Add toggle for parse_meta_item unsafe parsing

This makes it possible for the `unsafe(...)` syntax to only be
valid at the top level, and the `NestedMetaItem`s will automatically
reject `unsafe(...)`.

compiler/rustc_builtin_macros/src/cfg.rs

@@ -6,6 +6,7 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::PResult;
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacEager, MacroExpanderResult};
+use rustc_parse::parser::attr::AllowLeadingUnsafe;
 use rustc_span::Span;
 use {rustc_ast as ast, rustc_attr as attr};
 
@@ -42,7 +43,7 @@ fn parse_cfg<'a>(cx: &ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a,
         return Err(cx.dcx().create_err(errors::RequiresCfgPattern { span }));
     }
 
-    let cfg = p.parse_meta_item()?;
+    let cfg = p.parse_meta_item(AllowLeadingUnsafe::Yes)?;
 
     let _ = p.eat(&token::Comma);