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Merged
rust-bors merged 2 commits into from
Mar 18, 2026
Merged

Remove string content from panics #153677

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05ac9d4
Remove string content from panics
ConradIrwin Mar 10, 2026
7ec050a
Fix tests and tidy up wtf-8 failures
ConradIrwin Mar 11, 2026
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28 changes: 10 additions & 18 deletions library/alloctests/tests/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -612,14 +612,14 @@ mod slice_index {
data: "abcdef";
good: data[4..4] == "";
bad: data[4..3];
message: "begin > end (4 > 3)";
message: "byte range starts at 4 but ends at 3";
}

in mod rangeinclusive_neg_width {
data: "abcdef";
good: data[4..=3] == "";
bad: data[4..=2];
message: "begin > end (4 > 3)";
message: "byte range starts at 4 but ends at 3";
}
}

Expand Down Expand Up @@ -659,49 +659,49 @@ mod slice_index {
data: super::DATA;
bad: data[super::BAD_START..super::GOOD_END];
message:
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of";
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5 of string)";
}

in mod range_2 {
data: super::DATA;
bad: data[super::GOOD_START..super::BAD_END];
message:
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7) of";
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7 of string)";
}

in mod rangefrom {
data: super::DATA;
bad: data[super::BAD_START..];
message:
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of";
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5 of string)";
}

in mod rangeto {
data: super::DATA;
bad: data[..super::BAD_END];
message:
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7) of";
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7 of string)";
}

in mod rangeinclusive_1 {
data: super::DATA;
bad: data[super::BAD_START..=super::GOOD_END_INCL];
message:
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of";
"start byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5 of string)";
}

in mod rangeinclusive_2 {
data: super::DATA;
bad: data[super::GOOD_START..=super::BAD_END_INCL];
message:
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7) of";
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7 of string)";
}

in mod rangetoinclusive {
data: super::DATA;
bad: data[..=super::BAD_END_INCL];
message:
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7) of";
"end byte index 6 is not a char boundary; it is inside 'β' (bytes 5..7 of string)";
}
}
}
Expand All @@ -716,18 +716,10 @@ mod slice_index {

// check the panic includes the prefix of the sliced string
#[test]
#[should_panic(
expected = "end byte index 1024 is out of bounds of `Lorem ipsum dolor sit amet"
)]
#[should_panic(expected = "end byte index 1024 is out of bounds for string of length 416")]
fn test_slice_fail_truncated_1() {
let _ = &LOREM_PARAGRAPH[..1024];
}
// check the truncation in the panic message
#[test]
#[should_panic(expected = "luctus, im`[...]")]
fn test_slice_fail_truncated_2() {
let _ = &LOREM_PARAGRAPH[..1024];
}
}

#[test]
Expand Down
16 changes: 6 additions & 10 deletions library/core/src/str/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -81,25 +81,21 @@ const fn slice_error_fail_ct(_: &str, _: usize, _: usize) -> ! {

#[track_caller]
fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
const MAX_DISPLAY_LENGTH: usize = 256;
let trunc_len = s.floor_char_boundary(MAX_DISPLAY_LENGTH);
let s_trunc = &s[..trunc_len];
let ellipsis = if trunc_len < s.len() { "[...]" } else { "" };
let len = s.len();

// 1. begin is OOB.
if begin > len {
panic!("start byte index {begin} is out of bounds of `{s_trunc}`{ellipsis}");
panic!("start byte index {begin} is out of bounds for string of length {len}");
}

// 2. end is OOB.
if end > len {
panic!("end byte index {end} is out of bounds of `{s_trunc}`{ellipsis}");
panic!("end byte index {end} is out of bounds for string of length {len}");
}

// 3. range is backwards.
if begin > end {
panic!("begin > end ({begin} > {end}) when slicing `{s_trunc}`{ellipsis}")
panic!("byte range starts at {begin} but ends at {end}");
}

// 4. begin is inside a character.
Expand All @@ -109,7 +105,7 @@ fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
let range = floor..ceil;
let ch = s[floor..ceil].chars().next().unwrap();
panic!(
"start byte index {begin} is not a char boundary; it is inside {ch:?} (bytes {range:?}) of `{s_trunc}`{ellipsis}"
"start byte index {begin} is not a char boundary; it is inside {ch:?} (bytes {range:?} of string)"
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@RalfJung RalfJung Mar 11, 2026 •
edited
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Suggested change
"start byte index {begin} is not a char boundary; it is inside {ch:?} (bytes {range:?} of string)"
"start byte index {begin} is not a char boundary; it is inside {ch:?} (bytes {range:?})"

"of string" sounds odd to me -- is it needed? (Same for similar messages)

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@ConradIrwin ConradIrwin Mar 19, 2026

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I like including the word string because the problem is due to mis-use of a str/String API, which may not otherwise be obvious

)
}

Expand All @@ -120,15 +116,15 @@ fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
let range = floor..ceil;
let ch = s[floor..ceil].chars().next().unwrap();
panic!(
"end byte index {end} is not a char boundary; it is inside {ch:?} (bytes {range:?}) of `{s_trunc}`{ellipsis}"
"end byte index {end} is not a char boundary; it is inside {ch:?} (bytes {range:?} of string)"
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@TKanX TKanX Mar 11, 2026

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"of string" sounds odd to me -- is it needed? (Same for similar messages)
@RalfJung

Suggested change
"end byte index {end} is not a char boundary; it is inside {ch:?} (bytes {range:?} of string)"
"end byte index {end} is not a char boundary; it is inside {ch:?} (bytes {range:?})"

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@TKanX TKanX Mar 11, 2026

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1, 2, 6 describe the type so not redundant right?

)
}

// 6. end is OOB and range is inclusive (end == len).
// This test cannot be combined with 2. above because for cases like
// `"abcαβγ"[4..9]` the error is that 4 is inside 'α', not that 9 is OOB.
debug_assert_eq!(end, len);
panic!("end byte index {end} is out of bounds of `{s_trunc}`{ellipsis}");
panic!("end byte index {end} is out of bounds for string of length {len}");
}

impl str {
Expand Down
17 changes: 14 additions & 3 deletions library/core/src/wtf8.rs
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Expand Up @@ -484,11 +484,22 @@ unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
}
}

/// Copied from core::str::raw::slice_error_fail
#[inline(never)]
fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {

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@TKanX TKanX Mar 11, 2026

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warning: unused variable: `s`
     --> library/core/src/wtf8.rs:488:21
      |
  488 | fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
      |                     ^ help: if this is intentional, prefix it with an underscore: `_s`

@ConradIrwin

assert!(begin <= end);
panic!("index {begin} and/or {end} in `{s:?}` do not lie on character boundary");
let len = s.len();
if begin > len {
panic!("start byte index {begin} is out of bounds for string of length {len}");
}
if end > len {
panic!("end byte index {end} is out of bounds for string of length {len}");
}
if begin > end {
panic!("byte range starts at {begin} but ends at {end}");
}
if !s.is_code_point_boundary(begin) {
panic!("byte index {begin} is not a code point boundary");
}
panic!("byte index {end} is not a code point boundary");
Comment on lines -490 to +502
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@joshtriplett joshtriplett Mar 17, 2026

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🎉 This looks like a huge improvement to usability.

}
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/// Iterator for the code points of a WTF-8 string.
Expand Down
2 changes: 1 addition & 1 deletion tests/ui/intrinsics/const-eval-select-backtrace-std.run.stderr
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@@ -1,4 +1,4 @@

thread 'main' ($TID) panicked at $DIR/const-eval-select-backtrace-std.rs:6:8:
start byte index 1 is out of bounds of ``
start byte index 1 is out of bounds for string of length 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
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