Rollup merge of #71109 - lcnr:generics_in_const_fn, r=eddyb

allow const generics in const fn

This was explicitly forbidden before. As we were unable to think of a reason
why this should still be the case, this check has been removed.

r? @eddyb
cc @varkor @Centril

src/librustc_ast_passes/ast_validation.rs

@@ -561,28 +561,6 @@ impl<'a> AstValidator<'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 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
  .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

@@ -1,11 +1,11 @@
+// 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

@@ -1,23 +1,10 @@
-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