slice::get_mut() followed by slice::copy_from_slice generates unreachable panic branch #98294
Description
In code like the following, the compiler misses a possible optimization: in f1, the length of dst is equal to the length of bytes, yet the compiler generates a call to len_mismatch_fail.
pub fn f1(a: &mut [u8], offset: usize, bytes: &[u8]) {
if let Some(dst) = a.get_mut(offset..offset + bytes.len()) {
dst.copy_from_slice(bytes);
}
}The compiler knows the lengths are equal (and thus, the panic is unreachable), because the following snippet optimizes the panic away:
pub fn f2(a: &mut [u8], offset: usize, bytes: &[u8]) {
if let Some(dst) = a.get_mut(offset..offset + bytes.len()) {
assert!(dst.len() == bytes.len());
dst.copy_from_slice(bytes);
}
}This is a regression between rustc versions 1.51 and 1.52.