Use more specific panic message for &str slicing errors #38066

bluss · 2016-11-29T03:14:58Z

Separate out of bounds errors from character boundary errors, and print
more details for character boundary errors.

It reports the first error it finds in:

begin out of bounds
end out of bounds
begin <= end violated
begin not char boundary
end not char boundary.

Example:

&"abcαβγ"[..4]

thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not
a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`'

Fixes #38052

rust-highfive · 2016-11-29T03:15:10Z

r? @sfackler

(rust_highfive has picked a reviewer for you, use r? to override)

durka · 2016-11-29T17:59:02Z

Should we show all the problems instead of just the first one?

durka · 2016-11-29T18:07:20Z

src/libcore/str/mod.rs

    let ellipsis = if truncated { "[...]" } else { "" };

+    // 1. out of bounds
+    let index = if !s.is_char_boundary(begin) { begin } else { end };


This is confusing to me. It seems like you choose which index is going to be in the message before checking which one is out of bounds.

is_char_boundary is false if the index is out of bounds.

ok, it should be confusing, because it's not really right.

durka · 2016-11-29T18:11:01Z

src/libcore/str/mod.rs

+    while !s.is_char_boundary(char_start) {
+        char_start -= 1;
+    }
+    let ch = s[char_start..].chars().next().unwrap();


This can panic if the string is empty or if char_start is at the end. I think I've convinced myself this can't happen, but at least a comment would help, or just to be defensive

if let Some(ch) = s[char_start..].chars().next() { let char_range = char_start .. char_start + ch.len_utf8(); panic!("index {} is not a char boundary; it is inside {:?} (bytes {:?}) of `{}`{}", index, ch, char_range, s_trunc, ellipsis); } else { panic!("index {} is not a char boundary in `{}`{}", index, s_trunc, ellipsis); }

char_start is a character boundary, and then using it for slicing is ok. It could not be larger than len, that case has already been handled. It could not be equal to len, then it would not be an error in the first place.

durka · 2016-11-29T18:15:42Z

src/libcore/str/mod.rs

+    while !s.is_char_boundary(char_start) {
+        char_start -= 1;
+    }
+    let ch = s[char_start..].chars().next().unwrap();


Also, if the problematic index is beyond MAX_DISPLAY_LENGTH, the preview isn't going to be very useful. It could instead show the region of the string from max(0, index - MAX_DISPLAY_LENGTH/2) .. min(s.len(), index + MAX_DISPLAY_LENGTH/2) with ellipses.

I feel like this is unnecessary, it just wants to identify the string somehow.

killercup

Nice work! Handling the different cases with custom messages makes this much better.

As mentioned in #38052 I'd like to try and convey that indexing a string (slice) is done using byte offsets -- and not character offsets. I think this is one of those messages people will see this in failing tests and then have a hard time recognizing why this went wrong; and then have an even harder time to find a solution like the unicode-segmentation crate. UTF-8 is hard when people don't think about it.

I know putting all that in the panic message is not a solution; but maybe you can change it to "slicing at byte 4 which is not a char boundary" or something like that?

(By the way, is there a lint to catch statically known wrong string slices?)

killercup · 2016-11-29T19:40:12Z

src/libcore/str/mod.rs

+    // 1. out of bounds
+    if begin > s.len() || end > s.len() {
+        let oob_index = if begin > s.len() { begin } else { end };
+        panic!("index {} is out of bounds of `{}`{}", oob_index, s_trunc, ellipsis);


Can you add a test for this as well?

I'll change the existing test to match more of the panic message

durka · 2016-11-29T20:05:01Z

@killercup maybe just s/index/byte index/ in the panic messages.

bluss · 2016-11-29T20:14:32Z

I like byte index or something like that. I've gotten into the habit of saying byte offset too, but don't know which is best. char_indices says indices, and other docs say .. what? 😄

Separate out of bounds errors from character boundary errors, and print more details for character boundary errors. Example: &"abcαβγ"[..4] thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not a char boundary; it is inside `α` (bytes 3..5) of `abcαβγ`'

steveklabnik · 2017-01-03T16:41:44Z

ping @sfackler ; you're the reviewer on this PR but haven't commented. Should we move it to someone else?

bluss · 2017-01-03T17:30:51Z

Oh sorry, I also forgot about this. Thank you steveklabnik!

sfackler · 2017-01-03T17:46:29Z

Oops, missed this!

I'm a bit worried about including the entire string in the panic message. I can see that getting pretty incomprehensible for larger strings. Maybe just 20 or 30 characters of context around the index?

bluss · 2017-01-03T18:14:21Z

This PR keeps the previous behaviour that the string piece shown is truncated to 256 bytes or less. That was just some arbitrary limit that I picked in a previous that PR introduced some truncation at all.

sfackler · 2017-01-03T18:33:06Z

Oh great, my mistake!

sfackler · 2017-01-03T18:33:44Z

@bors r+

bors · 2017-01-03T18:33:45Z

📌 Commit d83fff3 has been approved by sfackler

bors · 2017-01-03T18:33:55Z

⌛ Testing commit d83fff3 with merge 9954355...

Use more specific panic message for &str slicing errors Separate out of bounds errors from character boundary errors, and print more details for character boundary errors. It reports the first error it finds in: 1. begin out of bounds 2. end out of bounds 3. begin <= end violated 3. begin not char boundary 5. end not char boundary. Example: &"abcαβγ"[..4] thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`' Fixes #38052

bors · 2017-01-03T20:54:16Z

💔 Test failed - status-travis

alexcrichton · 2017-01-03T23:29:23Z

@bors: retry * android timeout

…

On Tue, Jan 3, 2017 at 12:54 PM, bors ***@***.***> wrote: 💔 Test failed - status-travis <https://travis-ci.org/rust-lang/rust/builds/188617319> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#38066 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAD95CYOl3qSCEJ6itbP_hwP8FeXEANPks5rOrV8gaJpZM4K-hVh> .

bors · 2017-01-03T23:51:25Z

⌛ Testing commit d83fff3 with merge 4682271...

Use more specific panic message for &str slicing errors Separate out of bounds errors from character boundary errors, and print more details for character boundary errors. It reports the first error it finds in: 1. begin out of bounds 2. end out of bounds 3. begin <= end violated 3. begin not char boundary 5. end not char boundary. Example: &"abcαβγ"[..4] thread 'str::test_slice_fail_boundary_1' panicked at 'byte index 4 is not a char boundary; it is inside 'α' (bytes 3..5) of `abcαβγ`' Fixes #38052

bors · 2017-01-04T01:50:39Z

☀️ Test successful - status-appveyor, status-travis
Approved by: sfackler
Pushing 4682271 to master...

Version 1.16.0 (2017-03-16) =========================== Language -------- * Lifetimes in statics and consts default to `'static`. [RFC 1623] * [The compiler's `dead_code` lint now accounts for type aliases][38051]. * [Uninhabitable enums (those without any variants) no longer permit wildcard match patterns][38069] * [Clean up semantics of `self` in an import list][38313] * [`Self` may appear in `impl` headers][38920] * [`Self` may appear in struct expressions][39282] Compiler -------- * [`rustc` now supports `--emit=metadata`, which causes rustc to emit a `.rmeta` file containing only crate metadata][38571]. This can be used by tools like the Rust Language Service to perform metadata-only builds. * [Levenshtein based typo suggestions now work in most places, while previously they worked only for fields and sometimes for local variables][38927]. Together with the overhaul of "no resolution"/"unexpected resolution" errors (#[38154]) they result in large and systematic improvement in resolution diagnostics. * [Fix `transmute::<T, U>` where `T` requires a bigger alignment than `U`][38670] * [rustc: use -Xlinker when specifying an rpath with ',' in it][38798] * [`rustc` no longer attempts to provide "consider using an explicit lifetime" suggestions][37057]. They were inaccurate. Stabilized APIs --------------- * [`VecDeque::truncate`] * [`VecDeque::resize`] * [`String::insert_str`] * [`Duration::checked_add`] * [`Duration::checked_sub`] * [`Duration::checked_div`] * [`Duration::checked_mul`] * [`str::replacen`] * [`str::repeat`] * [`SocketAddr::is_ipv4`] * [`SocketAddr::is_ipv6`] * [`IpAddr::is_ipv4`] * [`IpAddr::is_ipv6`] * [`Vec::dedup_by`] * [`Vec::dedup_by_key`] * [`Result::unwrap_or_default`] * [`<*const T>::wrapping_offset`] * [`<*mut T>::wrapping_offset`] * `CommandExt::creation_flags` * [`File::set_permissions`] * [`String::split_off`] Libraries --------- * [`[T]::binary_search` and `[T]::binary_search_by_key` now take their argument by `Borrow` parameter][37761] * [All public types in std implement `Debug`][38006] * [`IpAddr` implements `From<Ipv4Addr>` and `From<Ipv6Addr>`][38327] * [`Ipv6Addr` implements `From<[u16; 8]>`][38131] * [Ctrl-Z returns from `Stdin.read()` when reading from the console on Windows][38274] * [std: Fix partial writes in `LineWriter`][38062] * [std: Clamp max read/write sizes on Unix][38062] * [Use more specific panic message for `&str` slicing errors][38066] * [`TcpListener::set_only_v6` is deprecated][38304]. This functionality cannot be achieved in std currently. * [`writeln!`, like `println!`, now accepts a form with no string or formatting arguments, to just print a newline][38469] * [Implement `iter::Sum` and `iter::Product` for `Result`][38580] * [Reduce the size of static data in `std_unicode::tables`][38781] * [`char::EscapeDebug`, `EscapeDefault`, `EscapeUnicode`, `CaseMappingIter`, `ToLowercase`, `ToUppercase`, implement `Display`][38909] * [`Duration` implements `Sum`][38712] * [`String` implements `ToSocketAddrs`][39048] Cargo ----- * [The `cargo check` command does a type check of a project without building it][cargo/3296] * [crates.io will display CI badges from Travis and AppVeyor, if specified in Cargo.toml][cargo/3546] * [crates.io will display categories listed in Cargo.toml][cargo/3301] * [Compilation profiles accept integer values for `debug`, in addition to `true` and `false`. These are passed to `rustc` as the value to `-C debuginfo`][cargo/3534] * [Implement `cargo --version --verbose`][cargo/3604] * [All builds now output 'dep-info' build dependencies compatible with make and ninja][cargo/3557] * [Build all workspace members with `build --all`][cargo/3511] * [Document all workspace members with `doc --all`][cargo/3515] * [Path deps outside workspace are not members][cargo/3443] Misc ---- * [`rustdoc` has a `--sysroot` argument that, like `rustc`, specifies the path to the Rust implementation][38589] * [The `armv7-linux-androideabi` target no longer enables NEON extensions, per Google's ABI guide][38413] * [The stock standard library can be compiled for Redox OS][38401] * [Rust has initial SPARC support][38726]. Tier 3. No builds available. * [Rust has experimental support for Nvidia PTX][38559]. Tier 3. No builds available. * [Fix backtraces on i686-pc-windows-gnu by disabling FPO][39379] Compatibility Notes ------------------- * [Uninhabitable enums (those without any variants) no longer permit wildcard match patterns][38069] * In this release, references to uninhabited types can not be pattern-matched. This was accidentally allowed in 1.15. * [The compiler's `dead_code` lint now accounts for type aliases][38051]. * [Ctrl-Z returns from `Stdin.read()` when reading from the console on Windows][38274] * [Clean up semantics of `self` in an import list][38313] [37057]: rust-lang/rust#37057 [37761]: rust-lang/rust#37761 [38006]: rust-lang/rust#38006 [38051]: rust-lang/rust#38051 [38062]: rust-lang/rust#38062 [38062]: rust-lang/rust#38622 [38066]: rust-lang/rust#38066 [38069]: rust-lang/rust#38069 [38131]: rust-lang/rust#38131 [38154]: rust-lang/rust#38154 [38274]: rust-lang/rust#38274 [38304]: rust-lang/rust#38304 [38313]: rust-lang/rust#38313 [38314]: rust-lang/rust#38314 [38327]: rust-lang/rust#38327 [38401]: rust-lang/rust#38401 [38413]: rust-lang/rust#38413 [38469]: rust-lang/rust#38469 [38559]: rust-lang/rust#38559 [38571]: rust-lang/rust#38571 [38580]: rust-lang/rust#38580 [38589]: rust-lang/rust#38589 [38670]: rust-lang/rust#38670 [38712]: rust-lang/rust#38712 [38726]: rust-lang/rust#38726 [38781]: rust-lang/rust#38781 [38798]: rust-lang/rust#38798 [38909]: rust-lang/rust#38909 [38920]: rust-lang/rust#38920 [38927]: rust-lang/rust#38927 [39048]: rust-lang/rust#39048 [39282]: rust-lang/rust#39282 [39379]: rust-lang/rust#39379 [`<*const T>::wrapping_offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset [`<*mut T>::wrapping_offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset [`Duration::checked_add`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_add [`Duration::checked_div`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_div [`Duration::checked_mul`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_mul [`Duration::checked_sub`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.checked_sub [`File::set_permissions`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.set_permissions [`IpAddr::is_ipv4`]: https://doc.rust-lang.org/std/net/enum.IpAddr.html#method.is_ipv4 [`IpAddr::is_ipv6`]: https://doc.rust-lang.org/std/net/enum.IpAddr.html#method.is_ipv6 [`Result::unwrap_or_default`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.unwrap_or_default [`SocketAddr::is_ipv4`]: https://doc.rust-lang.org/std/net/enum.SocketAddr.html#method.is_ipv4 [`SocketAddr::is_ipv6`]: https://doc.rust-lang.org/std/net/enum.SocketAddr.html#method.is_ipv6 [`String::insert_str`]: https://doc.rust-lang.org/std/string/struct.String.html#method.insert_str [`String::split_off`]: https://doc.rust-lang.org/std/string/struct.String.html#method.split_off [`Vec::dedup_by_key`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.dedup_by_key [`Vec::dedup_by`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.dedup_by [`VecDeque::resize`]: https://doc.rust-lang.org/std/collections/vec_deque/struct.VecDeque.html#method.resize [`VecDeque::truncate`]: https://doc.rust-lang.org/std/collections/vec_deque/struct.VecDeque.html#method.truncate [`str::repeat`]: https://doc.rust-lang.org/std/primitive.str.html#method.repeat [`str::replacen`]: https://doc.rust-lang.org/std/primitive.str.html#method.replacen [cargo/3296]: rust-lang/cargo#3296 [cargo/3301]: rust-lang/cargo#3301 [cargo/3443]: rust-lang/cargo#3443 [cargo/3511]: rust-lang/cargo#3511 [cargo/3515]: rust-lang/cargo#3515 [cargo/3534]: rust-lang/cargo#3534 [cargo/3546]: rust-lang/cargo#3546 [cargo/3557]: rust-lang/cargo#3557 [cargo/3604]: rust-lang/cargo#3604 [RFC 1623]: https://github.com/rust-lang/rfcs/blob/master/text/1623-static.md

rust-highfive assigned sfackler Nov 29, 2016

bluss force-pushed the string-slice-error branch 2 times, most recently from c29b730 to 46a26f2 Compare November 29, 2016 03:21

durka approved these changes Nov 29, 2016

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killercup reviewed Nov 29, 2016

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bluss force-pushed the string-slice-error branch from 9bc3cd4 to 95e8c41 Compare November 29, 2016 19:59

bluss force-pushed the string-slice-error branch from 9c659ea to d83fff3 Compare November 30, 2016 18:00

bors merged commit d83fff3 into rust-lang:master Jan 4, 2017

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Use more specific panic message for &str slicing errors #38066

Use more specific panic message for &str slicing errors #38066

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Use more specific panic message for &str slicing errors #38066

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