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, DerefAdjustKind};
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().skip_norm_wip(),
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().skip_norm_wip().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).instantiate_identity().skip_norm_wip()
86                .skip_binder()
87                .inputs_and_output
88            {
89                [arg, res] if !arg.is_mutable_ptr() && arg.peel_refs() == ty && res.is_bool() => NoChange,
90                _ => Lazy,
91            }
92        } else {
93            Lazy
94        }
95    } else {
96        Lazy
97    }
98}
99
100fn res_has_significant_drop(res: Res, cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
101    if let Res::Def(DefKind::Ctor(..) | DefKind::Variant | DefKind::Enum | DefKind::Struct, _)
102    | Res::SelfCtor(_)
103    | Res::SelfTyAlias { .. } = res
104    {
105        cx.typeck_results()
106            .expr_ty(e)
107            .has_significant_drop(cx.tcx, cx.typing_env())
108    } else {
109        false
110    }
111}
112
113#[expect(clippy::too_many_lines)]
114fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessSuggestion {
115    struct V<'cx, 'tcx> {
116        cx: &'cx LateContext<'tcx>,
117        eagerness: EagernessSuggestion,
118    }
119
120    impl<'tcx> Visitor<'tcx> for V<'_, 'tcx> {
121        fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
122            use EagernessSuggestion::{ForceNoChange, Lazy, NoChange};
123            if self.eagerness == ForceNoChange {
124                return;
125            }
126
127            // Autoderef through a user-defined `Deref` impl can have side-effects,
128            // so don't suggest changing it.
129            if self
130                .cx
131                .typeck_results()
132                .expr_adjustments(e)
133                .iter()
134                .any(|adj| matches!(adj.kind, Adjust::Deref(DerefAdjustKind::Overloaded(_))))
135            {
136                self.eagerness |= NoChange;
137                return;
138            }
139
140            match e.kind {
141                ExprKind::Call(
142                    &Expr {
143                        kind: ExprKind::Path(ref path),
144                        hir_id,
145                        ..
146                    },
147                    args,
148                ) => match self.cx.qpath_res(path, hir_id) {
149                    res @ (Res::Def(DefKind::Ctor(..) | DefKind::Variant, _) | Res::SelfCtor(_)) => {
150                        if res_has_significant_drop(res, self.cx, e) {
151                            self.eagerness = ForceNoChange;
152                            return;
153                        }
154                    },
155                    Res::Def(_, id) if self.cx.tcx.is_promotable_const_fn(id) => (),
156                    // No need to walk the arguments here, `is_const_evaluatable` already did
157                    Res::Def(..) if is_const_evaluatable(self.cx, e) => {
158                        self.eagerness |= NoChange;
159                        return;
160                    },
161                    Res::Def(_, id) => match path {
162                        QPath::Resolved(_, p) => {
163                            self.eagerness |=
164                                fn_eagerness(self.cx, id, p.segments.last().unwrap().ident.name, !args.is_empty());
165                        },
166                        QPath::TypeRelative(_, name) => {
167                            self.eagerness |= fn_eagerness(self.cx, id, name.ident.name, !args.is_empty());
168                        },
169                    },
170                    _ => self.eagerness = Lazy,
171                },
172                // No need to walk the arguments here, `is_const_evaluatable` already did
173                ExprKind::MethodCall(..) if is_const_evaluatable(self.cx, e) => {
174                    self.eagerness |= NoChange;
175                    return;
176                },
177                #[expect(clippy::match_same_arms)] // arm pattern can't be merged due to `ref`, see rust#105778
178                ExprKind::Struct(path, ..) => {
179                    if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
180                        self.eagerness = ForceNoChange;
181                        return;
182                    }
183                },
184                ExprKind::Path(ref path) => {
185                    if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
186                        self.eagerness = ForceNoChange;
187                        return;
188                    }
189                },
190                ExprKind::MethodCall(name, ..) => {
191                    self.eagerness |= self
192                        .cx
193                        .typeck_results()
194                        .type_dependent_def_id(e.hir_id)
195                        .map_or(Lazy, |id| fn_eagerness(self.cx, id, name.ident.name, true));
196                },
197                ExprKind::Index(_, e, _) => {
198                    let ty = self.cx.typeck_results().expr_ty_adjusted(e);
199                    if is_copy(self.cx, ty) && !ty.is_ref() {
200                        self.eagerness |= NoChange;
201                    } else {
202                        self.eagerness = Lazy;
203                    }
204                },
205
206                // `-i32::MIN` panics with overflow checks
207                ExprKind::Unary(UnOp::Neg, right) if ConstEvalCtxt::new(self.cx).eval(right).is_none() => {
208                    self.eagerness |= NoChange;
209                },
210
211                // Custom `Deref` impl might have side effects
212                ExprKind::Unary(UnOp::Deref, e)
213                    if self.cx.typeck_results().expr_ty(e).builtin_deref(true).is_none() =>
214                {
215                    self.eagerness |= NoChange;
216                },
217                // Dereferences should be cheap, but dereferencing a raw pointer earlier may not be safe.
218                ExprKind::Unary(UnOp::Deref, e) if !self.cx.typeck_results().expr_ty(e).is_raw_ptr() => (),
219                ExprKind::Unary(UnOp::Deref, _) => self.eagerness |= NoChange,
220                ExprKind::Unary(_, e)
221                    if matches!(
222                        self.cx.typeck_results().expr_ty(e).kind(),
223                        ty::Bool | ty::Int(_) | ty::Uint(_),
224                    ) => {},
225
226                // `>>` and `<<` panic when the right-hand side is greater than or equal to the number of bits in the
227                // type of the left-hand side, or is negative.
228                // We intentionally only check if the right-hand isn't a constant, because even if the suggestion would
229                // overflow with constants, the compiler emits an error for it and the programmer will have to fix it.
230                // Thus, we would realistically only delay the lint.
231                ExprKind::Binary(op, _, right)
232                    if matches!(op.node, BinOpKind::Shl | BinOpKind::Shr)
233                        && ConstEvalCtxt::new(self.cx).eval(right).is_none() =>
234                {
235                    self.eagerness |= NoChange;
236                },
237
238                ExprKind::Binary(op, left, right)
239                    if matches!(op.node, BinOpKind::Div | BinOpKind::Rem)
240                        && let right_ty = self.cx.typeck_results().expr_ty(right)
241                        && let ecx = ConstEvalCtxt::new(self.cx)
242                        && let left = ecx.eval(left)
243                        && let right = ecx.eval(right).and_then(|c| c.int_value(self.cx.tcx, right_ty))
244                        && matches!(
245                            (left, right),
246                            // `1 / x`: x might be zero
247                            (_, None)
248                            // `x / -1`: x might be T::MIN
249                            | (None, Some(FullInt::S(-1)))
250                        ) =>
251                {
252                    self.eagerness |= NoChange;
253                },
254
255                // Similar to `>>` and `<<`, we only want to avoid linting entirely if either side is unknown and the
256                // compiler can't emit an error for an overflowing expression.
257                // Suggesting eagerness for `true.then(|| i32::MAX + 1)` is okay because the compiler will emit an
258                // error and it's good to have the eagerness warning up front when the user fixes the logic error.
259                ExprKind::Binary(op, left, right)
260                    if matches!(op.node, BinOpKind::Add | BinOpKind::Sub | BinOpKind::Mul)
261                        && !self.cx.typeck_results().expr_ty(e).is_floating_point()
262                        && let ecx = ConstEvalCtxt::new(self.cx)
263                        && (ecx.eval(left).is_none() || ecx.eval(right).is_none()) =>
264                {
265                    self.eagerness |= NoChange;
266                },
267
268                ExprKind::Binary(_, lhs, rhs)
269                    if self.cx.typeck_results().expr_ty(lhs).is_primitive()
270                        && self.cx.typeck_results().expr_ty(rhs).is_primitive() => {},
271
272                // Can't be moved into a closure
273                ExprKind::Break(..)
274                | ExprKind::Continue(_)
275                | ExprKind::Ret(_)
276                | ExprKind::Become(_)
277                | ExprKind::InlineAsm(_)
278                | ExprKind::Yield(..)
279                | ExprKind::Err(_) => {
280                    self.eagerness = ForceNoChange;
281                    return;
282                },
283
284                // Memory allocation, custom operator, loop, or call to an unknown function
285                ExprKind::Unary(..) | ExprKind::Binary(..) | ExprKind::Loop(..) | ExprKind::Call(..) => {
286                    self.eagerness = Lazy;
287                },
288
289                ExprKind::ConstBlock(_)
290                | ExprKind::Array(_)
291                | ExprKind::Tup(_)
292                | ExprKind::Use(..)
293                | ExprKind::Lit(_)
294                | ExprKind::Cast(..)
295                | ExprKind::Type(..)
296                | ExprKind::DropTemps(_)
297                | ExprKind::Let(..)
298                | ExprKind::If(..)
299                | ExprKind::Match(..)
300                | ExprKind::Closure { .. }
301                | ExprKind::Field(..)
302                | ExprKind::AddrOf(..)
303                | ExprKind::Repeat(..)
304                | ExprKind::Block(Block { stmts: [], .. }, _)
305                | ExprKind::OffsetOf(..)
306                | ExprKind::UnsafeBinderCast(..) => (),
307
308                // Assignment might be to a local defined earlier, so don't eagerly evaluate.
309                // Blocks with multiple statements might be expensive, so don't eagerly evaluate.
310                // TODO: Actually check if either of these are true here.
311                ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Block(..) => self.eagerness |= NoChange,
312            }
313            walk_expr(self, e);
314        }
315    }
316
317    let mut v = V {
318        cx,
319        eagerness: EagernessSuggestion::Eager,
320    };
321    v.visit_expr(e);
322    v.eagerness
323}
324
325/// Whether the given expression should be changed to evaluate eagerly
326pub fn switch_to_eager_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
327    expr_eagerness(cx, expr) == EagernessSuggestion::Eager
328}
329
330/// Whether the given expression should be changed to evaluate lazily
331pub fn switch_to_lazy_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
332    expr_eagerness(cx, expr) == EagernessSuggestion::Lazy
333}
clippy_utils/eager_or_lazy.rs

clippy_utils/
eager_or_lazy.rs
