format #![feature(unsafe_attributes)]

Our diff-check job was failing in part due to removing `unsafe` from any
`#[unsafe(attributes)]`. To prevent that I added a quick implementation
for this.

src/attr.rs

@@ -353,10 +353,18 @@ fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String>
 
                 // 1 = `[`
                 let shape = shape.offset_left(prefix.len() + 1)?;
-                Some(
-                    meta.rewrite(context, shape)
-                        .map_or_else(|| snippet.to_owned(), |rw| format!("{}[{}]", prefix, rw)),
-                )
+                Some(meta.rewrite(context, shape).map_or_else(
+                    || snippet.to_owned(),
+                    |rw| match &self.kind {
+                        ast::AttrKind::Normal(normal_attr) => match normal_attr.item.unsafety {
+                            // For #![feature(unsafe_attributes)]
+                            // See https://github.com/rust-lang/rust/issues/123757
+                            ast::Safety::Unsafe(_) => format!("{}[unsafe({})]", prefix, rw),
+                            _ => format!("{}[{}]", prefix, rw),
+                        },
+                        _ => format!("{}[{}]", prefix, rw),
+                    },
+                ))
             } else {
                 Some(snippet.to_owned())
             }

tests/target/unsafe_attributes.rs

@@ -0,0 +1,34 @@
+#![feature(unsafe_attributes)]
+// https://github.com/rust-lang/rust/issues/123757
+//
+#![simple_ident]
+#![simple::path]
+#![simple_ident_expr = ""]
+#![simple::path::Expr = ""]
+#![simple_ident_tt(a b c)]
+#![simple_ident_tt[a b c]]
+#![simple_ident_tt{a b c}]
+#![simple::path::tt(a b c)]
+#![simple::path::tt[a b c]]
+#![simple::path::tt{a b c}]
+#![unsafe(simple_ident)]
+#![unsafe(simple::path)]
+#![unsafe(simple_ident_expr = "")]
+#![unsafe(simple::path::Expr = "")]
+#![unsafe(simple_ident_tt(a b c))]
+#![unsafe(simple_ident_tt[a b c])]
+#![unsafe(simple_ident_tt{a b c})]
+#![unsafe(simple::path::tt(a b c))]
+#![unsafe(simple::path::tt[a b c])]
+#![unsafe(simple::path::tt{a b c})]
+// I don't think `safe` attributes are a thing, but adding these formatting cases here just in case
+#![safe(simple_ident)]
+#![safe(simple::path)]
+#![safe(simple_ident_expr = "")]
+#![safe(simple::path::Expr = "")]
+#![safe(simple_ident_tt(a b c))]
+#![safe(simple_ident_tt[a b c])]
+#![safe(simple_ident_tt{a b c})]
+#![safe(simple::path::tt(a b c))]
+#![safe(simple::path::tt[a b c])]
+#![safe(simple::path::tt{a b c})]