WIP crater experiment: do not promote const fn calls implicitly ever

compiler/rustc_mir/src/transform/promote_consts.rs

@@ -692,14 +692,9 @@ impl<'tcx> Validator<'_, 'tcx> {
  ) -> Result<(), Unpromotable> {
  let fn_ty = callee.ty(self.body, self.tcx);
 
- // When doing explicit promotion and inside const/static items, we promote all (eligible) function calls.
+ // When doing explicit promotion, we promote all (eligible) function calls.
  // Everywhere else, we require `#[rustc_promotable]` on the callee.
- let promote_all_const_fn = self.explicit
- || matches!(
- self.const_kind,
- Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
- );
- if !promote_all_const_fn {
+ if !self.explicit {
  if let ty::FnDef(def_id, _) = *fn_ty.kind() {
  // Never promote runtime `const fn` calls of
  // functions without `#[rustc_promotable]`.

library/core/src/ptr/const_ptr.rs

@@ -43,6 +43,7 @@ impl<T: ?Sized> *const T {
  /// Casts to a pointer of another type.
  #[stable(feature = "ptr_cast", since = "1.38.0")]
  #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[inline]
  pub const fn cast<U>(self) -> *const U {
  self as _

library/core/src/ptr/mod.rs

@@ -196,7 +196,7 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 /// ```
 #[inline(always)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
+#[rustc_promotable] // Operation cannot fail, so it may be promoted.
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
 pub const fn null<T>() -> *const T {
  0 as *const T
@@ -214,7 +214,7 @@ pub const fn null<T>() -> *const T {
 /// ```
 #[inline(always)]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
+#[rustc_promotable] // Operation cannot fail, so it may be promoted.
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
 pub const fn null_mut<T>() -> *mut T {
  0 as *mut T

library/core/src/ptr/mut_ptr.rs

@@ -42,6 +42,7 @@ impl<T: ?Sized> *mut T {
  /// Casts to a pointer of another type.
  #[stable(feature = "ptr_cast", since = "1.38.0")]
  #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[inline]
  pub const fn cast<U>(self) -> *mut U {
  self as _

library/core/src/slice/mod.rs

@@ -404,6 +404,7 @@ impl<T> [T] {
  /// [`as_mut_ptr`]: #method.as_mut_ptr
  #[stable(feature = "rust1", since = "1.0.0")]
  #[rustc_const_stable(feature = "const_slice_as_ptr", since = "1.32.0")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[inline]
  pub const fn as_ptr(&self) -> *const T {
  self as *const [T] as *const T
@@ -432,6 +433,7 @@ impl<T> [T] {
  /// ```
  #[stable(feature = "rust1", since = "1.0.0")]
  #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[inline]
  pub const fn as_mut_ptr(&mut self) -> *mut T {
  self as *mut [T] as *mut T

library/std/src/net/ip.rs

@@ -302,6 +302,7 @@ impl Ipv4Addr {
  /// let addr = Ipv4Addr::new(127, 0, 0, 1);
  /// ```
  #[rustc_const_stable(feature = "const_ipv4", since = "1.32.0")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[stable(feature = "rust1", since = "1.0.0")]
  pub const fn new(a: u8, b: u8, c: u8, d: u8) -> Ipv4Addr {
  // `s_addr` is stored as BE on all machine and the array is in BE order.
@@ -1045,6 +1046,7 @@ impl Ipv6Addr {
  /// ```
  #[rustc_allow_const_fn_unstable(const_fn_transmute)]
  #[rustc_const_stable(feature = "const_ipv6", since = "1.32.0")]
+ #[rustc_promotable] // Operation cannot fail, so it may be promoted.
  #[stable(feature = "rust1", since = "1.0.0")]
  pub const fn new(a: u16, b: u16, c: u16, d: u16, e: u16, f: u16, g: u16, h: u16) -> Ipv6Addr {
  let addr16 = [

src/test/ui/consts/const-eval/ub-ref.rs

@@ -26,7 +26,7 @@ const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
 const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
 //~^ ERROR it is undefined behavior to use this value
 
-const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize], _>(&[mem::transmute(&0)]) };
+const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute(&[&0] as &[_]) };
 //~^ ERROR it is undefined behavior to use this value
 
 const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };

src/test/ui/consts/const-eval/ub-ref.stderr

@@ -49,8 +49,8 @@ LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
 error[E0080]: it is undefined behavior to use this value
  --> $DIR/ub-ref.rs:29:1
  |
-LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize], _>(&[mem::transmute(&0)]) };
- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer at .<deref>, but expected plain (non-pointer) bytes
+LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute(&[&0] as &[_]) };
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer at .<deref>, but expected plain (non-pointer) bytes
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
 

src/test/ui/consts/rfc-2203-const-array-repeat-exprs/fn-call-in-const.rs

@@ -1,7 +1,8 @@
 // run-pass
 
-#![allow(unused)]
+#![allow(unused, incomplete_features)]
 #![feature(const_in_array_repeat_expressions)]
+#![feature(inline_const)]
 
 // Some type that is not copyable.
 struct Bar;
@@ -16,8 +17,6 @@ const fn type_copy() -> u32 {
 
 const _: [u32; 2] = [type_copy(); 2];
 
-// This is allowed because all promotion contexts use the explicit rules for promotability when
-// inside an explicit const context.
-const _: [Option<Bar>; 2] = [type_no_copy(); 2];
+const _: [Option<Bar>; 2] = [const { type_no_copy() }; 2];
 
 fn main() {}