Add specific feature gate error for const-unstable features

src/librustc_mir/diagnostics.rs

@@ -2370,6 +2370,37 @@ let value = (&foo(), &foo());
 ```
 "##,
 
+E0723: r##"
+An feature unstable 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
+    ()
+}
+```
+
+To enable this feature on a nightly version of rustc, add the `const_fn`
+feature flag:
+
+```
+#![feature(const_fn)]
+
+trait T {}
+
+impl T for () {}
+
+const fn foo() -> impl T {
+    ()
+}
+```
+"##,
+
 }
 
 register_diagnostics! {

src/librustc_mir/transform/qualify_consts.rs

@@ -1206,7 +1206,17 @@ impl MirPass for QualifyAndPromoteConstants {
                         // enforce `min_const_fn` for stable const fns
                         use super::qualify_min_const_fn::is_min_const_fn;
                         if let Err((span, err)) = is_min_const_fn(tcx, def_id, mir) {
-                            tcx.sess.span_err(span, &err);
+                            let mut diag = struct_span_err!(
+                                tcx.sess,
+                                span,
+                                E0723,
+                                "{} (see issue #57563)",
+                                err,
+                            );
+                            diag.help(
+                                "add #![feature(const_fn)] to the crate attributes to enable",
+                            );
+                            diag.emit();
                         } else {
                             // this should not produce any errors, but better safe than sorry
                             // FIXME(#53819)

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

@@ -1,10 +1,12 @@
-error: heap allocations are not allowed in const fn
+error[E0723]: heap allocations are not allowed in const fn (see issue #57563)
   --> $DIR/bad_const_fn_body_ice.rs:2:5
    |
 LL |     vec![1, 2, 3] //~ ERROR heap allocations are not allowed in const fn
    |     ^^^^^^^^^^^^^
    |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.

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

@@ -1,32 +1,43 @@
-error: unsizing casts are not allowed in const fn
+error[E0723]: unsizing casts are not allowed in const fn (see issue #57563)
   --> $DIR/cast_errors.rs:3:41
    |
 LL | const fn unsize(x: &[u8; 3]) -> &[u8] { x }
    |                                         ^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:5:23
    |
 LL | const fn closure() -> fn() { || {} }
    |                       ^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:8:5
    |
 LL |     (|| {}) as fn();
    |     ^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:11:28
    |
 LL | const fn reify(f: fn()) -> unsafe fn() { f }
    |                            ^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cast_errors.rs:13:21
    |
 LL | const fn reify2() { main as unsafe fn(); }
    |                     ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to 5 previous errors
 
+For more information about this error, try `rustc --explain E0723`.

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

@@ -1,8 +1,11 @@
-error: function pointers in const fn are unstable
+error[E0723]: function pointers in const fn are unstable (see issue #57563)
   --> $DIR/cmp_fn_pointers.rs:1:14
    |
 LL | const fn cmp(x: fn(), y: fn()) -> bool { //~ ERROR function pointers in const fn are unstable
    |              ^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.

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

@@ -1,8 +1,11 @@
-error: loops are not allowed in const fn
+error[E0723]: loops are not allowed in const fn (see issue #57563)
   --> $DIR/loop_ice.rs:2:5
    |
 LL |     loop {} //~ ERROR loops are not allowed in const fn
    |     ^^^^^^^
+   |
+   = help: add #![feature(const_fn)] to the crate attributes to enable
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0723`.