Check for whether index is out of bound during compilation failed unexpectedly

[htw-yay]

I tried these two codes:

#[derive(Debug)]
struct Arr<T, const N: usize>([T; N]);

impl<T: Default + Copy, const N: usize> Arr<T, N> {
    fn from_slice(slice: &[T]) -> Option<Self> {
        if slice.len() == N {
            let mut arr = Self([T::default(); N]);
            arr.0.copy_from_slice(&slice);
            Some(arr)
        } else {
            None
        }
    }
}

fn main() {
    let slice: &[u8] = b"Hello, world";
    let arr = Arr::<u8, 12>::from_slice(slice).unwrap();
    println!("{:?}", arr.0[20]);
// let inner = arr.0;
// println!("{:?}", inner[20]);
}

...
// println!("{:?}", arr.0[20]);
    let inner = arr.0;
    println!("{:?}", inner[20]);
...

I expected to see this happen:
When running cargo build,
"index out of bounds: the length is 12 but the index is 20" should be indicated for both codes

Instead, this happened:
Only Code 2 had it

Meta

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-pc-windows-msvc
release: 1.78.0
LLVM version: 18.1.2

rustc 1.79.0-beta.3 (f5d04caa7 2024-05-03)
binary: rustc
commit-hash: f5d04caa74a1dfa5ffc4082c2c8f621f25336bbc
commit-date: 2024-05-03
host: x86_64-pc-windows-msvc
release: 1.79.0-beta.3
LLVM version: 18.1.4

rustc 1.80.0-nightly (faefc618c 2024-05-07)
binary: rustc
commit-hash: faefc618cf48bd794cbc808448df1bf3f59f36af
commit-date: 2024-05-07
host: x86_64-pc-windows-msvc
release: 1.80.0-nightly
LLVM version: 18.1.4

Backtrace

Building:

Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s

error: this operation will panic at runtime
  --> t1\src\main.rs:22:22
   |
22 |     println!("{:?}", inner[20])
   |                      ^^^^^^^^^ index out of bounds: the length is 12 but the index is 20
   |
   = note: `#[deny(unconditional_panic)]` on by default

error: could not compile `t1` (bin "t1") due to 1 previous error

Running(Code 1):

thread 'main' panicked at t1\src\main.rs:19:22:
index out of bounds: the len is 12 but the index is 20
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:645
   1: core::panicking::panic_fmt
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\panicking.rs:72
   2: core::panicking::panic_bounds_check
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\panicking.rs:209
   3: t1::main
             at .\t1\src\main.rs:19
   4: core::ops::function::FnOnce::call_once<void (*)(),tuple$<> >
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\core\src\ops\function.rs:250
   5: core::hint::black_box
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\core\src\hint.rs:337
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\t1.exe` (exit code: 101)

thread 'main' panicked at t1\src\main.rs:19:22:
index out of bounds: the len is 12 but the index is 20
stack backtrace:
   0:     0x7ff650fa73fa - std::backtrace_rs::backtrace::dbghelp64::trace
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\..\..\backtrace\src\backtrace\dbghelp64.rs:99
   1:     0x7ff650fa73fa - std::backtrace_rs::backtrace::trace_unsynchronized
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\..\..\backtrace\src\backtrace\mod.rs:66
   2:     0x7ff650fa73fa - std::sys_common::backtrace::_print_fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:68
   3:     0x7ff650fa73fa - std::sys_common::backtrace::_print::impl$0::fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:44
   4:     0x7ff650fb519b - core::fmt::rt::Argument::fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\fmt\rt.rs:142
   5:     0x7ff650fb519b - core::fmt::write
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\fmt\mod.rs:1153
   6:     0x7ff650fa5791 - std::io::Write::write_fmt<std::sys::pal::windows::stdio::Stderr>
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\io\mod.rs:1843
   7:     0x7ff650fa71e6 - std::sys_common::backtrace::print
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:34
   8:     0x7ff650fa8ccf - std::panicking::default_hook::closure$1
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:272
   9:     0x7ff650fa8967 - std::panicking::default_hook
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:292
  10:     0x7ff650fa920d - std::panicking::rust_panic_with_hook
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:779
  11:     0x7ff650fa90c9 - std::panicking::begin_panic_handler::closure$0
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:657
  12:     0x7ff650fa7a89 - std::sys_common::backtrace::__rust_end_short_backtrace<std::panicking::begin_panic_handler::closure_env$0,never$>
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\sys_common\backtrace.rs:171
  13:     0x7ff650fa8d86 - std::panicking::begin_panic_handler
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\panicking.rs:645
  14:     0x7ff650fba387 - core::panicking::panic_fmt
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\panicking.rs:72
  15:     0x7ff650fba534 - core::panicking::panic_bounds_check
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\core\src\panicking.rs:209
  16:     0x7ff650fa1768 - t1::main
                               at C:\Users\htwyay\Documents\LearnRust\t1\src\main.rs:19
  17:     0x7ff650fa11cb - core::ops::function::FnOnce::call_once<void (*)(),tuple$<> >
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\core\src\ops\function.rs:250
  18:     0x7ff650fa113e - core::hint::black_box
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\core\src\hint.rs:337
  19:     0x7ff650fa113e - std::sys_common::backtrace::__rust_begin_short_backtrace<void (*)(),tuple$<> >
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\std\src\sys_common\backtrace.rs:155
  20:     0x7ff650fa1061 - std::rt::lang_start::closure$0<tuple$<> >
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\std\src\rt.rs:166
  21:     0x7ff650fa3672 - std::rt::lang_start_internal
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library\std\src\rt.rs:148
  22:     0x7ff650fa103a - std::rt::lang_start<tuple$<> >
                               at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6\library\std\src\rt.rs:165
  23:     0x7ff650fa1789 - main
  24:     0x7ff650fb8950 - invoke_main
                               at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
  25:     0x7ff650fb8950 - __scrt_common_main_seh
                               at D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  26:     0x7ff95a3e1fd7 - BaseThreadInitThunk
  27:     0x7ff95c49d7d0 - RtlUserThreadStart
error: process didn't exit successfully: `target\debug\t1.exe` (exit code: 101)

[mkehrt]

This repros without the copying method.

This panics at runtime:

#[derive(Debug)]
struct Arr<T, const N: usize>([T; N]);

fn main() {
    let arr = Arr(*b"Hello, world");
    println!("{:?}", arr.0[20]);
    // let inner = arr.0;
    // println!("{:?}", inner[20]);
}

While this gives a compiler error:

#[derive(Debug)]
struct Arr<T, const N: usize>([T; N]);

fn main() {
    let arr = Arr(*b"Hello, world");
    // println!("{:?}", arr.0[20]);
    let inner = arr.0;
    println!("{:?}", inner[20]);
}

[mkehrt]

Interestingly this panics:

struct Arr([u8; 12]);

fn main() {
    let arr = Arr(*b"Hello, world");
    
    // Panics
    println!("{:?}", arr.0[20]);

But this (without the println) is a compiler error:

struct Arr([u8; 12]);

fn main() {
    let arr = Arr(*b"Hello, world");
    
    // Compiler errror
    arr.0[20];
}

Expanding the macros for the former println call (into HIR) gives:

 ::std::io::_print(format_arguments::new_v1(&["", "\n"],
                        &[format_argument::new_debug(&arr.0[20])]));
            };

I guessed that the reference before arr.0[20] was the issue. And, in fact, this panics:

struct Arr([u8; 12]);

fn main() {
    let arr = Arr(*b"Hello, world");
    
    // Panics
    (&arr.0)[20];
}

In fact, this happens even if you remove the enclosing struct:

fn main() {
    let arr: [u8; 12] = *b"Hello, world";

    // Panic
    (&arr)[20];

fn main() {
    let arr: [u8; 12] = *b"Hello, world";

    // Compile error
    arr[20];
}

So it appears that the issue is that the lint to check whether a constant index into a constant array or slice will be out of bounds occurs with arrays but not with slices. I don't know if this is a bug or not.

[mkehrt]

@rustbot label +A-lint