Auto merge of #55635 - oli-obk:min_const_unsafe_fn, r=nikomatsakis

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

cc #55607

r? @Centril

26 files changed, 560 insertions, 37 deletions

diff --git a/src/test/run-pass-fulldeps/newtype_index.rs b/src/test/run-pass-fulldeps/newtype_index.rs
index 3cd622a33b1..0fd7ccd55fb 100644
--- a/src/test/run-pass-fulldeps/newtype_index.rs
+++ b/src/test/run-pass-fulldeps/newtype_index.rs
@@ -1,4 +1,4 @@
-#![feature(rustc_attrs, rustc_private, step_trait)]
+#![feature(rustc_attrs, rustc_private, step_trait, min_const_unsafe_fn)]
 
 #[macro_use] extern crate rustc_data_structures;
 extern crate rustc_serialize;
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn.rs b/src/test/ui/consts/min_const_fn/min_const_fn.rs
index 0dba3a7de53..43ca9e75393 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn.rs
+++ b/src/test/ui/consts/min_const_fn/min_const_fn.rs
@@ -78,9 +78,9 @@ const fn i32_ops2(c: i32, d: i32) -> bool { c < d }
 const fn i32_ops3(c: i32, d: i32) -> bool { c != d }
 const fn i32_ops4(c: i32, d: i32) -> i32 { c + d }
 const fn char_cast(u: u8) -> char { u as char }
-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 unsafe fn ret_i32_no_unsafe() -> i32 { 42 }
+const unsafe fn ret_null_ptr_no_unsafe<T>() -> *const T { 0 as *const T }
+const unsafe fn ret_null_mut_ptr_no_unsafe<T>() -> *mut T { 0 as *mut T }
 
 // not ok
 const fn foo11<T: std::fmt::Display>(t: T) -> T { t }
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.rs b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.rs
index 67332c6d2cf..f11b43dcd86 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.rs
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.rs
@@ -8,23 +8,27 @@
 // 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 }
-const unsafe fn foo6<T>() -> *mut T { 0 as *mut T }
+const unsafe fn ret_i32_no_unsafe() -> i32 { 42 }
+const unsafe fn ret_null_ptr_no_unsafe<T>() -> *const T { 0 as *const T }
+const unsafe fn ret_null_mut_ptr_no_unsafe<T>() -> *mut T { 0 as *mut T }
 const fn no_unsafe() { unsafe {} }
 
 // not ok
-const fn foo8() -> i32 {
-    unsafe { foo4() } //~ ERROR unsafe operations are not allowed in const fn
+const fn call_unsafe_const_fn() -> i32 {
+    unsafe { ret_i32_no_unsafe() } //~ ERROR calls to `const unsafe fn` in const fns are unstable
 }
-const fn foo9() -> *const String {
-    unsafe { foo5::<String>() } //~ ERROR unsafe operations are not allowed in const fn
+const fn call_unsafe_generic_const_fn() -> *const String {
+    unsafe { ret_null_ptr_no_unsafe::<String>() }
+    //~^ ERROR calls to `const unsafe fn` in const fns are unstable
 }
-const fn foo10() -> *const Vec<std::cell::Cell<u32>> {
-    unsafe { foo6::<Vec<std::cell::Cell<u32>>>() } //~ ERROR not allowed in const fn
+const fn call_unsafe_generic_cell_const_fn() -> *const Vec<std::cell::Cell<u32>> {
+    unsafe { ret_null_mut_ptr_no_unsafe::<Vec<std::cell::Cell<u32>>>() }
+    //~^ ERROR calls to `const unsafe fn` in const fns
 }
-const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
+const unsafe fn deref_forbidden(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
 //~^ dereferencing raw pointers in constant functions
 
 fn main() {}
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.stderr b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.stderr
index 8cff0d491d8..922a7883b9f 100644
--- a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.stderr
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe.stderr
@@ -1,58 +1,60 @@
 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:31:59
    |
-LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
-   |                                                   ^^
+LL | const unsafe fn deref_forbidden(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.rs:34:5
+  --> $DIR/min_const_fn_unsafe.rs:38:5
    |
 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.rs:19:14
+error[E0658]: calls to `const unsafe fn` in const fns are unstable (see issue #55607)
+  --> $DIR/min_const_fn_unsafe.rs:21:14
    |
-LL |     unsafe { foo4() } //~ ERROR unsafe operations are not allowed in const fn
-   |              ^^^^^^ call to unsafe function
+LL |     unsafe { ret_i32_no_unsafe() } //~ ERROR calls to `const unsafe fn` in const fns are unstable
+   |              ^^^^^^^^^^^^^^^^^^^
    |
-   = note: consult the function's documentation for information on how to avoid undefined behavior
+   = help: add #![feature(min_const_unsafe_fn)] 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:22:14
+error[E0658]: calls to `const unsafe fn` in const fns are unstable (see issue #55607)
+  --> $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
+LL |     unsafe { ret_null_ptr_no_unsafe::<String>() }
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: consult the function's documentation for information on how to avoid undefined behavior
+   = help: add #![feature(min_const_unsafe_fn)] 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:25:14
+error[E0658]: calls to `const unsafe fn` in const fns are unstable (see issue #55607)
+  --> $DIR/min_const_fn_unsafe.rs:28:14
    |
-LL |     unsafe { foo6::<Vec<std::cell::Cell<u32>>>() } //~ ERROR not allowed in const fn
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+LL |     unsafe { ret_null_mut_ptr_no_unsafe::<Vec<std::cell::Cell<u32>>>() }
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: consult the function's documentation for information on how to avoid undefined behavior
+   = help: add #![feature(min_const_unsafe_fn)] to the crate attributes to enable
 
 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:31:59
    |
-LL | const unsafe fn foo30_3(x: *mut usize) -> usize { *x } //~ ERROR not allowed in const fn
-   |                                                   ^^ dereference of raw pointer
+LL | const unsafe fn deref_forbidden(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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
 
 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:38:5
    |
 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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
 
 error: aborting due to 7 previous errors
 
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.rs b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.rs
new file mode 100644
index 00000000000..8a6884bc6b9
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.rs
@@ -0,0 +1,62 @@
+// 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
+
+const unsafe fn foo30_4(x: *mut usize) -> &'static usize { &*x } //~ ERROR not allowed in const fn
+//~^ dereferencing raw pointers in constant functions
+
+const fn foo30_5(x: *mut usize) -> &'static usize { unsafe { &*x } } //~ ERROR not allowed
+//~^ 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
+}
diff --git a/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.stderr b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.stderr
new file mode 100644
index 00000000000..20c75afbe63
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_fn_unsafe_feature_gate.stderr
@@ -0,0 +1,97 @@
+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]: dereferencing raw pointers in constant functions is unstable (see issue #51911)
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:50:60
+   |
+LL | const unsafe fn foo30_4(x: *mut usize) -> &'static usize { &*x } //~ ERROR not allowed in const fn
+   |                                                            ^^^
+   |
+   = help: add #![feature(const_raw_ptr_deref)] to the crate attributes to enable
+
+error[E0658]: dereferencing raw pointers in constant functions is unstable (see issue #51911)
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:53:62
+   |
+LL | const fn foo30_5(x: *mut usize) -> &'static usize { unsafe { &*x } } //~ ERROR not allowed
+   |                                                              ^^^
+   |
+   = 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:60:5
+   |
+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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+error: dereference of raw pointer is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:50:60
+   |
+LL | const unsafe fn foo30_4(x: *mut usize) -> &'static 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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+error: dereference of raw pointer is unsafe and unsafe operations are not allowed in const fn
+  --> $DIR/min_const_fn_unsafe_feature_gate.rs:53:62
+   |
+LL | const fn foo30_5(x: *mut usize) -> &'static usize { unsafe { &*x } } //~ ERROR not allowed
+   |                                                              ^^^ 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:60:5
+   |
+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
+   = note: unsafe action within a `const unsafe fn` still require an `unsafe` block in contrast to regular `unsafe fn`.
+
+error: aborting due to 11 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.rs b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.rs
new file mode 100644
index 00000000000..f559c23ff0f
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.rs
@@ -0,0 +1,47 @@
+// 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.
+
+#![unstable(feature = "humans",
+            reason = "who ever let humans program computers,
+            we're apparently really bad at it",
+            issue = "0")]
+
+#![feature(rustc_const_unstable, const_fn, foo, foo2)]
+#![feature(min_const_unsafe_fn)]
+#![feature(staged_api)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature="foo")]
+const unsafe fn foo() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar() -> u32 { unsafe { foo() } } //~ ERROR can only call other `min_const_fn`
+
+#[unstable(feature = "rust1", issue="0")]
+const unsafe fn foo2() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar2() -> u32 { unsafe { foo2() } } //~ ERROR can only call other `min_const_fn`
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// conformity is required, even with `const_fn` feature gate
+const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` op
+
+// check whether this function cannot be called even with the feature gate active
+#[unstable(feature = "foo2", issue="0")]
+const unsafe fn foo2_gated() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar2_gated() -> u32 { unsafe { foo2_gated() } } //~ ERROR can only call other
+
+fn main() {}
diff --git a/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.stderr b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.stderr
new file mode 100644
index 00000000000..37be2889173
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability.stderr
@@ -0,0 +1,26 @@
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:26:41
+   |
+LL | const unsafe fn bar() -> u32 { unsafe { foo() } } //~ ERROR can only call other `min_const_fn`
+   |                                         ^^^^^
+
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:33:42
+   |
+LL | const unsafe fn bar2() -> u32 { unsafe { foo2() } } //~ ERROR can only call other `min_const_fn`
+   |                                          ^^^^^^
+
+error: only int, `bool` and `char` operations are stable in const fn
+  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:37:33
+   |
+LL | const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` op
+   |                                 ^^^^^^^^^^^^^
+
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability.rs:45:48
+   |
+LL | const unsafe fn bar2_gated() -> u32 { unsafe { foo2_gated() } } //~ ERROR can only call other
+   |                                                ^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
diff --git a/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.rs b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.rs
new file mode 100644
index 00000000000..131bc97c85a
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.rs
@@ -0,0 +1,43 @@
+// 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.
+
+#![unstable(feature = "humans",
+            reason = "who ever let humans program computers,
+            we're apparently really bad at it",
+            issue = "0")]
+
+#![feature(rustc_const_unstable, const_fn, foo, foo2)]
+#![feature(min_const_unsafe_fn)]
+#![feature(staged_api)]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature="foo")]
+const fn foo() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar() -> u32 { foo() } //~ ERROR can only call other `min_const_fn`
+
+#[unstable(feature = "rust1", issue="0")]
+const fn foo2() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar2() -> u32 { foo2() } //~ ERROR can only call other `min_const_fn`
+
+// check whether this function cannot be called even with the feature gate active
+#[unstable(feature = "foo2", issue="0")]
+const fn foo2_gated() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const unsafe fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `min_const_fn`
+
+fn main() {}
diff --git a/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.stderr b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.stderr
new file mode 100644
index 00000000000..0b58dc1294b
--- /dev/null
+++ b/src/test/ui/consts/min_const_fn/min_const_unsafe_fn_libstd_stability2.stderr
@@ -0,0 +1,20 @@
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:26:32
+   |
+LL | const unsafe fn bar() -> u32 { foo() } //~ ERROR can only call other `min_const_fn`
+   |                                ^^^^^
+
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:33:33
+   |
+LL | const unsafe fn bar2() -> u32 { foo2() } //~ ERROR can only call other `min_const_fn`
+   |                                 ^^^^^^
+
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_unsafe_fn_libstd_stability2.rs:41:39
+   |
+LL | const unsafe fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `min_const_fn`
+   |                                       ^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
diff --git a/src/test/ui/unsafe/ranged_ints.rs b/src/test/ui/unsafe/ranged_ints.rs
new file mode 100644
index 00000000000..0fa2da917e9
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints.rs
@@ -0,0 +1,8 @@
+#![feature(rustc_attrs)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {
+    let _x = NonZero(0); //~ ERROR initializing type with `rustc_layout_scalar_valid_range` attr
+}
diff --git a/src/test/ui/unsafe/ranged_ints.stderr b/src/test/ui/unsafe/ranged_ints.stderr
new file mode 100644
index 00000000000..f59a930b5a9
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints.stderr
@@ -0,0 +1,11 @@
+error[E0133]: initializing type with `rustc_layout_scalar_valid_range` attr is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints.rs:7:14
+   |
+LL |     let _x = NonZero(0); //~ ERROR initializing type with `rustc_layout_scalar_valid_range` attr
+   |              ^^^^^^^^^^ initializing type with `rustc_layout_scalar_valid_range` attr
+   |
+   = note: initializing a layout restricted type's field with a value outside the valid range is undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints2.rs b/src/test/ui/unsafe/ranged_ints2.rs
new file mode 100644
index 00000000000..68ba120b279
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints2.rs
@@ -0,0 +1,9 @@
+#![feature(rustc_attrs)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {
+    let mut x = unsafe { NonZero(1) };
+    let y = &mut x.0; //~ ERROR mutation of layout constrained field is unsafe
+}
diff --git a/src/test/ui/unsafe/ranged_ints2.stderr b/src/test/ui/unsafe/ranged_ints2.stderr
new file mode 100644
index 00000000000..ae63f47ed74
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints2.stderr
@@ -0,0 +1,11 @@
+error[E0133]: mutation of layout constrained field is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints2.rs:8:13
+   |
+LL |     let y = &mut x.0; //~ ERROR mutation of layout constrained field is unsafe
+   |             ^^^^^^^^ mutation of layout constrained field
+   |
+   = note: mutating layout constrained fields cannot statically be checked for valid values
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints2_const.rs b/src/test/ui/unsafe/ranged_ints2_const.rs
new file mode 100644
index 00000000000..a61e3329bdc
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints2_const.rs
@@ -0,0 +1,20 @@
+#![feature(rustc_attrs, const_let, const_fn)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {
+}
+
+const fn foo() -> NonZero<u32> {
+    let mut x = unsafe { NonZero(1) };
+    let y = &mut x.0; //~ ERROR references in constant functions may only refer to immutable
+    //~^ ERROR mutation of layout constrained field is unsafe
+    unsafe { NonZero(1) }
+}
+
+const fn bar() -> NonZero<u32> {
+    let mut x = unsafe { NonZero(1) };
+    let y = unsafe { &mut x.0 }; //~ ERROR references in constant functions may only refer to immut
+    unsafe { NonZero(1) }
+}
diff --git a/src/test/ui/unsafe/ranged_ints2_const.stderr b/src/test/ui/unsafe/ranged_ints2_const.stderr
new file mode 100644
index 00000000000..f79792ffba9
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints2_const.stderr
@@ -0,0 +1,24 @@
+error[E0017]: references in constant functions may only refer to immutable values
+  --> $DIR/ranged_ints2_const.rs:11:13
+   |
+LL |     let y = &mut x.0; //~ ERROR references in constant functions may only refer to immutable
+   |             ^^^^^^^^ constant functions require immutable values
+
+error[E0017]: references in constant functions may only refer to immutable values
+  --> $DIR/ranged_ints2_const.rs:18:22
+   |
+LL |     let y = unsafe { &mut x.0 }; //~ ERROR references in constant functions may only refer to immut
+   |                      ^^^^^^^^ constant functions require immutable values
+
+error[E0133]: mutation of layout constrained field is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints2_const.rs:11:13
+   |
+LL |     let y = &mut x.0; //~ ERROR references in constant functions may only refer to immutable
+   |             ^^^^^^^^ mutation of layout constrained field
+   |
+   = note: mutating layout constrained fields cannot statically be checked for valid values
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0017, E0133.
+For more information about an error, try `rustc --explain E0017`.
diff --git a/src/test/ui/unsafe/ranged_ints3.rs b/src/test/ui/unsafe/ranged_ints3.rs
new file mode 100644
index 00000000000..47d67fac678
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints3.rs
@@ -0,0 +1,11 @@
+#![feature(rustc_attrs)]
+
+use std::cell::Cell;
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {
+    let mut x = unsafe { NonZero(Cell::new(1)) };
+    let y = &x.0; //~ ERROR borrow of layout constrained field with interior mutability
+}
diff --git a/src/test/ui/unsafe/ranged_ints3.stderr b/src/test/ui/unsafe/ranged_ints3.stderr
new file mode 100644
index 00000000000..311a058fdb0
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints3.stderr
@@ -0,0 +1,11 @@
+error[E0133]: borrow of layout constrained field with interior mutability is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints3.rs:10:13
+   |
+LL |     let y = &x.0; //~ ERROR borrow of layout constrained field with interior mutability
+   |             ^^^^ borrow of layout constrained field with interior mutability
+   |
+   = note: references to fields of layout constrained fields lose the constraints. Coupled with interior mutability, the field can be changed to invalid values
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints3_const.rs b/src/test/ui/unsafe/ranged_ints3_const.rs
new file mode 100644
index 00000000000..6497b611224
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints3_const.rs
@@ -0,0 +1,21 @@
+#![feature(rustc_attrs, const_let, const_fn)]
+
+use std::cell::Cell;
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {}
+
+const fn foo() -> NonZero<Cell<u32>> {
+    let mut x = unsafe { NonZero(Cell::new(1)) };
+    let y = &x.0; //~ ERROR cannot borrow a constant which may contain interior mutability
+    //~^ ERROR borrow of layout constrained field with interior mutability
+    unsafe { NonZero(Cell::new(1)) }
+}
+
+const fn bar() -> NonZero<Cell<u32>> {
+    let mut x = unsafe { NonZero(Cell::new(1)) };
+    let y = unsafe { &x.0 }; //~ ERROR cannot borrow a constant which may contain interior mut
+    unsafe { NonZero(Cell::new(1)) }
+}
diff --git a/src/test/ui/unsafe/ranged_ints3_const.stderr b/src/test/ui/unsafe/ranged_ints3_const.stderr
new file mode 100644
index 00000000000..d83d75787d9
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints3_const.stderr
@@ -0,0 +1,24 @@
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/ranged_ints3_const.rs:12:13
+   |
+LL |     let y = &x.0; //~ ERROR cannot borrow a constant which may contain interior mutability
+   |             ^^^^
+
+error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
+  --> $DIR/ranged_ints3_const.rs:19:22
+   |
+LL |     let y = unsafe { &x.0 }; //~ ERROR cannot borrow a constant which may contain interior mut
+   |                      ^^^^
+
+error[E0133]: borrow of layout constrained field with interior mutability is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints3_const.rs:12:13
+   |
+LL |     let y = &x.0; //~ ERROR cannot borrow a constant which may contain interior mutability
+   |             ^^^^ borrow of layout constrained field with interior mutability
+   |
+   = note: references to fields of layout constrained fields lose the constraints. Coupled with interior mutability, the field can be changed to invalid values
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0133, E0492.
+For more information about an error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints4.rs b/src/test/ui/unsafe/ranged_ints4.rs
new file mode 100644
index 00000000000..d8632c48434
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints4.rs
@@ -0,0 +1,9 @@
+#![feature(rustc_attrs)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {
+    let mut x = unsafe { NonZero(1) };
+    x.0 = 0; //~ ERROR mutation of layout constrained field is unsafe
+}
diff --git a/src/test/ui/unsafe/ranged_ints4.stderr b/src/test/ui/unsafe/ranged_ints4.stderr
new file mode 100644
index 00000000000..c6468b643b4
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints4.stderr
@@ -0,0 +1,11 @@
+error[E0133]: mutation of layout constrained field is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints4.rs:8:5
+   |
+LL |     x.0 = 0; //~ ERROR mutation of layout constrained field is unsafe
+   |     ^^^^^^^ mutation of layout constrained field
+   |
+   = note: mutating layout constrained fields cannot statically be checked for valid values
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints4_const.rs b/src/test/ui/unsafe/ranged_ints4_const.rs
new file mode 100644
index 00000000000..f589e4739ba
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints4_const.rs
@@ -0,0 +1,19 @@
+#![feature(rustc_attrs, const_let, const_fn)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {}
+
+const fn foo() -> NonZero<u32> {
+    let mut x = unsafe { NonZero(1) };
+    x.0 = 0;
+    //~^ ERROR mutation of layout constrained field is unsafe
+    x
+}
+
+const fn bar() -> NonZero<u32> {
+    let mut x = unsafe { NonZero(1) };
+    unsafe { x.0 = 0 }; // this is UB
+    x
+}
diff --git a/src/test/ui/unsafe/ranged_ints4_const.stderr b/src/test/ui/unsafe/ranged_ints4_const.stderr
new file mode 100644
index 00000000000..fe83b15ce5c
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints4_const.stderr
@@ -0,0 +1,11 @@
+error[E0133]: mutation of layout constrained field is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints4_const.rs:10:5
+   |
+LL |     x.0 = 0;
+   |     ^^^^^^^ mutation of layout constrained field
+   |
+   = note: mutating layout constrained fields cannot statically be checked for valid values
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/unsafe/ranged_ints_const.rs b/src/test/ui/unsafe/ranged_ints_const.rs
new file mode 100644
index 00000000000..8477772867e
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints_const.rs
@@ -0,0 +1,11 @@
+#![feature(rustc_attrs)]
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[repr(transparent)]
+pub(crate) struct NonZero<T>(pub(crate) T);
+fn main() {}
+
+const fn foo() -> NonZero<u32> { NonZero(0) }
+//~^ ERROR initializing type with `rustc_layout_scalar_valid_range` attr is unsafe
+
+const fn bar() -> NonZero<u32> { unsafe { NonZero(0) } }
diff --git a/src/test/ui/unsafe/ranged_ints_const.stderr b/src/test/ui/unsafe/ranged_ints_const.stderr
new file mode 100644
index 00000000000..584ad40a92b
--- /dev/null
+++ b/src/test/ui/unsafe/ranged_ints_const.stderr
@@ -0,0 +1,11 @@
+error[E0133]: initializing type with `rustc_layout_scalar_valid_range` attr is unsafe and requires unsafe function or block
+  --> $DIR/ranged_ints_const.rs:8:34
+   |
+LL | const fn foo() -> NonZero<u32> { NonZero(0) }
+   |                                  ^^^^^^^^^^ initializing type with `rustc_layout_scalar_valid_range` attr
+   |
+   = note: initializing a layout restricted type's field with a value outside the valid range is undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.