remove E0723 error code

compiler/rustc_error_codes/src/error_codes.rs

@@ -416,7 +416,6 @@ E0716: include_str!("./error_codes/E0716.md"),
 E0718: include_str!("./error_codes/E0718.md"),
 E0719: include_str!("./error_codes/E0719.md"),
 E0720: include_str!("./error_codes/E0720.md"),
-E0723: include_str!("./error_codes/E0723.md"),
 E0724: include_str!("./error_codes/E0724.md"),
 E0725: include_str!("./error_codes/E0725.md"),
 E0727: include_str!("./error_codes/E0727.md"),
@@ -636,6 +635,7 @@ E0781: include_str!("./error_codes/E0781.md"),
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
     E0722, // Malformed `#[optimize]` attribute
+//    E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
     E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`

compiler/rustc_error_codes/src/error_codes/E0723.md

@@ -1,20 +0,0 @@
-An unstable feature in `const` contexts was used.
-
-Erroneous code example:
-
-```compile_fail,E0723
-const fn foo<T: Copy>(_: T) { // error!
-   // ...
-}
-```
-
-To enable this feature on a nightly version of rustc, add the `const_fn`
-feature flag:
-
-```
-#![feature(const_fn)]
-
-const fn foo<T: Copy>(_: T) { // ok!
-   // ...
-}
-```

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

@@ -1,6 +1,6 @@
 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
 
-use rustc_errors::{struct_span_err, Applicability, Diagnostic, ErrorReported};
+use rustc_errors::{Applicability, Diagnostic, ErrorReported};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, HirId, LangItem};
 use rustc_index::bit_set::BitSet;
@@ -234,13 +234,11 @@ impl Validator<'mir, 'tcx> {
             if self.is_const_stable_const_fn() {
                 let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
                 if crate::const_eval::is_parent_const_impl_raw(tcx, hir_id) {
-                    struct_span_err!(
-                        self.ccx.tcx.sess,
-                        self.span,
-                        E0723,
-                        "trait methods cannot be stable const fn"
-                    )
-                    .emit();
+                    self.ccx
+                        .tcx
+                        .sess
+                        .struct_span_err(self.span, "trait methods cannot be stable const fn")
+                        .emit();
                 }
             }
 

src/test/ui/rfc-2632-const-trait-impl/stability.stderr

@@ -1,4 +1,4 @@
-error[E0723]: trait methods cannot be stable const fn
+error: trait methods cannot be stable const fn
   --> $DIR/stability.rs:14:5
    |
 LL | /     fn sub(self, rhs: Self) -> Self {
@@ -17,4 +17,3 @@ LL |     Int(1i32) + Int(2i32)
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0723`.