Add feature for const pointers

Author: filtsin

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -22,6 +22,7 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
 use std::mem;
 use std::ops::DerefMut;
+use rustc_session::parse::feature_err;
 
 const MORE_EXTERN: &str =
     "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
@@ -824,6 +825,16 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     )
                     .emit();
                 });
+                if !self.session.features_untracked().const_fn_pointer {
+                    if let Const::Yes(span) = bfty.constness {
+                        feature_err(
+                            &self.session.parse_sess,
+                            sym::const_fn_pointer,
+                            span,
+                            "`const fn` pointer type is unstable"
+                        ).emit()
+                    }
+                }
                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
             }
             TyKind::TraitObject(ref bounds, ..) => {

compiler/rustc_feature/src/active.rs

@@ -602,6 +602,9 @@ declare_features! (
     /// Allows `#[instruction_set(_)]` attribute
     (active, isa_attribute, "1.48.0", Some(74727), None),
 
+    /// Allow const fn pointer
+    (active, const_fn_pointer, "1.49.0", Some(63997), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_middle/src/ty/relate.rs

@@ -225,7 +225,7 @@ impl<'tcx> Relate<'tcx> for ast::Constness {
         a: ast::Constness,
         b: ast::Constness
     ) -> RelateResult<'tcx, ast::Constness> {
-        if a == b || (a == ast::Constness::Const && b == ast::Constness::NotConst) { Ok(a) }
+        if a == b || (a == ast::Constness::Const && b == ast::Constness::NotConst) { Ok(b) }
         else { Err(TypeError::ConstnessMismatch(expected_found(relation, a, b))) }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -357,6 +357,7 @@ symbols! {
         const_fn,
         const_fn_floating_point_arithmetic,
         const_fn_fn_ptr_basics,
+        const_fn_pointer,
         const_fn_transmute,
         const_fn_union,
         const_generics,