`split_at_mut` should `#[track_caller]`

[ruza-net]

When split_at_mut is called with mid > len, an internal assertion fails. However, it doesn't track its caller, so RUST_BACKTRACE=1 is required to determine the call site.

I tried this code:

fn main() {
    let mut x = [1];
    
    x.split_at_mut(2);
}

I expected to see this happen:

An error of the form thread 'main' panicked at 'split index (is 2) should be <= len (is 1)'.

Instead, this happened:

thread 'main' panicked at 'assertion failed: mid <= self.len()', /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b/library/core/src/slice/mod.rs:1279:9

Meta

This occurs on stable (1.50.0), beta (1.51.0-beta.8) and nightly (1.53.0-nightly 2021-03-21).

Tested on playground.

[jyn514]

@ruza-net are you interested in adding this yourself? It should be pretty simple :) just add it to library/core/src/slice/mod.rs like it says in the panic.

[Rustin170506]

@rustbot claim

I would like to try this.

[slightlyoutofphase]

I think you need to alter the actual assertion to produce formatted output to truly get what you stated as "expected" though, don't you? Something like this:

let len = self.len();
assert!(
    mid <= len,
    "split index (is {}) should be <= len (is {}))",
    mid,
    len
);

All #[track_caller] alone will do is make the existing assertion print out like this: thread 'main' panicked at 'assertion failed: mid <= self.len()', src/your_file.rs:line:column.

[Nugine]

This issue has been solved by

• Track caller of slice split and swap #91961