clippy_utils/
eager_or_lazy.rs

1//! Utilities for evaluating whether eagerly evaluated expressions can be made lazy and vice versa.
2//!
3//! Things to consider:
4//!  - does the expression have side-effects?
5//!  - is the expression computationally expensive?
6//!
7//! See lints:
8//!  - unnecessary-lazy-evaluations
9//!  - or-fun-call
10//!  - option-if-let-else
11
12use crate::consts::{ConstEvalCtxt, FullInt};
13use crate::sym;
14use crate::ty::{all_predicates_of, is_copy};
15use crate::visitors::is_const_evaluatable;
16use rustc_hir::def::{DefKind, Res};
17use rustc_hir::def_id::DefId;
18use rustc_hir::intravisit::{Visitor, walk_expr};
19use rustc_hir::{BinOpKind, Block, Expr, ExprKind, QPath, UnOp};
20use rustc_lint::LateContext;
21use rustc_middle::ty;
22use rustc_middle::ty::adjustment::Adjust;
23use rustc_span::Symbol;
24use std::{cmp, ops};
25
26#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
27enum EagernessSuggestion {
28    // The expression is cheap and should be evaluated eagerly
29    Eager,
30    // The expression may be cheap, so don't suggested lazy evaluation; or the expression may not be safe to switch to
31    // eager evaluation.
32    NoChange,
33    // The expression is likely expensive and should be evaluated lazily.
34    Lazy,
35    // The expression cannot be placed into a closure.
36    ForceNoChange,
37}
38impl ops::BitOr for EagernessSuggestion {
39    type Output = Self;
40    fn bitor(self, rhs: Self) -> Self {
41        cmp::max(self, rhs)
42    }
43}
44impl ops::BitOrAssign for EagernessSuggestion {
45    fn bitor_assign(&mut self, rhs: Self) {
46        *self = *self | rhs;
47    }
48}
49
50/// Determine the eagerness of the given function call.
51fn fn_eagerness(cx: &LateContext<'_>, fn_id: DefId, name: Symbol, have_one_arg: bool) -> EagernessSuggestion {
52    use EagernessSuggestion::{Eager, Lazy, NoChange};
53
54    let ty = match cx.tcx.impl_of_assoc(fn_id) {
55        Some(id) => cx.tcx.type_of(id).instantiate_identity(),
56        None => return Lazy,
57    };
58
59    if (matches!(name, sym::is_empty | sym::len) || name.as_str().starts_with("as_")) && have_one_arg {
60        if matches!(
61            cx.tcx.crate_name(fn_id.krate),
62            sym::std | sym::core | sym::alloc | sym::proc_macro
63        ) {
64            Eager
65        } else {
66            NoChange
67        }
68    } else if let ty::Adt(def, subs) = ty.kind() {
69        // Types where the only fields are generic types (or references to) with no trait bounds other
70        // than marker traits.
71        // Due to the limited operations on these types functions should be fairly cheap.
72        if def.variants().iter().flat_map(|v| v.fields.iter()).any(|x| {
73            matches!(
74                cx.tcx.type_of(x.did).instantiate_identity().peel_refs().kind(),
75                ty::Param(_)
76            )
77        }) && all_predicates_of(cx.tcx, fn_id).all(|(pred, _)| match pred.kind().skip_binder() {
78            ty::ClauseKind::Trait(pred) => cx.tcx.trait_def(pred.trait_ref.def_id).is_marker,
79            _ => true,
80        }) && subs.types().all(|x| matches!(x.peel_refs().kind(), ty::Param(_)))
81        {
82            // Limit the function to either `(self) -> bool` or `(&self) -> bool`
83            match &**cx
84                .tcx
85                .fn_sig(fn_id)
86                .instantiate_identity()
87                .skip_binder()
88                .inputs_and_output
89            {
90                [arg, res] if !arg.is_mutable_ptr() && arg.peel_refs() == ty && res.is_bool() => NoChange,
91                _ => Lazy,
92            }
93        } else {
94            Lazy
95        }
96    } else {
97        Lazy
98    }
99}
100
101fn res_has_significant_drop(res: Res, cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
102    if let Res::Def(DefKind::Ctor(..) | DefKind::Variant | DefKind::Enum | DefKind::Struct, _)
103    | Res::SelfCtor(_)
104    | Res::SelfTyAlias { .. } = res
105    {
106        cx.typeck_results()
107            .expr_ty(e)
108            .has_significant_drop(cx.tcx, cx.typing_env())
109    } else {
110        false
111    }
112}
113
114#[expect(clippy::too_many_lines)]
115fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessSuggestion {
116    struct V<'cx, 'tcx> {
117        cx: &'cx LateContext<'tcx>,
118        eagerness: EagernessSuggestion,
119    }
120
121    impl<'tcx> Visitor<'tcx> for V<'_, 'tcx> {
122        fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
123            use EagernessSuggestion::{ForceNoChange, Lazy, NoChange};
124            if self.eagerness == ForceNoChange {
125                return;
126            }
127
128            // Autoderef through a user-defined `Deref` impl can have side-effects,
129            // so don't suggest changing it.
130            if self
131                .cx
132                .typeck_results()
133                .expr_adjustments(e)
134                .iter()
135                .any(|adj| matches!(adj.kind, Adjust::Deref(Some(_))))
136            {
137                self.eagerness |= NoChange;
138                return;
139            }
140
141            match e.kind {
142                ExprKind::Call(
143                    &Expr {
144                        kind: ExprKind::Path(ref path),
145                        hir_id,
146                        ..
147                    },
148                    args,
149                ) => match self.cx.qpath_res(path, hir_id) {
150                    res @ (Res::Def(DefKind::Ctor(..) | DefKind::Variant, _) | Res::SelfCtor(_)) => {
151                        if res_has_significant_drop(res, self.cx, e) {
152                            self.eagerness = ForceNoChange;
153                            return;
154                        }
155                    },
156                    Res::Def(_, id) if self.cx.tcx.is_promotable_const_fn(id) => (),
157                    // No need to walk the arguments here, `is_const_evaluatable` already did
158                    Res::Def(..) if is_const_evaluatable(self.cx, e) => {
159                        self.eagerness |= NoChange;
160                        return;
161                    },
162                    Res::Def(_, id) => match path {
163                        QPath::Resolved(_, p) => {
164                            self.eagerness |=
165                                fn_eagerness(self.cx, id, p.segments.last().unwrap().ident.name, !args.is_empty());
166                        },
167                        QPath::TypeRelative(_, name) => {
168                            self.eagerness |= fn_eagerness(self.cx, id, name.ident.name, !args.is_empty());
169                        },
170                    },
171                    _ => self.eagerness = Lazy,
172                },
173                // No need to walk the arguments here, `is_const_evaluatable` already did
174                ExprKind::MethodCall(..) if is_const_evaluatable(self.cx, e) => {
175                    self.eagerness |= NoChange;
176                    return;
177                },
178                #[expect(clippy::match_same_arms)] // arm pattern can't be merged due to `ref`, see rust#105778
179                ExprKind::Struct(path, ..) => {
180                    if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
181                        self.eagerness = ForceNoChange;
182                        return;
183                    }
184                },
185                ExprKind::Path(ref path) => {
186                    if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
187                        self.eagerness = ForceNoChange;
188                        return;
189                    }
190                },
191                ExprKind::MethodCall(name, ..) => {
192                    self.eagerness |= self
193                        .cx
194                        .typeck_results()
195                        .type_dependent_def_id(e.hir_id)
196                        .map_or(Lazy, |id| fn_eagerness(self.cx, id, name.ident.name, true));
197                },
198                ExprKind::Index(_, e, _) => {
199                    let ty = self.cx.typeck_results().expr_ty_adjusted(e);
200                    if is_copy(self.cx, ty) && !ty.is_ref() {
201                        self.eagerness |= NoChange;
202                    } else {
203                        self.eagerness = Lazy;
204                    }
205                },
206
207                // `-i32::MIN` panics with overflow checks
208                ExprKind::Unary(UnOp::Neg, right) if ConstEvalCtxt::new(self.cx).eval(right).is_none() => {
209                    self.eagerness |= NoChange;
210                },
211
212                // Custom `Deref` impl might have side effects
213                ExprKind::Unary(UnOp::Deref, e)
214                    if self
215                        .cx
216                        .typeck_results()
217                        .expr_ty(e)
218                        .builtin_deref(true)
219                        .is_none() =>
220                {
221                    self.eagerness |= NoChange;
222                },
223                // Dereferences should be cheap, but dereferencing a raw pointer earlier may not be safe.
224                ExprKind::Unary(UnOp::Deref, e) if !self.cx.typeck_results().expr_ty(e).is_raw_ptr() => (),
225                ExprKind::Unary(UnOp::Deref, _) => self.eagerness |= NoChange,
226                ExprKind::Unary(_, e)
227                    if matches!(
228                        self.cx.typeck_results().expr_ty(e).kind(),
229                        ty::Bool | ty::Int(_) | ty::Uint(_),
230                    ) => {},
231
232                // `>>` and `<<` panic when the right-hand side is greater than or equal to the number of bits in the
233                // type of the left-hand side, or is negative.
234                // We intentionally only check if the right-hand isn't a constant, because even if the suggestion would
235                // overflow with constants, the compiler emits an error for it and the programmer will have to fix it.
236                // Thus, we would realistically only delay the lint.
237                ExprKind::Binary(op, _, right)
238                    if matches!(op.node, BinOpKind::Shl | BinOpKind::Shr)
239                        && ConstEvalCtxt::new(self.cx).eval(right).is_none() =>
240                {
241                    self.eagerness |= NoChange;
242                },
243
244                ExprKind::Binary(op, left, right)
245                    if matches!(op.node, BinOpKind::Div | BinOpKind::Rem)
246                        && let right_ty = self.cx.typeck_results().expr_ty(right)
247                        && let ecx = ConstEvalCtxt::new(self.cx)
248                        && let left = ecx.eval(left)
249                        && let right = ecx.eval(right).and_then(|c| c.int_value(self.cx.tcx, right_ty))
250                        && matches!(
251                            (left, right),
252                            // `1 / x`: x might be zero
253                            (_, None)
254                            // `x / -1`: x might be T::MIN
255                            | (None, Some(FullInt::S(-1)))
256                        ) =>
257                {
258                    self.eagerness |= NoChange;
259                },
260
261                // Similar to `>>` and `<<`, we only want to avoid linting entirely if either side is unknown and the
262                // compiler can't emit an error for an overflowing expression.
263                // Suggesting eagerness for `true.then(|| i32::MAX + 1)` is okay because the compiler will emit an
264                // error and it's good to have the eagerness warning up front when the user fixes the logic error.
265                ExprKind::Binary(op, left, right)
266                    if matches!(op.node, BinOpKind::Add | BinOpKind::Sub | BinOpKind::Mul)
267                        && !self.cx.typeck_results().expr_ty(e).is_floating_point()
268                        && let ecx = ConstEvalCtxt::new(self.cx)
269                        && (ecx.eval(left).is_none() || ecx.eval(right).is_none()) =>
270                {
271                    self.eagerness |= NoChange;
272                },
273
274                ExprKind::Binary(_, lhs, rhs)
275                    if self.cx.typeck_results().expr_ty(lhs).is_primitive()
276                        && self.cx.typeck_results().expr_ty(rhs).is_primitive() => {},
277
278                // Can't be moved into a closure
279                ExprKind::Break(..)
280                | ExprKind::Continue(_)
281                | ExprKind::Ret(_)
282                | ExprKind::Become(_)
283                | ExprKind::InlineAsm(_)
284                | ExprKind::Yield(..)
285                | ExprKind::Err(_) => {
286                    self.eagerness = ForceNoChange;
287                    return;
288                },
289
290                // Memory allocation, custom operator, loop, or call to an unknown function
291                ExprKind::Unary(..) | ExprKind::Binary(..) | ExprKind::Loop(..) | ExprKind::Call(..) => {
292                    self.eagerness = Lazy;
293                },
294
295                ExprKind::ConstBlock(_)
296                | ExprKind::Array(_)
297                | ExprKind::Tup(_)
298                | ExprKind::Use(..)
299                | ExprKind::Lit(_)
300                | ExprKind::Cast(..)
301                | ExprKind::Type(..)
302                | ExprKind::DropTemps(_)
303                | ExprKind::Let(..)
304                | ExprKind::If(..)
305                | ExprKind::Match(..)
306                | ExprKind::Closure { .. }
307                | ExprKind::Field(..)
308                | ExprKind::AddrOf(..)
309                | ExprKind::Repeat(..)
310                | ExprKind::Block(Block { stmts: [], .. }, _)
311                | ExprKind::OffsetOf(..)
312                | ExprKind::UnsafeBinderCast(..) => (),
313
314                // Assignment might be to a local defined earlier, so don't eagerly evaluate.
315                // Blocks with multiple statements might be expensive, so don't eagerly evaluate.
316                // TODO: Actually check if either of these are true here.
317                ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Block(..) => self.eagerness |= NoChange,
318            }
319            walk_expr(self, e);
320        }
321    }
322
323    let mut v = V {
324        cx,
325        eagerness: EagernessSuggestion::Eager,
326    };
327    v.visit_expr(e);
328    v.eagerness
329}
330
331/// Whether the given expression should be changed to evaluate eagerly
332pub fn switch_to_eager_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
333    expr_eagerness(cx, expr) == EagernessSuggestion::Eager
334}
335
336/// Whether the given expression should be changed to evaluate lazily
337pub fn switch_to_lazy_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
338    expr_eagerness(cx, expr) == EagernessSuggestion::Lazy
339}
clippy_utils/eager_or_lazy.rs

clippy_utils/
eager_or_lazy.rs
