allow const generics in const fn

src/librustc_ast_passes/ast_validation.rs

@@ -561,28 +561,6 @@ fn check_c_varadic_type(&self, fk: FnKind<'a>) {
         }
     }
 
-    /// We currently do not permit const generics in `const fn`,
-    /// as this is tantamount to allowing compile-time dependent typing.
-    ///
-    /// FIXME(const_generics): Is this really true / necessary? Discuss with @varkor.
-    /// At any rate, the restriction feels too syntactic. Consider moving it to e.g. typeck.
-    fn check_const_fn_const_generic(&self, span: Span, sig: &FnSig, generics: &Generics) {
-        if let Const::Yes(const_span) = sig.header.constness {
-            // Look for const generics and error if we find any.
-            for param in &generics.params {
-                if let GenericParamKind::Const { .. } = param.kind {
-                    self.err_handler()
-                        .struct_span_err(
-                            span,
-                            "const parameters are not permitted in const functions",
-                        )
-                        .span_label(const_span, "`const` because of this")
-                        .emit();
-                }
-            }
-        }
-    }
-
     fn check_item_named(&self, ident: Ident, kind: &str) {
         if ident.name != kw::Underscore {
             return;
@@ -966,9 +944,8 @@ fn visit_item(&mut self, item: &'a Item) {
                         .emit();
                 }
             }
-            ItemKind::Fn(def, ref sig, ref generics, ref body) => {
+            ItemKind::Fn(def, _, _, ref body) => {
                 self.check_defaultness(item.span, def);
-                self.check_const_fn_const_generic(item.span, sig, generics);
 
                 if body.is_none() {
                     let msg = "free function without a body";

src/test/ui/const-generics/const-fn-with-const-param.rs

+// run-pass
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
 
 const fn const_u32_identity<const X: u32>() -> u32 {
-    //~^ ERROR const parameters are not permitted in const functions
     X
 }
 
 fn main() {
-    println!("{:?}", const_u32_identity::<18>());
+    assert_eq!(const_u32_identity::<18>(), 18);
 }

src/test/ui/const-generics/const-fn-with-const-param.stderr

-error: const parameters are not permitted in const functions
-  --> $DIR/const-fn-with-const-param.rs:4:1
-   |
-LL |   const fn const_u32_identity<const X: u32>() -> u32 {
-   |   ^----
-   |   |
-   |  _`const` because of this
-   | |
-LL | |
-LL | |     X
-LL | | }
-   | |_^
-
 warning: the feature `const_generics` is incomplete and may cause the compiler to crash
-  --> $DIR/const-fn-with-const-param.rs:1:12
+  --> $DIR/const-fn-with-const-param.rs:2:12
    |
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
    |
    = note: `#[warn(incomplete_features)]` on by default
 
-error: aborting due to previous error; 1 warning emitted
+warning: 1 warning emitted