Allow calling const unsafe fn in const fn behind a feature gate

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

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// gate-test-min_const_unsafe_fn
+
 // ok
 const unsafe fn foo4() -> i32 { 42 }
 const unsafe fn foo5<T>() -> *const T { 0 as *const T }

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

@@ -1,5 +1,5 @@
 error[E0658]: dereferencing raw pointers in constant functions is unstable (see issue #51911)
-  --> $DIR/min_const_fn_unsafe.rs:27:51
+  --> $DIR/min_const_fn_unsafe.rs:29:51
    |
 LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
    |                                                   ^^
@@ -7,7 +7,7 @@ LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowe
    = help: add #![feature(const_raw_ptr_deref)] to the crate attributes to enable
 
 error[E0658]: unions in const fn are unstable (see issue #51909)
-  --> $DIR/min_const_fn_unsafe.rs:34:5
+  --> $DIR/min_const_fn_unsafe.rs:36:5
    |
 LL |     Foo { x: () }.y //~ ERROR not allowed in const fn
    |     ^^^^^^^^^^^^^^^
@@ -15,7 +15,7 @@ LL |     Foo { x: () }.y //~ ERROR not allowed in const fn
    = help: add #![feature(const_fn_union)] to the crate attributes to enable
 
 error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
-  --> $DIR/min_const_fn_unsafe.rs:19:14
+  --> $DIR/min_const_fn_unsafe.rs:21:14
    |
 LL |     unsafe { foo4() } //~ ERROR unsafe operations are not allowed in const fn
    |              ^^^^^^ call to unsafe function
@@ -23,7 +23,7 @@ LL |     unsafe { foo4() } //~ ERROR unsafe operations are not allowed in const
    = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
-  --> $DIR/min_const_fn_unsafe.rs:22:14
+  --> $DIR/min_const_fn_unsafe.rs:24:14
    |
 LL |     unsafe { foo5::<String>() } //~ ERROR unsafe operations are not allowed in const fn
    |              ^^^^^^^^^^^^^^^^ call to unsafe function
@@ -31,7 +31,7 @@ LL |     unsafe { foo5::<String>() } //~ ERROR unsafe operations are not allowed
    = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
-  --> $DIR/min_const_fn_unsafe.rs:25:14
+  --> $DIR/min_const_fn_unsafe.rs:27:14
    |
 LL |     unsafe { foo6::<Vec<std::cell::Cell<u32>>>() } //~ ERROR not allowed in const fn
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
@@ -39,7 +39,7 @@ LL |     unsafe { foo6::<Vec<std::cell::Cell<u32>>>() } //~ ERROR not allowed in
    = note: consult the function's documentation for information on how to avoid undefined behavior
 
 error: dereference of raw pointer is unsafe and unsafe operations are not allowed in const fn
-  --> $DIR/min_const_fn_unsafe.rs:27:51
+  --> $DIR/min_const_fn_unsafe.rs:29:51
    |
 LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
    |                                                   ^^ dereference of raw pointer
@@ -47,7 +47,7 @@ LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowe
    = note: raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules and cause data races: all of these are undefined behavior
 
 error: access to union field is unsafe and unsafe operations are not allowed in const fn
-  --> $DIR/min_const_fn_unsafe.rs:34:5
+  --> $DIR/min_const_fn_unsafe.rs:36:5
    |
 LL |     Foo { x: () }.y //~ ERROR not allowed in const fn
    |     ^^^^^^^^^^^^^^^ access to union field

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

@@ -0,0 +1,56 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(min_const_unsafe_fn)]
+
+// ok
+const unsafe fn foo4() -> i32 { 42 }
+const unsafe fn foo5<T>() -> *const T { 0 as *const T }
+const unsafe fn foo6<T>() -> *mut T { 0 as *mut T }
+const fn no_unsafe() { unsafe {} }
+
+const fn foo8() -> i32 {
+    unsafe { foo4() }
+}
+const fn foo9() -> *const String {
+    unsafe { foo5::<String>() }
+}
+const fn foo10() -> *const Vec<std::cell::Cell<u32>> {
+    unsafe { foo6::<Vec<std::cell::Cell<u32>>>() }
+}
+const unsafe fn foo8_3() -> i32 {
+    unsafe { foo4() }
+}
+const unsafe fn foo9_3() -> *const String {
+    unsafe { foo5::<String>() }
+}
+const unsafe fn foo10_3() -> *const Vec<std::cell::Cell<u32>> {
+    unsafe { foo6::<Vec<std::cell::Cell<u32>>>() }
+}
+// not ok
+const unsafe fn foo8_2() -> i32 {
+    foo4() //~ ERROR not allowed in const fn
+}
+const unsafe fn foo9_2() -> *const String {
+    foo5::<String>() //~ ERROR not allowed in const fn
+}
+const unsafe fn foo10_2() -> *const Vec<std::cell::Cell<u32>> {
+    foo6::<Vec<std::cell::Cell<u32>>>() //~ ERROR not allowed in const fn
+}
+const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
+//~^ dereferencing raw pointers in constant functions
+
+fn main() {}
+
+const unsafe fn no_union() {
+    union Foo { x: (), y: () }
+    Foo { x: () }.y //~ ERROR not allowed in const fn
+    //~^ unions in const fn
+}

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

@@ -0,0 +1,59 @@
+error[E0658]: dereferencing raw pointers in constant functions is unstable (see issue #51911)
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:47:51
+   |
+LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
+   |                                                   ^^
+   |
+   = help: add #![feature(const_raw_ptr_deref)] to the crate attributes to enable
+
+error[E0658]: unions in const fn are unstable (see issue #51909)
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:54:5
+   |
+LL |     Foo { x: () }.y //~ ERROR not allowed in const fn
+   |     ^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn_union)] to the crate attributes to enable
+
+error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:39:5
+   |
+LL |     foo4() //~ ERROR not allowed in const fn
+   |     ^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:42:5
+   |
+LL |     foo5::<String>() //~ ERROR not allowed in const fn
+   |     ^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:45:5
+   |
+LL |     foo6::<Vec<std::cell::Cell<u32>>>() //~ ERROR not allowed in const fn
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: dereference of raw pointer is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:47:51
+   |
+LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
+   |                                                   ^^ dereference of raw pointer
+   |
+   = note: raw pointers may be NULL, dangling or unaligned; they can violate aliasing rules and cause data races: all of these are undefined behavior
+
+error: access to union field is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:54:5
+   |
+LL |     Foo { x: () }.y //~ ERROR not allowed in const fn
+   |     ^^^^^^^^^^^^^^^ access to union field
+   |
+   = note: the field may not be properly initialized: using uninitialized data will cause undefined behavior
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0658`.