider any other candidates if this lookup fails
        // so we can freely treat opaque types as inference variables here
        // to allow more code to compile.
        let treat_opaques = TreatNotYetDefinedOpaques::AsInfer;
        let method = self.lookup_method_for_operator(
            cause.clone(),
            opname,
            trait_did,
            lhs_ty,
            opt_rhs_ty,
            treat_opaques,
        );
        match method {
            Some(ok) => {
                let method = self.register_infer_ok_obligations(ok);
                self.select_obligations_where_possible(|_| {});
                Ok(method)
            }
            None => {
                // This path may do some inference, so make sure we've really
                // doomed compilation so as to not accidentally stabilize new
                // inference or something here...
                self.dcx().span_delayed_bug(span, "this path really should be doomed...");
                // Guide inference for the RHS expression if it's provided --
                // this will allow us to better error reporting, at the expense
                // of making some error messages a bit more specific.
                if let Some((rhs_expr, rhs_ty)) = opt_rhs
                    && rhs_ty.is_ty_var()
                {
                    self.check_expr_coercible_to_type(rhs_expr, rhs_ty, None);
                }

                // Construct an obligation `self_ty : Trait<input_tys>`
                let args =
                    ty::GenericArgs::for_item(self.tcx, trait_did, |param, _| match param.kind {
                        ty::GenericParamDefKind::Lifetime
                        | ty::GenericParamDefKind::Const { .. } => {
                            unreachable!("did not expect operand trait to have lifetime/const args")
                        }
                        ty::GenericParamDefKind::Type { .. } => {
                            if param.index == 0 {
                                lhs_ty.into()
                            } else {
                                opt_rhs_ty.expect("expected RHS for binop").into()
                            }
                        }
                    });
                let obligation = Obligation::new(
                    self.tcx,
                    cause,
                    self.param_env,
                    ty::TraitRef::new_from_args(self.tcx, trait_did, args),
                );
                let ocx = ObligationCtxt::new_with_diagnostics(&self.infcx);
                ocx.register_obligation(obligation);
                Err(ocx.evaluate_obligations_error_on_ambiguity())
            }
        }
    }
}

fn lang_item_for_binop(tcx: TyCtxt<'_>, op: Op) -> (Symbol, Option<DefId>) {
    let lang = tcx.lang_items();
    match op {
        Op::AssignOp(op) => match op.node {
            AssignOpKind::AddAssign => (sym::add_assign, lang.add_assign_trait()),
            AssignOpKind::SubAssign => (sym::sub_assign, lang.sub_assign_trait()),
            AssignOpKind::MulAssign => (sym::mul_assign, lang.mul_assign_trait()),
            AssignOpKind::DivAssign => (sym::div_assign, lang.div_assign_trait()),
            AssignOpKind::RemAssign => (sym::rem_assign, lang.rem_assign_trait()),
            AssignOpKind::BitXorAssign => (sym::bitxor_assign, lang.bitxor_assign_trait()),
            AssignOpKind::BitAndAssign => (sym::bitand_assign, lang.bitand_assign_trait()),
            AssignOpKind::BitOrAssign => (sym::bitor_assign, lang.bitor_assign_trait()),
            AssignOpKind::ShlAssign => (sym::shl_assign, lang.shl_assign_trait()),
            AssignOpKind::ShrAssign => (sym::shr_assign, lang.shr_assign_trait()),
        },
        Op::BinOp(op) => match op.node {
            BinOpKind::Add => (sym::add, lang.add_trait()),
            BinOpKind::Sub => (sym::sub, lang.sub_trait()),
            BinOpKind::Mul => (sym::mul, lang.mul_trait()),
            BinOpKind::Div => (sym::div, lang.div_trait()),
            BinOpKind::Rem => (sym::rem, lang.rem_trait()),
            BinOpKind::BitXor => (sym::bitxor, lang.bitxor_trait()),
            BinOpKind::BitAnd => (sym::bitand, lang.bitand_trait()),
            BinOpKind::BitOr => (sym::bitor, lang.bitor_trait()),
            BinOpKind::Shl => (sym::shl, lang.shl_trait()),
            BinOpKind::Shr => (sym::shr, lang.shr_trait()),
            BinOpKind::Lt => (sym::lt, lang.partial_ord_trait()),
            BinOpKind::Le => (sym::le, lang.partial_ord_trait()),
            BinOpKind::Ge => (sym::ge, lang.partial_ord_trait()),
            BinOpKind::Gt => (sym::gt, lang.partial_ord_trait()),
            BinOpKind::Eq => (sym::eq, lang.eq_trait()),
            BinOpKind::Ne => (sym::ne, lang.eq_trait()),
            BinOpKind::And | BinOpKind::Or => {
                bug!("&& and || are not overloadable")
            }
        },
    }
}

fn lang_item_for_unop(tcx: TyCtxt<'_>, op: hir::UnOp) -> (Symbol, Option<hir::def_id::DefId>) {
    let lang = tcx.lang_items();
    match op {
        hir::UnOp::Not => (sym::not, lang.not_trait()),
        hir::UnOp::Neg => (sym::neg, lang.neg_trait()),
        hir::UnOp::Deref => bug!("Deref is not overloadable"),
    }
}

/// Check if `expr` contains a `let` or `&&`, indicating presence of a let-chain
pub(crate) fn contains_let_in_chain(expr: &Expr<'_>) -> bool {
    match &expr.kind {
        ExprKind::Let(..) => true,
        ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, left, right) => {
            contains_let_in_chain(left) || contains_let_in_chain(right)
        }
        _ => false,
    }
}

// Binary operator categories. These categories summarize the behavior
// with respect to the builtin operations supported.
#[derive(Clone, Copy)]
enum BinOpCategory {
    /// &&, || -- cannot be overridden
    Shortcircuit,

    /// <<, >> -- when shifting a single integer, rhs can be any
    /// integer type. For simd, types must match.
    Shift,

    /// +, -, etc -- takes equal types, produces same type as input,
    /// applicable to ints/floats/simd
    Math,

    /// &, |, ^ -- takes equal types, produces same type as input,
    /// applicable to ints/floats/simd/bool
    Bitwise,

    /// ==, !=, etc -- takes equal types, produces bools, except for simd,
    /// which produce the input type
    Comparison,
}

impl From<BinOpKind> for BinOpCategory {
    fn from(op: BinOpKind) -> BinOpCategory {
        use hir::BinOpKind::*;
        match op {
            Shl | Shr => BinOpCategory::Shift,
            Add | Sub | Mul | Div | Rem => BinOpCategory::Math,
            BitXor | BitAnd | BitOr => BinOpCategory::Bitwise,
            Eq | Ne | Lt | Le | Ge | Gt => BinOpCategory::Comparison,
            And | Or => BinOpCategory::Shortcircuit,
        }
    }
}

impl From<AssignOpKind> for BinOpCategory {
    fn from(op: AssignOpKind) -> BinOpCategory {
        use hir::AssignOpKind::*;
        match op {
            ShlAssign | ShrAssign => BinOpCategory::Shift,
            AddAssign | SubAssign | MulAssign | DivAssign | RemAssign => BinOpCategory::Math,
            BitXorAssign | BitAndAssign | BitOrAssign => BinOpCategory::Bitwise,
        }
    }
}

/// An assignment op (e.g. `a += b`), or a binary op (e.g. `a + b`).
#[derive(Clone, Copy, Debug, PartialEq)]
enum Op {
    BinOp(hir::BinOp),
    AssignOp(hir::AssignOp),
}

impl Op {
    fn span(&self) -> Span {
        match self {
            Op::BinOp(op) => op.span,
            Op::AssignOp(op) => op.span,
        }
    }

    fn as_str(&self) -> &'static str {
        match self {
            Op::BinOp(op) => op.node.as_str(),
            Op::AssignOp(op) => op.node.as_str(),
        }
    }

    fn is_by_value(&self) -> bool {
        match self {
            Op::BinOp(op) => op.node.is_by_value(),
            Op::AssignOp(op) => op.node.is_by_value(),
        }
    }
}

/// Dereferences a single level of immutable referencing.
fn deref_ty_if_possible(ty: Ty<'_>) -> Ty<'_> {
    match ty.kind() {
        ty::Ref(_, ty, hir::Mutability::Not) => *ty,
        _ => ty,
    }
}

/// Returns `true` if this is a built-in arithmetic operation (e.g.,
/// u32 + u32, i16x4 == i16x4) and false if these types would have to be
/// overloaded to be legal. There are two reasons that we distinguish
/// builtin operations from overloaded ones (vs trying to drive
/// everything uniformly through the trait system and intrinsics or
/// something like that):
///
/// 1. Builtin operations can trivially be evaluated in constants.
/// 2. For comparison operators applied to SIMD types the result is
///    not of type `bool`. For example, `i16x4 == i16x4` yields a
///    type like `i16x4`. This means that the overloaded trait
///    `PartialEq` is not applicable.
///
/// Reason #2 is the killer. I tried for a while to always use
/// overloaded logic and just check the types in constants/codegen after
/// the fact, and it worked fine, except for SIMD types. -nmatsakis
fn is_builtin_binop<'tcx>(lhs: Ty<'tcx>, rhs: Ty<'tcx>, category: BinOpCategory) -> bool {
    // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
    // (See https://github.com/rust-lang/rust/issues/57447.)
    let (lhs, rhs) = (deref_ty_if_possible(lhs), deref_ty_if_possible(rhs));

    match category {
        BinOpCategory::Shortcircuit => true,
        BinOpCategory::Shift => {
            lhs.references_error()
                || rhs.references_error()
                || lhs.is_integral() && rhs.is_integral()
        }
        BinOpCategory::Math => {
            lhs.references_error()
                || rhs.references_error()
                || lhs.is_integral() && rhs.is_integral()
                || lhs.is_floating_point() && rhs.is_floating_point()
        }
        BinOpCategory::Bitwise => {
            lhs.references_error()
                || rhs.references_error()
                || lhs.is_integral() && rhs.is_integral()
                || lhs.is_floating_point() && rhs.is_floating_point()
                || lhs.is_bool() && rhs.is_bool()
        }
        BinOpCategory::Comparison => {
            lhs.references_error() || rhs.references_error() || lhs.is_scalar() && rhs.is_scalar()
        }
    }
}