Also put comparing raw pointers behind a feature gate

Author: oli-obk

src/librustc_mir/diagnostics.rs

@@ -1113,36 +1113,6 @@ fn main() {
 ```
 "##,
 
-E0395: r##"
-The value assigned to a constant scalar must be known at compile time,
-which is not the case when comparing raw pointers.
-
-Erroneous code example:
-
-```compile_fail,E0395
-static FOO: i32 = 42;
-static BAR: i32 = 42;
-
-static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
-// error: raw pointers cannot be compared in statics!
-```
-
-The address assigned by the linker to `FOO` and `BAR` may or may not
-be identical, so the value of `BAZ` can't be determined.
-
-If you want to do the comparison, please do it at run-time.
-
-For example:
-
-```
-static FOO: i32 = 42;
-static BAR: i32 = 42;
-
-let baz: bool = { (&FOO as *const i32) == (&BAR as *const i32) };
-// baz isn't a constant expression so it's ok
-```
-"##,
-
 E0161: r##"
 A value was moved. However, its size was not known at compile time, and only
 values of a known size can be moved.

src/librustc_mir/transform/qualify_consts.rs

@@ -752,14 +752,13 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 
                     self.add(Qualif::NOT_CONST);
                     if self.mode != Mode::Fn {
-                        struct_span_err!(
-                            self.tcx.sess, self.span, E0395,
-                            "raw pointers cannot be compared in {}s",
-                            self.mode)
-                        .span_label(
+                        emit_feature_err(
+                            &self.tcx.sess.parse_sess,
+                            "const_compare_raw_pointers",
                             self.span,
-                            "comparing raw pointers in static")
-                        .emit();
+                            GateIssue::Language,
+                            &format!("comparing raw pointers inside {}", self.mode),
+                        );
                     }
                 }
             }

src/libsyntax/feature_gate.rs

@@ -225,6 +225,9 @@ declare_features! (
     // Allows dereferencing raw pointers during const eval
     (active, const_raw_ptr_deref, "1.27.0", Some(51911), None),
 
+    // Allows comparing raw pointers during const eval
+    (active, const_compare_raw_pointers, "1.27.0", Some(53020), None),
+
     // Allows using #[prelude_import] on glob `use` items.
     //
     // rustc internal

src/test/ui/error-codes/E0395.rs

@@ -11,6 +11,6 @@
 static FOO: i32 = 42;
 static BAR: i32 = 42;
 
-static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) }; //~ ERROR E0395
+static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) }; //~ ERROR issue #53020
 fn main() {
 }

src/test/ui/error-codes/E0395.stderr

@@ -1,9 +1,11 @@
-error[E0395]: raw pointers cannot be compared in statics
+error[E0658]: comparing raw pointers inside static (see issue #53020)
   --> $DIR/E0395.rs:14:22
    |
-LL | static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) }; //~ ERROR E0395
-   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ comparing raw pointers in static
+LL | static BAZ: bool = { (&FOO as *const i32) == (&BAR as *const i32) }; //~ ERROR issue #53020
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_compare_raw_pointers)] to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0395`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/issue-25826.rs

@@ -11,6 +11,6 @@
 fn id<T>(t: T) -> T { t }
 fn main() {
     const A: bool = id::<u8> as *const () < id::<u16> as *const ();
-    //~^ ERROR raw pointers cannot be compared in constants [E0395]
+    //~^ ERROR comparing raw pointers inside constant
     println!("{}", A);
 }

src/test/ui/issue-25826.stderr

@@ -1,9 +1,11 @@
-error[E0395]: raw pointers cannot be compared in constants
+error[E0658]: comparing raw pointers inside constant (see issue #53020)
   --> $DIR/issue-25826.rs:13:21
    |
 LL |     const A: bool = id::<u8> as *const () < id::<u16> as *const ();
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ comparing raw pointers in static
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_compare_raw_pointers)] to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0395`.
+For more information about this error, try `rustc --explain E0658`.