Fix: Preserve panic behavior in needless_range_loop suggestions

Aadi11z · Aadi11z · commit 83d7f69dd0f9 · 2026-05-07T20:52:38.000+04:00
Preserve panic semantics in `needless_range_loop` iterator suggestions
by using slicing instead of `.take(n)` where the original range would
panic on out-of-bounds access.

Also:
- use `snippet_with_applicability` for generated slice bounds
- emit the panic-behavior note only for affected suggestions
- update UI tests accordingly
diff --git a/clippy_lints/src/loops/needless_range_loop.rs b/clippy_lints/src/loops/needless_range_loop.rs
@@ -1,6 +1,6 @@
 use super::NEEDLESS_RANGE_LOOP;
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::source::snippet;
+use clippy_utils::source::{snippet, snippet_with_applicability};
 use clippy_utils::ty::has_iter_method;
 use clippy_utils::visitors::is_local_used;
 use clippy_utils::{SpanlessEq, contains_name, higher, is_integer_const, peel_hir_expr_while, sugg};
@@ -165,10 +165,24 @@ pub(super) fn check<'tcx>(
                     },
                 );
             } else {
-                let repl = if starts_at_zero && take_is_empty {
-                    format!("&{ref_mut}{indexed}")
+                let mut applicability = Applicability::HasPlaceholders;
+                let (repl, note) = if starts_at_zero && take_is_empty {
+                    (format!("&{ref_mut}{indexed}"), None)
+                } else if !take_is_empty {
+                    // Adding condition for when 'take' is not empty Fixing `.take(n)` with slicing to preserve panic
+                    // semantics
+                    (
+                        format!(
+                            "{indexed}[..{}].{method}(){method_2}",
+                            snippet_with_applicability(cx, end.unwrap().span, "..", &mut applicability)
+                        ),
+                        Some(
+                            "this suggestion preserves panic behavior, but the panic will occur \
+                            before iteration if the upper bound exceeds the collection length",
+                        ),
+                    )
                 } else {
-                    format!("{indexed}.{method}(){method_1}{method_2}")
+                    (format!("{indexed}.{method}(){method_1}{method_2}"), None)
                 };
 
                 span_lint_and_then(
@@ -180,8 +194,11 @@ pub(super) fn check<'tcx>(
                         diag.multipart_suggestion(
                             "consider using an iterator",
                             vec![(pat.span, "<item>".to_string()), (span, repl)],
-                            Applicability::HasPlaceholders,
+                            applicability,
                         );
+                        if let Some(note) = note {
+                            diag.note(note);
+                        }
                     },
                 );
             }
diff --git a/tests/ui/needless_range_loop.stderr b/tests/ui/needless_range_loop.stderr
@@ -66,10 +66,11 @@ error: the loop variable `i` is only used to index `vec2`
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 0..vec.len() {
-LL +     for <item> in vec2.iter().take(vec.len()) {
+LL +     for <item> in vec2[..vec.len()].iter() {
    |
 
 error: the loop variable `i` is only used to index `vec`
@@ -90,10 +91,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in 0..MAX_LEN {
    |              ^^^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 0..MAX_LEN {
-LL +     for <item> in vec.iter().take(MAX_LEN) {
+LL +     for <item> in vec[..MAX_LEN].iter() {
    |
 
 error: the loop variable `i` is only used to index `vec`
@@ -102,10 +104,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in 0..=MAX_LEN {
    |              ^^^^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 0..=MAX_LEN {
-LL +     for <item> in vec.iter().take(MAX_LEN + 1) {
+LL +     for <item> in vec[..MAX_LEN].iter() {
    |
 
 error: the loop variable `i` is only used to index `vec`
@@ -114,10 +117,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in 5..10 {
    |              ^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 5..10 {
-LL +     for <item> in vec.iter().take(10).skip(5) {
+LL +     for <item> in vec[..10].iter().skip(5) {
    |
 
 error: the loop variable `i` is only used to index `vec`
@@ -126,10 +130,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in 5..=10 {
    |              ^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 5..=10 {
-LL +     for <item> in vec.iter().take(10 + 1).skip(5) {
+LL +     for <item> in vec[..10].iter().skip(5) {
    |
 
 error: the loop variable `i` is used to index `vec`
@@ -186,10 +191,11 @@ error: the loop variable `i` is only used to index `a`
 LL |     for i in 0..MAX_LEN {
    |              ^^^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 0..MAX_LEN {
-LL +     for <item> in a.iter().take(MAX_LEN) {
+LL +     for <item> in a[..MAX_LEN].iter() {
    |
 
 error: aborting due to 16 previous errors
diff --git a/tests/ui/needless_range_loop2.stderr b/tests/ui/needless_range_loop2.stderr
@@ -4,12 +4,13 @@ error: the loop variable `i` is only used to index `ns`
 LL |     for i in 3..10 {
    |              ^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
    = note: `-D clippy::needless-range-loop` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::needless_range_loop)]`
 help: consider using an iterator
    |
 LL -     for i in 3..10 {
-LL +     for <item> in ns.iter().take(10).skip(3) {
+LL +     for <item> in ns[..10].iter().skip(3) {
    |
 
 error: the loop variable `i` is only used to index `ms`
@@ -42,10 +43,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in x..x + 4 {
    |              ^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in x..x + 4 {
-LL +     for <item> in vec.iter_mut().skip(x).take(4) {
+LL +     for <item> in vec[..x + 4].iter_mut().take(4) {
    |
 
 error: the loop variable `i` is only used to index `vec`
@@ -54,10 +56,11 @@ error: the loop variable `i` is only used to index `vec`
 LL |     for i in x..=x + 4 {
    |              ^^^^^^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in x..=x + 4 {
-LL +     for <item> in vec.iter_mut().skip(x).take(4 + 1) {
+LL +     for <item> in vec[..x + 4].iter_mut().take(4 + 1) {
    |
 
 error: the loop variable `i` is only used to index `arr`
@@ -78,10 +81,11 @@ error: the loop variable `i` is only used to index `arr`
 LL |     for i in 0..2 {
    |              ^^^^
    |
+   = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length
 help: consider using an iterator
    |
 LL -     for i in 0..2 {
-LL +     for <item> in arr.iter().take(2) {
+LL +     for <item> in arr[..2].iter() {
    |
 
 error: the loop variable `i` is only used to index `arr`

Original file line number	Diff line number	Diff line change
@@ -66,10 +66,11 @@ error: the loop variable `i` is only used to index `vec2`
`66`	`66`	`LL \| for i in 0..vec.len() {`
`67`	`67`	`\| ^^^^^^^^^^^^`
`68`	`68`	`\|`
	`69`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`69`	`70`	`help: consider using an iterator`
`70`	`71`	`\|`
`71`	`72`	`LL - for i in 0..vec.len() {`
`72`		`-LL + for <item> in vec2.iter().take(vec.len()) {`
	`73`	`+LL + for <item> in vec2[..vec.len()].iter() {`
`73`	`74`	`\|`
`74`	`75`
`75`	`76`	error: the loop variable `i` is only used to index `vec`
@@ -90,10 +91,11 @@ error: the loop variable `i` is only used to index `vec`
`90`	`91`	`LL \| for i in 0..MAX_LEN {`
`91`	`92`	`\| ^^^^^^^^^^`
`92`	`93`	`\|`
	`94`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`93`	`95`	`help: consider using an iterator`
`94`	`96`	`\|`
`95`	`97`	`LL - for i in 0..MAX_LEN {`
`96`		`-LL + for <item> in vec.iter().take(MAX_LEN) {`
	`98`	`+LL + for <item> in vec[..MAX_LEN].iter() {`
`97`	`99`	`\|`
`98`	`100`
`99`	`101`	error: the loop variable `i` is only used to index `vec`
@@ -102,10 +104,11 @@ error: the loop variable `i` is only used to index `vec`
`102`	`104`	`LL \| for i in 0..=MAX_LEN {`
`103`	`105`	`\| ^^^^^^^^^^^`
`104`	`106`	`\|`
	`107`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`105`	`108`	`help: consider using an iterator`
`106`	`109`	`\|`
`107`	`110`	`LL - for i in 0..=MAX_LEN {`
`108`		`-LL + for <item> in vec.iter().take(MAX_LEN + 1) {`
	`111`	`+LL + for <item> in vec[..MAX_LEN].iter() {`
`109`	`112`	`\|`
`110`	`113`
`111`	`114`	error: the loop variable `i` is only used to index `vec`
@@ -114,10 +117,11 @@ error: the loop variable `i` is only used to index `vec`
`114`	`117`	`LL \| for i in 5..10 {`
`115`	`118`	`\| ^^^^^`
`116`	`119`	`\|`
	`120`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`117`	`121`	`help: consider using an iterator`
`118`	`122`	`\|`
`119`	`123`	`LL - for i in 5..10 {`
`120`		`-LL + for <item> in vec.iter().take(10).skip(5) {`
	`124`	`+LL + for <item> in vec[..10].iter().skip(5) {`
`121`	`125`	`\|`
`122`	`126`
`123`	`127`	error: the loop variable `i` is only used to index `vec`
@@ -126,10 +130,11 @@ error: the loop variable `i` is only used to index `vec`
`126`	`130`	`LL \| for i in 5..=10 {`
`127`	`131`	`\| ^^^^^^`
`128`	`132`	`\|`
	`133`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`129`	`134`	`help: consider using an iterator`
`130`	`135`	`\|`
`131`	`136`	`LL - for i in 5..=10 {`
`132`		`-LL + for <item> in vec.iter().take(10 + 1).skip(5) {`
	`137`	`+LL + for <item> in vec[..10].iter().skip(5) {`
`133`	`138`	`\|`
`134`	`139`
`135`	`140`	error: the loop variable `i` is used to index `vec`
@@ -186,10 +191,11 @@ error: the loop variable `i` is only used to index `a`
`186`	`191`	`LL \| for i in 0..MAX_LEN {`
`187`	`192`	`\| ^^^^^^^^^^`
`188`	`193`	`\|`
	`194`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`189`	`195`	`help: consider using an iterator`
`190`	`196`	`\|`
`191`	`197`	`LL - for i in 0..MAX_LEN {`
`192`		`-LL + for <item> in a.iter().take(MAX_LEN) {`
	`198`	`+LL + for <item> in a[..MAX_LEN].iter() {`
`193`	`199`	`\|`
`194`	`200`
`195`	`201`	`error: aborting due to 16 previous errors`
Original file line number	Diff line number	Diff line change
@@ -4,12 +4,13 @@ error: the loop variable `i` is only used to index `ns`
`4`	`4`	`LL \| for i in 3..10 {`
`5`	`5`	`\| ^^^^^`
`6`	`6`	`\|`
	`7`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`7`	`8`	= note: `-D clippy::needless-range-loop` implied by `-D warnings`
`8`	`9`	= help: to override `-D warnings` add `#[allow(clippy::needless_range_loop)]`
`9`	`10`	`help: consider using an iterator`
`10`	`11`	`\|`
`11`	`12`	`LL - for i in 3..10 {`
`12`		`-LL + for <item> in ns.iter().take(10).skip(3) {`
	`13`	`+LL + for <item> in ns[..10].iter().skip(3) {`
`13`	`14`	`\|`
`14`	`15`
`15`	`16`	error: the loop variable `i` is only used to index `ms`
@@ -42,10 +43,11 @@ error: the loop variable `i` is only used to index `vec`
`42`	`43`	`LL \| for i in x..x + 4 {`
`43`	`44`	`\| ^^^^^^^^`
`44`	`45`	`\|`
	`46`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`45`	`47`	`help: consider using an iterator`
`46`	`48`	`\|`
`47`	`49`	`LL - for i in x..x + 4 {`
`48`		`-LL + for <item> in vec.iter_mut().skip(x).take(4) {`
	`50`	`+LL + for <item> in vec[..x + 4].iter_mut().take(4) {`
`49`	`51`	`\|`
`50`	`52`
`51`	`53`	error: the loop variable `i` is only used to index `vec`
@@ -54,10 +56,11 @@ error: the loop variable `i` is only used to index `vec`
`54`	`56`	`LL \| for i in x..=x + 4 {`
`55`	`57`	`\| ^^^^^^^^^`
`56`	`58`	`\|`
	`59`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`57`	`60`	`help: consider using an iterator`
`58`	`61`	`\|`
`59`	`62`	`LL - for i in x..=x + 4 {`
`60`		`-LL + for <item> in vec.iter_mut().skip(x).take(4 + 1) {`
	`63`	`+LL + for <item> in vec[..x + 4].iter_mut().take(4 + 1) {`
`61`	`64`	`\|`
`62`	`65`
`63`	`66`	error: the loop variable `i` is only used to index `arr`
@@ -78,10 +81,11 @@ error: the loop variable `i` is only used to index `arr`
`78`	`81`	`LL \| for i in 0..2 {`
`79`	`82`	`\| ^^^^`
`80`	`83`	`\|`
	`84`	`+ = note: this suggestion preserves panic behavior, but the panic will occur before iteration if the upper bound exceeds the collection length`
`81`	`85`	`help: consider using an iterator`
`82`	`86`	`\|`
`83`	`87`	`LL - for i in 0..2 {`
`84`		`-LL + for <item> in arr.iter().take(2) {`
	`88`	`+LL + for <item> in arr[..2].iter() {`
`85`	`89`	`\|`
`86`	`90`
`87`	`91`	error: the loop variable `i` is only used to index `arr`