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Explicit warning on nested unsafe blocks #39144

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@Wilfred

When refactoring code, it's easy to accidentally end up with nested unsafe blocks. This can lead to confusing rustc errors.

unsafe fn very_dangerous() -> u64 {
    1
}

fn main() {
    unsafe {
        // lots of code...
        
        unsafe {
            very_dangerous();
        }
    }

The compiler says:

rustc 1.16.0-nightly (ff591b6dc 2017-01-15)
warning: unnecessary `unsafe` block, #[warn(unused_unsafe)] on by default
  --> <anon>:9:9
   |
9  |           unsafe {
   |  _________^ starting here...
10 | |             very_dangerous();
11 | |         }
   | |_________^ ...ending here

This is surprising at first glance, since very_dangerous requires an unsafe block but the message implies otherwise. Could we warn about nested unsafe blocks?

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