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

Unsafe extern blocks

This implements RFC 3484.

Tracking issue #123743 and RFC https://github.com/rust-lang/rfcs/pull/3484

This is better reviewed commit by commit.

compiler/rustc_expand/src/build.rs

@@ -631,7 +631,10 @@ impl<'a> ExtCtxt<'a> {
             span,
             name,
             AttrVec::new(),
-            ast::ItemKind::Static(ast::StaticItem { ty, mutability, expr: Some(expr) }.into()),
+            ast::ItemKind::Static(
+                ast::StaticItem { ty, safety: ast::Safety::Default, mutability, expr: Some(expr) }
+                    .into(),
+            ),
         )
     }