Auto merge of #124482 - spastorino:unsafe-extern-blocks, r=oli-obk

Unsafe extern blocks

This implements RFC 3484.

Tracking issue #123743 and RFC rust-lang/rfcs#3484

This is better reviewed commit by commit.

src/utils.rs

@@ -111,6 +111,7 @@ pub(crate) fn format_defaultness(defaultness: ast::Defaultness) -> &'static str
 pub(crate) fn format_safety(unsafety: ast::Safety) -> &'static str {
     match unsafety {
         ast::Safety::Unsafe(..) => "unsafe ",
+        ast::Safety::Safe(..) => "safe ",
         ast::Safety::Default => "",
     }
 }