clippy_utils/eager_or_lazy.rs
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//! Utilities for evaluating whether eagerly evaluated expressions can be made lazy and vice versa.
//!
//! Things to consider:
//! - does the expression have side-effects?
//! - is the expression computationally expensive?
//!
//! See lints:
//! - unnecessary-lazy-evaluations
//! - or-fun-call
//! - option-if-let-else
use crate::consts::{ConstEvalCtxt, FullInt};
use crate::ty::{all_predicates_of, is_copy};
use crate::visitors::is_const_evaluatable;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{Visitor, walk_expr};
use rustc_hir::{BinOpKind, Block, Expr, ExprKind, QPath, UnOp};
use rustc_lint::LateContext;
use rustc_middle::ty;
use rustc_middle::ty::adjustment::Adjust;
use rustc_span::{Symbol, sym};
use std::{cmp, ops};
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
enum EagernessSuggestion {
// The expression is cheap and should be evaluated eagerly
Eager,
// The expression may be cheap, so don't suggested lazy evaluation; or the expression may not be safe to switch to
// eager evaluation.
NoChange,
// The expression is likely expensive and should be evaluated lazily.
Lazy,
// The expression cannot be placed into a closure.
ForceNoChange,
}
impl ops::BitOr for EagernessSuggestion {
type Output = Self;
fn bitor(self, rhs: Self) -> Self {
cmp::max(self, rhs)
}
}
impl ops::BitOrAssign for EagernessSuggestion {
fn bitor_assign(&mut self, rhs: Self) {
*self = *self | rhs;
}
}
/// Determine the eagerness of the given function call.
fn fn_eagerness(cx: &LateContext<'_>, fn_id: DefId, name: Symbol, have_one_arg: bool) -> EagernessSuggestion {
use EagernessSuggestion::{Eager, Lazy, NoChange};
let name = name.as_str();
let ty = match cx.tcx.impl_of_method(fn_id) {
Some(id) => cx.tcx.type_of(id).instantiate_identity(),
None => return Lazy,
};
if (name.starts_with("as_") || name == "len" || name == "is_empty") && have_one_arg {
if matches!(
cx.tcx.crate_name(fn_id.krate),
sym::std | sym::core | sym::alloc | sym::proc_macro
) {
Eager
} else {
NoChange
}
} else if let ty::Adt(def, subs) = ty.kind() {
// Types where the only fields are generic types (or references to) with no trait bounds other
// than marker traits.
// Due to the limited operations on these types functions should be fairly cheap.
if def.variants().iter().flat_map(|v| v.fields.iter()).any(|x| {
matches!(
cx.tcx.type_of(x.did).instantiate_identity().peel_refs().kind(),
ty::Param(_)
)
}) && all_predicates_of(cx.tcx, fn_id).all(|(pred, _)| match pred.kind().skip_binder() {
ty::ClauseKind::Trait(pred) => cx.tcx.trait_def(pred.trait_ref.def_id).is_marker,
_ => true,
}) && subs.types().all(|x| matches!(x.peel_refs().kind(), ty::Param(_)))
{
// Limit the function to either `(self) -> bool` or `(&self) -> bool`
match &**cx
.tcx
.fn_sig(fn_id)
.instantiate_identity()
.skip_binder()
.inputs_and_output
{
[arg, res] if !arg.is_mutable_ptr() && arg.peel_refs() == ty && res.is_bool() => NoChange,
_ => Lazy,
}
} else {
Lazy
}
} else {
Lazy
}
}
fn res_has_significant_drop(res: Res, cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
if let Res::Def(DefKind::Ctor(..) | DefKind::Variant | DefKind::Enum | DefKind::Struct, _)
| Res::SelfCtor(_)
| Res::SelfTyAlias { .. } = res
{
cx.typeck_results()
.expr_ty(e)
.has_significant_drop(cx.tcx, cx.typing_env())
} else {
false
}
}
#[expect(clippy::too_many_lines)]
fn expr_eagerness<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> EagernessSuggestion {
struct V<'cx, 'tcx> {
cx: &'cx LateContext<'tcx>,
eagerness: EagernessSuggestion,
}
impl<'tcx> Visitor<'tcx> for V<'_, 'tcx> {
fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
use EagernessSuggestion::{ForceNoChange, Lazy, NoChange};
if self.eagerness == ForceNoChange {
return;
}
// Autoderef through a user-defined `Deref` impl can have side-effects,
// so don't suggest changing it.
if self
.cx
.typeck_results()
.expr_adjustments(e)
.iter()
.any(|adj| matches!(adj.kind, Adjust::Deref(Some(_))))
{
self.eagerness |= NoChange;
return;
}
match e.kind {
ExprKind::Call(
&Expr {
kind: ExprKind::Path(ref path),
hir_id,
..
},
args,
) => match self.cx.qpath_res(path, hir_id) {
res @ (Res::Def(DefKind::Ctor(..) | DefKind::Variant, _) | Res::SelfCtor(_)) => {
if res_has_significant_drop(res, self.cx, e) {
self.eagerness = ForceNoChange;
return;
}
},
Res::Def(_, id) if self.cx.tcx.is_promotable_const_fn(id) => (),
// No need to walk the arguments here, `is_const_evaluatable` already did
Res::Def(..) if is_const_evaluatable(self.cx, e) => {
self.eagerness |= NoChange;
return;
},
Res::Def(_, id) => match path {
QPath::Resolved(_, p) => {
self.eagerness |=
fn_eagerness(self.cx, id, p.segments.last().unwrap().ident.name, !args.is_empty());
},
QPath::TypeRelative(_, name) => {
self.eagerness |= fn_eagerness(self.cx, id, name.ident.name, !args.is_empty());
},
QPath::LangItem(..) => self.eagerness = Lazy,
},
_ => self.eagerness = Lazy,
},
// No need to walk the arguments here, `is_const_evaluatable` already did
ExprKind::MethodCall(..) if is_const_evaluatable(self.cx, e) => {
self.eagerness |= NoChange;
return;
},
#[expect(clippy::match_same_arms)] // arm pattern can't be merged due to `ref`, see rust#105778
ExprKind::Struct(path, ..) => {
if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
self.eagerness = ForceNoChange;
return;
}
},
ExprKind::Path(ref path) => {
if res_has_significant_drop(self.cx.qpath_res(path, e.hir_id), self.cx, e) {
self.eagerness = ForceNoChange;
return;
}
},
ExprKind::MethodCall(name, ..) => {
self.eagerness |= self
.cx
.typeck_results()
.type_dependent_def_id(e.hir_id)
.map_or(Lazy, |id| fn_eagerness(self.cx, id, name.ident.name, true));
},
ExprKind::Index(_, e, _) => {
let ty = self.cx.typeck_results().expr_ty_adjusted(e);
if is_copy(self.cx, ty) && !ty.is_ref() {
self.eagerness |= NoChange;
} else {
self.eagerness = Lazy;
}
},
// `-i32::MIN` panics with overflow checks
ExprKind::Unary(UnOp::Neg, right) if ConstEvalCtxt::new(self.cx).eval(right).is_none() => {
self.eagerness |= NoChange;
},
// Custom `Deref` impl might have side effects
ExprKind::Unary(UnOp::Deref, e)
if self.cx.typeck_results().expr_ty(e).builtin_deref(true).is_none() =>
{
self.eagerness |= NoChange;
},
// Dereferences should be cheap, but dereferencing a raw pointer earlier may not be safe.
ExprKind::Unary(UnOp::Deref, e) if !self.cx.typeck_results().expr_ty(e).is_unsafe_ptr() => (),
ExprKind::Unary(UnOp::Deref, _) => self.eagerness |= NoChange,
ExprKind::Unary(_, e)
if matches!(
self.cx.typeck_results().expr_ty(e).kind(),
ty::Bool | ty::Int(_) | ty::Uint(_),
) => {},
// `>>` and `<<` panic when the right-hand side is greater than or equal to the number of bits in the
// type of the left-hand side, or is negative.
// We intentionally only check if the right-hand isn't a constant, because even if the suggestion would
// overflow with constants, the compiler emits an error for it and the programmer will have to fix it.
// Thus, we would realistically only delay the lint.
ExprKind::Binary(op, _, right)
if matches!(op.node, BinOpKind::Shl | BinOpKind::Shr)
&& ConstEvalCtxt::new(self.cx).eval(right).is_none() =>
{
self.eagerness |= NoChange;
},
ExprKind::Binary(op, left, right)
if matches!(op.node, BinOpKind::Div | BinOpKind::Rem)
&& let right_ty = self.cx.typeck_results().expr_ty(right)
&& let ecx = ConstEvalCtxt::new(self.cx)
&& let left = ecx.eval(left)
&& let right = ecx.eval(right).and_then(|c| c.int_value(self.cx.tcx, right_ty))
&& matches!(
(left, right),
// `1 / x`: x might be zero
(_, None)
// `x / -1`: x might be T::MIN
| (None, Some(FullInt::S(-1)))
) =>
{
self.eagerness |= NoChange;
},
// Similar to `>>` and `<<`, we only want to avoid linting entirely if either side is unknown and the
// compiler can't emit an error for an overflowing expression.
// Suggesting eagerness for `true.then(|| i32::MAX + 1)` is okay because the compiler will emit an
// error and it's good to have the eagerness warning up front when the user fixes the logic error.
ExprKind::Binary(op, left, right)
if matches!(op.node, BinOpKind::Add | BinOpKind::Sub | BinOpKind::Mul)
&& !self.cx.typeck_results().expr_ty(e).is_floating_point()
&& let ecx = ConstEvalCtxt::new(self.cx)
&& (ecx.eval(left).is_none() || ecx.eval(right).is_none()) =>
{
self.eagerness |= NoChange;
},
ExprKind::Binary(_, lhs, rhs)
if self.cx.typeck_results().expr_ty(lhs).is_primitive()
&& self.cx.typeck_results().expr_ty(rhs).is_primitive() => {},
// Can't be moved into a closure
ExprKind::Break(..)
| ExprKind::Continue(_)
| ExprKind::Ret(_)
| ExprKind::Become(_)
| ExprKind::InlineAsm(_)
| ExprKind::Yield(..)
| ExprKind::Err(_) => {
self.eagerness = ForceNoChange;
return;
},
// Memory allocation, custom operator, loop, or call to an unknown function
ExprKind::Unary(..) | ExprKind::Binary(..) | ExprKind::Loop(..) | ExprKind::Call(..) => {
self.eagerness = Lazy;
},
ExprKind::ConstBlock(_)
| ExprKind::Array(_)
| ExprKind::Tup(_)
| ExprKind::Lit(_)
| ExprKind::Cast(..)
| ExprKind::Type(..)
| ExprKind::DropTemps(_)
| ExprKind::Let(..)
| ExprKind::If(..)
| ExprKind::Match(..)
| ExprKind::Closure { .. }
| ExprKind::Field(..)
| ExprKind::AddrOf(..)
| ExprKind::Repeat(..)
| ExprKind::Block(Block { stmts: [], .. }, _)
| ExprKind::OffsetOf(..) => (),
// Assignment might be to a local defined earlier, so don't eagerly evaluate.
// Blocks with multiple statements might be expensive, so don't eagerly evaluate.
// TODO: Actually check if either of these are true here.
ExprKind::Assign(..) | ExprKind::AssignOp(..) | ExprKind::Block(..) => self.eagerness |= NoChange,
}
walk_expr(self, e);
}
}
let mut v = V {
cx,
eagerness: EagernessSuggestion::Eager,
};
v.visit_expr(e);
v.eagerness
}
/// Whether the given expression should be changed to evaluate eagerly
pub fn switch_to_eager_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
expr_eagerness(cx, expr) == EagernessSuggestion::Eager
}
/// Whether the given expression should be changed to evaluate lazily
pub fn switch_to_lazy_eval<'tcx>(cx: &'_ LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
expr_eagerness(cx, expr) == EagernessSuggestion::Lazy
}