symbols: add is_preinterned fn to check if symbol was preinterned in compiler

Author: klensy

compiler/rustc_macros/src/symbols.rs

@@ -195,10 +195,10 @@ fn symbols_with_errors(input: TokenStream) -> (TokenStream, Vec<syn::Error>) {
             #n,
         });
     }
-    let _ = counter; // for future use
 
     let output = quote! {
         const SYMBOL_DIGITS_BASE: u32 = #digits_base;
+        const PREINTERNED_SYMBOLS_COUNT: u32 = #counter;
 
         #[doc(hidden)]
         #[allow(non_upper_case_globals)]

compiler/rustc_span/src/symbol.rs

@@ -2027,6 +2027,11 @@ impl Symbol {
     pub fn can_be_raw(self) -> bool {
         self != kw::Empty && self != kw::Underscore && !self.is_path_segment_keyword()
     }
+
+    /// Is this symbol was interned in compiler's `symbols!` macro
+    pub fn is_preinterned(self) -> bool {
+        self.as_u32() < PREINTERNED_SYMBOLS_COUNT
+    }
 }
 
 impl Ident {