Only if you actively disable panics being triggered if unsafe preconditions are triggered. In most code, the program will crash instead. Enabling default panic on up violation in production code was done last year, IIRC.

> Its unclear how the compiler could check anything about preconditions

It can't. This is done at runtime, by default and without manually needed programmer interaction.

You can see an example of this in the `ptr`module, here: https://doc.rust-lang.org/beta/src/core/ptr/mod.rs.html#1071

Some are only enabled for `debug_assert` (which is enabled by default), see `ptr::read`, here: https://doc.rust-lang.org/beta/src/core/ptr/mod.rs.html#1370