move 'trait bounds on const fn' to separate feature gate

Author: RalfJung

compiler/rustc_feature/src/active.rs

@@ -576,6 +576,9 @@ declare_features! (
     /// Allows using and casting function pointers in a `const fn`.
     (active, const_fn_fn_ptr_basics, "1.48.0", Some(57563), None),
 
+    /// Allows trait bounds in `const fn`.
+    (active, const_fn_trait_bound, "1.53.0", Some(57563), None),
+
     /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
     (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
 

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

@@ -642,12 +642,17 @@ pub mod ty {
         }
 
         fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            mcf_status_in_item(ccx)
+            if ccx.const_kind() != hir::ConstContext::ConstFn {
+                Status::Allowed
+            } else {
+                Status::Unstable(sym::const_fn_trait_bound)
+            }
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            mcf_build_error(
-                ccx,
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_fn_trait_bound,
                 span,
                 "trait bounds other than `Sized` on const fn parameters are unstable",
             )

compiler/rustc_span/src/symbol.rs

@@ -382,6 +382,7 @@ symbols! {
         const_fn,
         const_fn_floating_point_arithmetic,
         const_fn_fn_ptr_basics,
+        const_fn_trait_bound,
         const_fn_transmute,
         const_fn_union,
         const_generic_defaults,

library/alloc/src/lib.rs

@@ -90,6 +90,7 @@
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]
 #![feature(destructuring_assignment)]

library/core/src/lib.rs

@@ -86,6 +86,7 @@
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]
 #![feature(const_ptr_offset)]

src/test/ui/consts/const-eval/issue-49296.rs

@@ -1,7 +1,7 @@
 // issue-49296: Unsafe shenigans in constants can result in missing errors
 
-#![feature(const_fn)]
 #![feature(const_fn_union)]
+#![feature(const_fn_trait_bound)]
 
 const unsafe fn transmute<T: Copy, U: Copy>(t: T) -> U {
     #[repr(C)]

src/test/ui/consts/const-fn.rs

@@ -3,7 +3,8 @@
 
 // A very basic test of const fn functionality.
 
-#![feature(const_fn, const_indexing)]
+#![feature(const_indexing)]
+#![feature(const_fn_trait_bound)]
 
 const fn add(x: u32, y: u32) -> u32 {
     x + y

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

@@ -130,23 +130,23 @@ LL |     const fn get_mut_sq(&mut self) -> &mut T { &mut self.0 }
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:84:16
    |
 LL | const fn foo11<T: std::fmt::Display>(t: T) -> T { t }
    |                ^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:86:18
    |
 LL | const fn foo11_2<T: Send>(t: T) -> T { t }
    |                  ^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
 error[E0013]: constant functions cannot refer to statics
   --> $DIR/min_const_fn.rs:90:27
@@ -209,41 +209,41 @@ LL | const fn inc(x: &mut i32) { *x += 1 }
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:110:6
    |
 LL | impl<T: std::fmt::Debug> Foo<T> {
    |      ^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:115:6
    |
 LL | impl<T: std::fmt::Debug + Sized> Foo<T> {
    |      ^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:120:6
    |
 LL | impl<T: Sync + Sized> Foo<T> {
    |      ^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:126:34
    |
 LL | const fn no_apit2(_x: 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
 error[E0493]: destructors cannot be evaluated at compile-time
   --> $DIR/min_const_fn.rs:126:19
@@ -253,14 +253,14 @@ LL | const fn no_apit2(_x: AlanTuring<impl std::fmt::Debug>) {}
    |                   |
    |                   constant functions cannot evaluate destructors
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:129:22
    |
 LL | const fn no_apit(_x: 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
 error[E0493]: destructors cannot be evaluated at compile-time
   --> $DIR/min_const_fn.rs:129:18
@@ -270,23 +270,23 @@ LL | const fn no_apit(_x: impl std::fmt::Debug) {}
    |                  |
    |                  constant functions cannot evaluate destructors
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:132:23
    |
 LL | const fn no_dyn_trait(_x: &dyn 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn.rs:134:32
    |
 LL | const fn no_dyn_trait_ret() -> &'static dyn 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
 error[E0723]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn.rs:134:63

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

@@ -1,11 +1,11 @@
-error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
+error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn_dyn.rs:9:5
    |
 LL |     x.0.field;
    |     ^^^^^^^^^
    |
    = 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
+   = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
 
 error[E0723]: unsizing casts to types besides slices are not allowed in const fn
   --> $DIR/min_const_fn_dyn.rs:12:66
@@ -18,4 +18,5 @@ LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0723`.
+Some errors have detailed explanations: E0658, E0723.
+For more information about an error, try `rustc --explain E0658`.

src/test/ui/consts/unstable-const-fn-in-libcore.rs

@@ -6,7 +6,7 @@
 #![stable(feature = "core", since = "1.6.0")]
 #![feature(rustc_const_unstable)]
 #![feature(staged_api)]
-#![feature(const_fn)]
+#![feature(const_fn_trait_bound)]
 
 enum Opt<T> {
     Some(T),