Auto merge of #77464 - ecstatic-morse:const-fn-impl-trait, r=oli-obk

Give `impl Trait` in a `const fn` its own feature gate

...previously it was gated under `#![feature(const_fn)]`.

I think we actually want to do this in all const-contexts? If so, this should be `#![feature(const_impl_trait)]` instead. I don't think there's any way to make use of `impl Trait` within a `const` initializer.

cc #77463

r? `@oli-obk`

compiler/rustc_error_codes/src/error_codes/E0723.md

@@ -3,12 +3,8 @@ An unstable feature in `const` contexts was used.
 Erroneous code example:
 
 ```compile_fail,E0723
-trait T {}
-
-impl T for () {}
-
-const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
-    ()
+const fn foo<T: Copy>(_: T) { // error!
+   // ...
 }
 ```
 
@@ -18,11 +14,7 @@ feature flag:
 ```
 #![feature(const_fn)]
 
-trait T {}
-
-impl T for () {}
-
-const fn foo() -> impl T {
-    ()
+const fn foo<T: Copy>(_: T) { // ok!
+   // ...
 }
 ```

compiler/rustc_feature/src/active.rs

@@ -596,6 +596,9 @@ declare_features! (
     /// Allows rustc to inject a default alloc_error_handler
     (active, default_alloc_error_handler, "1.48.0", Some(66741), None),
 
+    /// Allows argument and return position `impl Trait` in a `const fn`.
+    (active, const_impl_trait, "1.48.0", Some(77463), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -558,12 +558,17 @@ pub mod ty {
     #[derive(Debug)]
     pub struct ImplTrait;
     impl NonConstOp for ImplTrait {
-        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            mcf_status_in_item(ccx)
+        fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_impl_trait)
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            mcf_build_error(ccx, span, "`impl Trait` in const fn is unstable")
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_impl_trait,
+                span,
+                &format!("`impl Trait` is not allowed in {}s", ccx.const_kind()),
+            )
         }
     }
 

compiler/rustc_span/src/symbol.rs

@@ -359,6 +359,7 @@ symbols! {
         const_fn_union,
         const_generics,
         const_if_match,
+        const_impl_trait,
         const_in_array_repeat_expressions,
         const_indexing,
         const_let,

library/core/src/lib.rs

@@ -83,6 +83,7 @@
 #![feature(const_fn)]
 #![feature(const_fn_union)]
 #![feature(const_assume)]
+#![cfg_attr(not(bootstrap), feature(const_impl_trait))]
 #![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(const_generics)]

src/test/ui/consts/min_const_fn/min_const_fn_impl_trait.rs

@@ -1,9 +1,10 @@
+// gate-test-const_impl_trait
+
 struct AlanTuring<T>(T);
-const fn no_rpit2() -> AlanTuring<impl std::fmt::Debug> {
-    //~^ ERROR `impl Trait` in const fn is unstable
+const fn no_rpit2() -> AlanTuring<impl std::fmt::Debug> { //~ `impl Trait`
     AlanTuring(0)
 }
 
-const fn no_rpit() -> impl std::fmt::Debug {} //~ ERROR `impl Trait` in const fn is unstable
+const fn no_rpit() -> impl std::fmt::Debug {} //~ `impl Trait`
 
 fn main() {}

src/test/ui/consts/min_const_fn/min_const_fn_impl_trait.stderr

@@ -1,21 +1,21 @@
-error[E0723]: `impl Trait` in const fn is unstable
-  --> $DIR/min_const_fn_impl_trait.rs:2:24
+error[E0658]: `impl Trait` is not allowed in constant functions
+  --> $DIR/min_const_fn_impl_trait.rs:4:24
    |
 LL | const fn no_rpit2() -> AlanTuring<impl std::fmt::Debug> {
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+   = note: see issue #77463 <https://github.com/rust-lang/rust/issues/77463> for more information
+   = help: add `#![feature(const_impl_trait)]` to the crate attributes to enable
 
-error[E0723]: `impl Trait` in const fn is unstable
-  --> $DIR/min_const_fn_impl_trait.rs:7:23
+error[E0658]: `impl Trait` is not allowed in constant functions
+  --> $DIR/min_const_fn_impl_trait.rs:8:23
    |
 LL | const fn no_rpit() -> impl std::fmt::Debug {}
    |                       ^^^^^^^^^^^^^^^^^^^^
    |
-   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
-   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+   = note: see issue #77463 <https://github.com/rust-lang/rust/issues/77463> for more information
+   = help: add `#![feature(const_impl_trait)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0723`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/type-alias-impl-trait/issue-53096.rs

@@ -1,5 +1,5 @@
 // check-pass
-#![feature(const_fn, const_fn_fn_ptr_basics)]
+#![feature(const_impl_trait, const_fn_fn_ptr_basics)]
 #![feature(type_alias_impl_trait)]
 
 type Foo = impl Fn() -> usize;

src/test/ui/type-alias-impl-trait/issue-53678-generator-and-const-fn.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(const_fn, generators, generator_trait, type_alias_impl_trait)]
+#![feature(const_impl_trait, generators, generator_trait, type_alias_impl_trait)]
 
 use std::ops::Generator;
 

src/test/ui/type-alias-impl-trait/structural-match-no-leak.rs

@@ -1,4 +1,4 @@
-#![feature(const_fn, type_alias_impl_trait)]
+#![feature(const_impl_trait, type_alias_impl_trait)]
 
 type Bar = impl Send;
 

src/test/ui/type-alias-impl-trait/structural-match.rs

@@ -1,4 +1,4 @@
-#![feature(const_fn, type_alias_impl_trait)]
+#![feature(const_impl_trait, type_alias_impl_trait)]
 
 type Foo = impl Send;