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unnecessary_fold improvements #13475

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705ba03
Add lints for associated function variants of addition and multiplica…
viliml Sep 28, 2024
882e37c
Simplify operator function matching
viliml Sep 28, 2024
19f11b6
Add tests
viliml Sep 28, 2024
895a0f9
Improve comments
viliml Sep 28, 2024
966dadc
Run rustfmt
viliml Sep 28, 2024
b474275
Fix turbofish tests to do what they say they do, and add negative var…
viliml Sep 28, 2024
1c2870f
Update clippy_lints/src/methods/unnecessary_fold.rs
viliml Sep 4, 2025
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61 changes: 59 additions & 2 deletions clippy_lints/src/methods/unnecessary_fold.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ use rustc_hir as hir;
use rustc_hir::PatKind;
use rustc_lint::LateContext;
use rustc_middle::ty;
use rustc_span::{Span, sym};
use rustc_span::{Span, Symbol, sym};

use super::UNNECESSARY_FOLD;

Expand Down Expand Up @@ -40,6 +40,19 @@ fn needs_turbofish(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
return false;
}

// - the final expression in the body of a function with a simple return type
if let hir::Node::Block(block) = parent
&& let grandparent = cx.tcx.parent_hir_node(block.hir_id)
&& let hir::Node::Expr(grandparent_expr) = grandparent
&& let ggparent = cx.tcx.parent_hir_node(grandparent_expr.hir_id)
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@Centri3 Centri3 Nov 3, 2024

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Probably better to iterate over the parents? is there any reason simply getting the containing item doesn't work?

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@viliml viliml Sep 4, 2025

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I don't know, I don't understand this infrastructure so well so my first try was probably not the best way to do it.

&& let hir::Node::Item(enclosing_item) = ggparent
&& let hir::ItemKind::Fn(fn_sig, _, _) = enclosing_item.kind
&& let hir::FnRetTy::Return(fn_return_ty) = fn_sig.decl.output
&& matches!(fn_return_ty.kind, hir::TyKind::Path(..))
{
return false;
}

// if it's neither of those, stay on the safe side and suggest turbofish,
// even if it could work!
true
Expand Down Expand Up @@ -80,7 +93,7 @@ fn check_fold_with_op(
let mut applicability = Applicability::MachineApplicable;

let turbofish = if replacement.has_generic_return {
format!("::<{}>", cx.typeck_results().expr_ty_adjusted(right_expr).peel_refs())
format!("::<{}>", cx.typeck_results().expr_ty(expr))
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@Centri3 Centri3 Nov 3, 2024

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Does this change make any difference? More specifically was the original code unnecessary

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@viliml viliml Sep 4, 2025

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I think I did it to be consistent with the new function I made, where there is no right_expr. And it is more straightforward and sure to work, as the type argument to sum and product is exactly the return type, which is the type of the whole expression.

} else {
String::new()
};
Expand All @@ -107,6 +120,40 @@ fn check_fold_with_op(
}
}

fn check_fold_with_fn(
cx: &LateContext<'_>,
expr: &hir::Expr<'_>,
acc: &hir::Expr<'_>,
fold_span: Span,
op: Symbol,
replacement: Replacement,
) {
if let hir::ExprKind::Path(hir::QPath::Resolved(None, p)) = acc.kind
// Extract the name of the function passed to fold
&& let hir::def::Res::Def(hir::def::DefKind::AssocFn, fn_did) = p.res
// Check if the function belongs to the operator
&& cx.tcx.is_diagnostic_item(op, fn_did)
{
let applicability = Applicability::MachineApplicable;

let turbofish = if replacement.has_generic_return {
format!("::<{}>", cx.typeck_results().expr_ty(expr))
} else {
String::new()
};

span_lint_and_sugg(
cx,
UNNECESSARY_FOLD,
fold_span.with_hi(expr.span.hi()),
"this `.fold` can be written more succinctly using another method",
"try",
format!("{method}{turbofish}()", method = replacement.method_name,),
applicability,
);
}
}

pub(super) fn check(
cx: &LateContext<'_>,
expr: &hir::Expr<'_>,
Expand Down Expand Up @@ -142,13 +189,23 @@ pub(super) fn check(
has_generic_return: needs_turbofish(cx, expr),
method_name: "sum",
});
check_fold_with_fn(cx, expr, acc, fold_span, sym::add, Replacement {
has_args: false,
has_generic_return: needs_turbofish(cx, expr),
method_name: "sum",
});
},
ast::LitKind::Int(Pu128(1), _) => {
check_fold_with_op(cx, expr, acc, fold_span, hir::BinOpKind::Mul, Replacement {
has_args: false,
has_generic_return: needs_turbofish(cx, expr),
method_name: "product",
});
check_fold_with_fn(cx, expr, acc, fold_span, sym::mul, Replacement {
has_args: false,
has_generic_return: needs_turbofish(cx, expr),
method_name: "product",
});
},
_ => (),
}
Expand Down
24 changes: 24 additions & 0 deletions tests/ui/unnecessary_fold.fixed
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Original file line number Diff line number Diff line change
Expand Up @@ -14,8 +14,10 @@ fn unnecessary_fold() {
let _ = (0..3).all(|x| x > 2);
// Can be replaced by .sum
let _: i32 = (0..3).sum();
let _: i32 = (0..3).sum();
// Can be replaced by .product
let _: i32 = (0..3).product();
let _: i32 = (0..3).product();
}

/// Should trigger the `UNNECESSARY_FOLD` lint, with an error span including exactly `.fold(...)`
Expand Down Expand Up @@ -56,6 +58,7 @@ fn unnecessary_fold_over_multiple_lines() {
fn issue10000() {
use std::collections::HashMap;
use std::hash::BuildHasher;
use std::ops::{Add, Mul};

fn anything<T>(_: T) {}
fn num(_: i32) {}
Expand All @@ -65,16 +68,37 @@ fn issue10000() {

// more cases:
let _ = map.values().sum::<i32>();
let _ = map.values().sum::<i32>();
let _ = map.values().product::<i32>();
let _ = map.values().product::<i32>();
let _: i32 = map.values().sum();
let _: i32 = map.values().sum();
let _: i32 = map.values().product();
let _: i32 = map.values().product();
anything(map.values().sum::<i32>());
anything(map.values().sum::<i32>());
anything(map.values().product::<i32>());
anything(map.values().product::<i32>());
num(map.values().sum());
num(map.values().sum());
num(map.values().product());
num(map.values().product());
}

smoketest_map(HashMap::new());

fn add_turbofish_not_necessary() -> i32 {
(0..3).sum()
}
fn mul_turbofish_not_necessary() -> i32 {
(0..3).product()
}
fn add_turbofish_necessary() -> impl Add {
(0..3).sum::<i32>()
}
fn mul_turbofish_necessary() -> impl Mul {
(0..3).product::<i32>()
}
}

fn main() {}
24 changes: 24 additions & 0 deletions tests/ui/unnecessary_fold.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,8 +14,10 @@ fn unnecessary_fold() {
let _ = (0..3).fold(true, |acc, x| acc && x > 2);
// Can be replaced by .sum
let _: i32 = (0..3).fold(0, |acc, x| acc + x);
let _: i32 = (0..3).fold(0, std::ops::Add::add);
// Can be replaced by .product
let _: i32 = (0..3).fold(1, |acc, x| acc * x);
let _: i32 = (0..3).fold(1, std::ops::Mul::mul);
}

/// Should trigger the `UNNECESSARY_FOLD` lint, with an error span including exactly `.fold(...)`
Expand Down Expand Up @@ -56,6 +58,7 @@ fn unnecessary_fold_over_multiple_lines() {
fn issue10000() {
use std::collections::HashMap;
use std::hash::BuildHasher;
use std::ops::{Add, Mul};

fn anything<T>(_: T) {}
fn num(_: i32) {}
Expand All @@ -65,16 +68,37 @@ fn issue10000() {

// more cases:
let _ = map.values().fold(0, |x, y| x + y);
let _ = map.values().fold(0, Add::add);
let _ = map.values().fold(1, |x, y| x * y);
let _ = map.values().fold(1, Mul::mul);
let _: i32 = map.values().fold(0, |x, y| x + y);
let _: i32 = map.values().fold(0, Add::add);
let _: i32 = map.values().fold(1, |x, y| x * y);
let _: i32 = map.values().fold(1, Mul::mul);
anything(map.values().fold(0, |x, y| x + y));
anything(map.values().fold(0, Add::add));
anything(map.values().fold(1, |x, y| x * y));
anything(map.values().fold(1, Mul::mul));
num(map.values().fold(0, |x, y| x + y));
num(map.values().fold(0, Add::add));
num(map.values().fold(1, |x, y| x * y));
num(map.values().fold(1, Mul::mul));
}

smoketest_map(HashMap::new());

fn add_turbofish_not_necessary() -> i32 {
(0..3).fold(0, |acc, x| acc + x)
}
fn mul_turbofish_not_necessary() -> i32 {
(0..3).fold(1, |acc, x| acc * x)
}
fn add_turbofish_necessary() -> impl Add {
(0..3).fold(0, |acc, x| acc + x)
}
fn mul_turbofish_necessary() -> impl Mul {
(0..3).fold(1, |acc, x| acc * x)
}
}

fn main() {}
110 changes: 97 additions & 13 deletions tests/ui/unnecessary_fold.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -29,76 +29,160 @@ LL | let _: i32 = (0..3).fold(0, |acc, x| acc + x);
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:18:25
--> tests/ui/unnecessary_fold.rs:17:25
|
LL | let _: i32 = (0..3).fold(0, std::ops::Add::add);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:19:25
|
LL | let _: i32 = (0..3).fold(1, |acc, x| acc * x);
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:23:41
--> tests/ui/unnecessary_fold.rs:20:25
|
LL | let _: i32 = (0..3).fold(1, std::ops::Mul::mul);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:25:41
|
LL | let _: bool = (0..3).map(|x| 2 * x).fold(false, |acc, x| acc || x > 2);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `any(|x| x > 2)`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:53:10
--> tests/ui/unnecessary_fold.rs:55:10
|
LL | .fold(false, |acc, x| acc || x > 2);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `any(|x| x > 2)`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:64:33
--> tests/ui/unnecessary_fold.rs:67:33
|
LL | assert_eq!(map.values().fold(0, |x, y| x + y), 0);
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:67:30
--> tests/ui/unnecessary_fold.rs:70:30
|
LL | let _ = map.values().fold(0, |x, y| x + y);
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:68:30
--> tests/ui/unnecessary_fold.rs:71:30
|
LL | let _ = map.values().fold(0, Add::add);
| ^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:72:30
|
LL | let _ = map.values().fold(1, |x, y| x * y);
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:69:35
--> tests/ui/unnecessary_fold.rs:73:30
|
LL | let _ = map.values().fold(1, Mul::mul);
| ^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:74:35
|
LL | let _: i32 = map.values().fold(0, |x, y| x + y);
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:70:35
--> tests/ui/unnecessary_fold.rs:75:35
|
LL | let _: i32 = map.values().fold(0, Add::add);
| ^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:76:35
|
LL | let _: i32 = map.values().fold(1, |x, y| x * y);
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:71:31
--> tests/ui/unnecessary_fold.rs:77:35
|
LL | let _: i32 = map.values().fold(1, Mul::mul);
| ^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:78:31
|
LL | anything(map.values().fold(0, |x, y| x + y));
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:72:31
--> tests/ui/unnecessary_fold.rs:79:31
|
LL | anything(map.values().fold(0, Add::add));
| ^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:80:31
|
LL | anything(map.values().fold(1, |x, y| x * y));
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:73:26
--> tests/ui/unnecessary_fold.rs:81:31
|
LL | anything(map.values().fold(1, Mul::mul));
| ^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:82:26
|
LL | num(map.values().fold(0, |x, y| x + y));
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:74:26
--> tests/ui/unnecessary_fold.rs:83:26
|
LL | num(map.values().fold(0, Add::add));
| ^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:84:26
|
LL | num(map.values().fold(1, |x, y| x * y));
| ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`

error: aborting due to 16 previous errors
error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:85:26
|
LL | num(map.values().fold(1, Mul::mul));
| ^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:91:16
|
LL | (0..3).fold(0, |acc, x| acc + x)
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:94:16
|
LL | (0..3).fold(1, |acc, x| acc * x)
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:97:16
|
LL | (0..3).fold(0, |acc, x| acc + x)
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`

error: this `.fold` can be written more succinctly using another method
--> tests/ui/unnecessary_fold.rs:100:16
|
LL | (0..3).fold(1, |acc, x| acc * x)
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`

error: aborting due to 30 previous errors