Add constness qualifier to fn pointer

Author: filtsin

compiler/rustc_ast/src/ast.rs

@@ -1900,6 +1900,7 @@ pub struct BareFnTy {
     pub ext: Extern,
     pub generic_params: Vec<GenericParam>,
     pub decl: P<FnDecl>,
+    pub constness: Const
 }
 
 /// The various kinds of type recognized by the compiler.

compiler/rustc_ast/src/mut_visit.rs

@@ -462,7 +462,7 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
             vis.visit_mt(mt);
         }
         TyKind::BareFn(bft) => {
-            let BareFnTy { unsafety: _, ext: _, generic_params, decl } = bft.deref_mut();
+            let BareFnTy { unsafety: _, ext: _, constness: _, generic_params, decl } = bft.deref_mut();
             generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
             vis.visit_fn_decl(decl);
         }

compiler/rustc_parse/src/parser/ty.rs

@@ -330,13 +330,10 @@ impl<'a> Parser<'a> {
         let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
         let decl = self.parse_fn_decl(|_| false, AllowPlus::No)?;
         let whole_span = lo.to(self.prev_token.span);
-        if let ast::Const::Yes(span) = constness {
-            self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
-        }
         if let ast::Async::Yes { span, .. } = asyncness {
             self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
         }
-        Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
+        Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl, constness })))
     }
 
     /// Emit an error for the given bad function pointer qualifier.