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
75                    .type_of(x.did)
76                    .instantiate_identity()
77                    .skip_norm_wip()
78                    .peel_refs()
79                    .kind(),
80                ty::Param(_)
81            )
82        }) && all_predicates_of(cx.tcx, fn_id).all(|(pred, _)| match pred.kind().skip_binder() {
83            ty::ClauseKind::Trait(pred) => cx.tcx.trait_def(pred.trait_ref.def_id).is_marker,
84            _ => true,
85        }) && subs.types().all(|x| matches!(x.peel_refs().kind(), ty::Param(_)))
86        {
87            // Limit the function to either `(self) -> bool` or `(&self) -> bool`
88            match &**cx
89                .tcx
90                .fn_sig(fn_id)
91                .instantiate_identity()
92                .skip_norm_wip()
93                .skip_binder()
94                .inputs_and_output
95            {
96                [arg, res] if !arg.is_mutable_ptr() && arg.peel_refs() == ty && res.is_bool() => NoChange,
97                _ => Lazy,
98            }
99        } else {
100            Lazy
101        }
102    } else {
103        Lazy
104    }
105}
106
107fn res_has_significant_drop(res: Res, cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
108    if let Res::Def(DefKind::Ctor(..) | DefKind::Variant | DefKind::Enum | DefKind::Struct, _)
109    | Res::SelfCtor(_)
110    | Res::SelfTyAlias { .. } = res
111    {
112        cx.typeck_results()
113            .expr_ty(e)
114            .has_significant_drop(cx.tcx, cx.typing_env())
115    } else {
116        false
117    }
118}
119
120#[expect(clippy::too_many_lines)]
121fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessSuggestion {
122    struct V<'cx, 'tcx> {
123        cx: &'cx LateContext<'tcx>,
124        eagerness: EagernessSuggestion,
125    }
126
127    impl<'tcx> Visitor<'tcx> for V<'_, 'tcx> {
128        fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
129            use EagernessSuggestion::{ForceNoChange, Lazy, NoChange};
130            if self.eagerness == ForceNoChange {
131                return;
132            }
133
134            // Autoderef through a user-defined `Deref` impl can have side-effects,
135            // so don't suggest changing it.
136            if self
137                .cx
138                .typeck_results()
139                .expr_adjustments(e)
140                .iter()
141                .any(|adj| matches!(adj.kind, Adjust::Deref(DerefAdjustKind::Overloaded(_))))
142            {
143                self.eagerness |= NoChange;
144                return;
145            }
146
147            match e.kind {
148                ExprKind::Call(
149                    &Expr {
150                        kind: ExprKind::Path(ref path),
151                        hir_id,
152                        ..
153                    },
154                    args,
155                ) => match self.cx.qpath_res(path, hir_id) {
156                    res @ (Res::Def(DefKind::Ctor(..) | DefKind::Variant, _) | Res::SelfCtor(_)) => {
157                        if res_has_significant_drop(res, self.cx, e) {
158                            self.eagerness = ForceNoChange;
159                            return;
160                        }
161                    },
162                    Res::Def(_, id) if self.cx.tcx.is_promotable_const_fn(id) => (),
163                    // No need to walk the arguments here, `is_const_evaluatable` already did
164                    Res::Def(..) if is_const_evaluatable(self.cx, e) => {
165                        self.eagerness |= NoChange;
166                        return;
167                    },
168                    Res::Def(_, id) => match path {
169                        QPath::Resolved(_, p) => {
170                            self.eagerness |=
171                                fn_eagerness(self.cx, id, p.segments.last().unwrap().ident.name, !args.is_empty());
172                        },
173                        QPath::TypeRelative(_, name) => {
174                            self.eagerness |= fn_eagerness(self.cx, id, name.ident.name, !args.is_empty());
175                        },
176                    },
177                    _ => self.eagerness = Lazy,
178                },
179                // No need to walk the arguments here, `is_const_evaluatable` already did
180                ExprKind::MethodCall(..) if is_const_evaluatable(self.cx, e) => {
181                    self.eagerness |= NoChange;
182                    return;
183                },
184                #[expect(clippy::match_same_arms)] // arm pattern can't be merged due to `ref`, see rust#105778
185                ExprKind::Struct(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::Path(ref path) => {
192                    if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
193                        self.eagerness = ForceNoChange;
194                        return;
195                    }
196                },
197                ExprKind::MethodCall(name, ..) => {
198                    self.eagerness |= self
199                        .cx
200                        .typeck_results()
201                        .type_dependent_def_id(e.hir_id)
202                        .map_or(Lazy, |id| fn_eagerness(self.cx, id, name.ident.name, true));
203                },
204                ExprKind::Index(_, e, _) => {
205                    let ty = self.cx.typeck_results().expr_ty_adjusted(e);
206                    if is_copy(self.cx, ty) && !ty.is_ref() {
207                        self.eagerness |= NoChange;
208                    } else {
209                        self.eagerness = Lazy;
210                    }
211                },
212
213                // `-i32::MIN` panics with overflow checks
214                ExprKind::Unary(UnOp::Neg, right) if ConstEvalCtxt::new(self.cx).eval(right).is_none() => {
215                    self.eagerness |= NoChange;
216                },
217
218                // Custom `Deref` impl might have side effects
219                ExprKind::Unary(UnOp::Deref, e)
220                    if self.cx.typeck_results().expr_ty(e).builtin_deref(true).is_none() =>
221                {
222                    self.eagerness |= NoChange;
223                },
224                // Dereferences should be cheap, but dereferencing a raw pointer earlier may not be safe.
225                ExprKind::Unary(UnOp::Deref, e) if !self.cx.typeck_results().expr_ty(e).is_raw_ptr() => (),
226                ExprKind::Unary(UnOp::Deref, _) => self.eagerness |= NoChange,
227                ExprKind::Unary(_, e)
228                    if matches!(
229                        self.cx.typeck_results().expr_ty(e).kind(),
230                        ty::Bool | ty::Int(_) | ty::Uint(_),
231                    ) => {},
232
233                // `>>` and `<<` panic when the right-hand side is greater than or equal to the number of bits in the
234                // type of the left-hand side, or is negative.
235                // We intentionally only check if the right-hand isn't a constant, because even if the suggestion would
236                // overflow with constants, the compiler emits an error for it and the programmer will have to fix it.
237                // Thus, we would realistically only delay the lint.
238                ExprKind::Binary(op, _, right)
239                    if matches!(op.node, BinOpKind::Shl | BinOpKind::Shr)
240                        && ConstEvalCtxt::new(self.cx).eval(right).is_none() =>
241                {
242                    self.eagerness |= NoChange;
243                },
244
245                ExprKind::Binary(op, left, right)
246                    if matches!(op.node, BinOpKind::Div | BinOpKind::Rem)
247                        && let right_ty = self.cx.typeck_results().expr_ty(right)
248                        && let ecx = ConstEvalCtxt::new(self.cx)
249                        && let left = ecx.eval(left)
250                        && let right = ecx.eval(right).and_then(|c| c.int_value(self.cx.tcx, right_ty))
251                        && matches!(
252                            (left, right),
253                            // `1 / x`: x might be zero
254                            (_, None)
255                            // `x / -1`: x might be T::MIN
256                            | (None, Some(FullInt::S(-1)))
257                        ) =>
258                {
259                    self.eagerness |= NoChange;
260                },
261
262                // Similar to `>>` and `<<`, we only want to avoid linting entirely if either side is unknown and the
263                // compiler can't emit an error for an overflowing expression.
264                // Suggesting eagerness for `true.then(|| i32::MAX + 1)` is okay because the compiler will emit an
265                // error and it's good to have the eagerness warning up front when the user fixes the logic error.
266                ExprKind::Binary(op, left, right)
267                    if matches!(op.node, BinOpKind::Add | BinOpKind::Sub | BinOpKind::Mul)
268                        && !self.cx.typeck_results().expr_ty(e).is_floating_point()
269                        && let ecx = ConstEvalCtxt::new(self.cx)
270                        && (ecx.eval(left).is_none() || ecx.eval(right).is_none()) =>
271                {
272                    self.eagerness |= NoChange;
273                },
274
275                ExprKind::Binary(_, lhs, rhs)
276                    if self.cx.typeck_results().expr_ty(lhs).is_primitive()
277                        && self.cx.typeck_results().expr_ty(rhs).is_primitive() => {},
278
279                // Can't be moved into a closure
280                ExprKind::Break(..)
281                | ExprKind::Continue(_)
282                | ExprKind::Ret(_)
283                | ExprKind::Become(_)
284                | ExprKind::InlineAsm(_)
285                | ExprKind::Yield(..)
286                | ExprKind::Err(_) => {
287                    self.eagerness = ForceNoChange;
288                    return;
289                },
290
291                // Memory allocation, custom operator, loop, or call to an unknown function
292                ExprKind::Unary(..) | ExprKind::Binary(..) | ExprKind::Loop(..) | ExprKind::Call(..) => {
293                    self.eagerness = Lazy;
294                },
295
296                ExprKind::ConstBlock(_)
297                | ExprKind::Array(_)
298                | ExprKind::Tup(_)
299                | ExprKind::Use(..)
300                | ExprKind::Lit(_)
301                | ExprKind::Cast(..)
302                | ExprKind::Type(..)
303                | ExprKind::DropTemps(_)
304                | ExprKind::Let(..)
305                | ExprKind::If(..)
306                | ExprKind::Match(..)
307                | ExprKind::Closure { .. }
308                | ExprKind::Field(..)
309                | ExprKind::AddrOf(..)
310                | ExprKind::Repeat(..)
311                | ExprKind::Block(Block { stmts: [], .. }, _)
312                | ExprKind::OffsetOf(..)
313                | ExprKind::UnsafeBinderCast(..) => (),
314
315                // Assignment might be to a local defined earlier, so don't eagerly evaluate.
316                // Blocks with multiple statements might be expensive, so don't eagerly evaluate.
317                // TODO: Actually check if either of these are true here.
318                ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Block(..) => self.eagerness |= NoChange,
319            }
320            walk_expr(self, e);
321        }
322    }
323
324    let mut v = V {
325        cx,
326        eagerness: EagernessSuggestion::Eager,
327    };
328    v.visit_expr(e);
329    v.eagerness
330}
331
332/// Whether the given expression should be changed to evaluate eagerly
333pub fn switch_to_eager_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
334    expr_eagerness(cx, expr) == EagernessSuggestion::Eager
335}
336
337/// Whether the given expression should be changed to evaluate lazily
338pub fn switch_to_lazy_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
339    expr_eagerness(cx, expr) == EagernessSuggestion::Lazy
340}
clippy_utils/eager_or_lazy.rs

clippy_utils/
eager_or_lazy.rs
