Rollup merge of #51765 - jonas-schievink:patch-1, r=KodrAus

Rollup merge of #51765 - jonas-schievink:patch-1, r=KodrAus

Use assert_eq! in copy_from_slice

This will print both lengths when the assertion fails instead of just saying that they're different.

Output of current stable and nightly (modulo the exact line number):
```
thread 'main' panicked at 'destination and source slices have different lengths', libcore/slice/mod.rs:1645:9
```

Output after this PR:
```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `123`,
 right: `456`: destination and source slices have different lengths', libcore/slice/mod.rs:1645:9
```

Note that I have not run the tests locally.